Analysis of previous "Instrumentation" competition topics

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Analysis of previous "Instrumentation" competition topics [Copy link]

 

In all previous electronic design competitions , " instrumentation " is the most common type of competition topic :

If we also include the signal source category questions ( 4 questions) [1] , such as : Design and production of practical signal sources ( Question B in 1995 ) ; Waveform generator ( Question A in 2001 ) ; Sine signal generator ( Question A in the seventh session of 2005 ) ; Signal generator ( Question H in 2007 ) [Higher vocational and technical school group] , the number of “ instrumentation and metering ” questions will reach 18 .

1. Simple resistance, capacitance and inductance tester ( 1995 D question )
2. Simple digital frequency meter ( 1997 B question )
3. Digital Power Frequency Effective Value Multimeter ( 1999 Question B )
4. Frequency characteristics tester ( 1999 C )
5. Simple Digital Storage Oscilloscope ( 2001 Question B )
6. Low frequency digital phase measuring instrument ( 2003 C topic )
7. Simple Logic Analyzer ( 2003 D Question)
8. Integrated operational amplifier parameter tester ( 2005 B question)
9. Simple spectrum analyzer ( 2005 C question)
10. Audio Signal Analyzer ( 2007 A ) [ Undergraduate Group]
11. Digital Oscilloscope ( 2007 Topic C ) [ Undergraduate Group]
12. Integrating DC Digital Voltmeter ( 2007 G ) [Higher Vocational and Technical College Group ]
13. Simple digital signal transmission performance analyzer ( 2011 E topic ) [Undergraduate group]
14. Simple automatic resistance tester ( 2011 G topic ) [Higher vocational and technical college group]

It can be seen that instrumentation questions are the most common type of questions that appear in electronic design competitions.

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Previous "Instrumentation" competition requirements and design solutions

Simple resistance, capacitance and inductance tester ( 1995 D question )

Design requirements for simple resistance, capacitance and inductance testers

Design and make a resistance, capacitance and inductance parameter tester with digital display, with measurement range: resistance 100Ω ~ 1MΩ ; capacitance 100pF ~ 10000pF ; inductance 100μH ~ 10mH . The measurement accuracy is ±5% . Make a 4 -digit digital tube display to display the measured value, and use light-emitting diodes to indicate the type and unit of the measured component.

Simple resistance, capacitance and inductance tester system design

1. Resistance measurement

( 1 ) Voltammetry : The theoretical basis of voltammetry is Ohm's law, that is, R=U / I . The specific method is to directly measure the terminal voltage and current flowing through the resistor to be measured, and then calculate the resistance value. This method seems simple and easy to use, but to measure accurately, it is necessary to select appropriate instruments and measurement methods according to specific circumstances.

( 2 ) Resistance measurement method in digital multimeter : A multi-value constant current source is formed by using a DC power supply, input resistor and operational amplifier to achieve multi-range resistance measurement. The current and voltage values of each range can be set . The constant current I ( I =E/R ) passes through the measured resistor Rx , and its terminal voltage Ux is measured by the digital voltage meter (DVM) , then Rx = Ux / I .

( 3 ) Measurement of high-value resistors : The voltage source voltage division method can be used to measure high-value resistors. Since the high-value resistor Rx is very large, the actual measurement requires: ① The buffer amplifier must have an extremely high input impedance. The DC input buffer of the instrument uses a cascade field effect transistor as a high input impedance stage. ② The circuit insulation is good. In order to reduce leakage from the buffer amplifier, printed circuit board, etc., measures must be taken on the printed circuit board materials, processes, moisture resistance, etc. ③ Use error correction technology to eliminate the voltage division error through calculation.

( 4 ) Bridge method : The bridge method is also called the zero indication method. It uses a zero-pointing circuit as a measurement indicator. It has a wide operating frequency and can largely eliminate or weaken the influence of system errors. It has a high accuracy of 10-4 . It should be pointed out that in practical applications, a DC double-arm bridge (also called a Kelvin bridge) is used to measure resistance. The signal source is a DC power supply, usually a large-capacity battery. This DC bridge can eliminate the measurement error caused by wiring resistance and contact resistance, and the accuracy of measuring small resistance can reach 10-5 .

2. Measurement of inductance and capacitance

( 1 ) Bridge method : In fact, impedance measuring instruments using the bridge method are multifunctional instruments, often called universal bridges. It is an AC bridge that can measure resistance, inductance and capacitance, the Q value of a coil, and the loss of a capacitor. It is a multi-purpose, wide-range portable instrument. The bridge consists of three parts: the bridge body, the signal source ( 1000Hz oscillator), and the transistor nulling instrument . The bridge body is the core part of the bridge, which consists of standard resistance, standard capacitance, and a conversion switch. By switching the conversion switch, different bridge circuits can be formed to measure resistance, capacitance, and inductance.

( 2 ) Resonance method ( Q meter) : The resonance method is another basic method for measuring impedance. It is a measurement method based on the resonance characteristics of a tuned circuit. Although the measurement accuracy is not as high as that of the AC bridge method, the technical difficulty is smaller than that of the high-frequency bridge method (mainly due to the influence of stray coupling) because the measurement circuit is simple and convenient. In addition, most high-frequency circuit components are used in tuned circuits, so the resonance method is more in line with the actual working conditions. Therefore, the resonance method is an important means for measuring high-frequency circuit parameters (such as capacitance, inductance, quality factor, effective impedance, etc.). Adjust the circuit to the resonant state, and according to the known circuit relationship and the known component values, the parameters of the unknown components can be obtained.

( 3 ) L and C measurement in portable digital multimeter : In portable digital multimeter, the time constant method is used to reduce costs. Time constant τ = RC .

3. Resistance, capacitance and inductance parameter tester system solution

The system structure of a resistance, capacitance and inductance parameter tester is as follows : resistance, capacitance and inductance use RC oscillators and LC oscillators to make their R , C , L values related to the oscillation frequency. According to the selected channel, the AT89S52 single-chip microcomputer sends two-bit address signals to the analog switch to obtain the oscillation frequency of the RC oscillator (using a 555 timer circuit ) or the LC oscillator ( capacitor three-point oscillation circuit ) , and then determines whether to switch the range according to the measured frequency. After processing the data, it is sent to the digital tube to display the corresponding measured R , C , L value parameter value.

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sigma published on 2019-6-24 14:02 Previous "Instrumentation" competition requirements and design solutions 1 Simple resistance, capacitance and inductance tester (1995...

Previous " Instrumentation " competition requirements and design solutions 2

Simple digital frequency meter ( 1997 B question )

Design requirements for a simple digital frequency meter: Design and make a simple frequency meter with digital display.

1. Design requirements

( 1 ) Basic requirements
　　① Frequency measurement
　　a. Measurement range Signal: square wave, sine wave; Amplitude: 0.5V～5V; Frequency: 1Hz～1MHz;
　　b. Measurement error ≤0.1%.
　　② Period measurement
　　a. Measurement range Signal: square wave, sine wave; Amplitude: 0.5V～5V; Frequency: 1Hz～1MHz;
　　b. Measurement error ≤0.1%.
　　③ Pulse width measurement
　　a. Measurement range Signal: pulse wave; Amplitude: 0.5V～5V; Pulse width ≥100μs;
　　b. Measurement error ≤1%.
　　④ Display
　　Decimal digital display, display refresh time 1～10 seconds continuously adjustable, different color LEDs are used to indicate the above three measurement functions.
　　⑤ With self-calibration function, the time scale signal frequency is 1MHz.
　　⑥ Design and make a voltage-stabilized power supply that meets the requirements of this design task.

( 2 ) Development part
　　① Extend the frequency measurement range to 0.1Hz～10MHz (signal amplitude 0.5V～5V), and reduce the measurement error to 0.01% (maximum gate time ≤10s).
　　② Measure and display the duty cycle of periodic pulse signals (amplitude 0.5V～5V, frequency 1Hz～1kHz), the duty cycle range is 10%～90%, and the measurement error is ≤1%.
　　③ Perform frequency measurement of small signals within the range of 1Hz～1MHz and under the condition that the measurement error is ≤1%, and propose and implement anti-interference measures.

Design of Simple Digital Frequency Meter System

1. Frequency measurement method

( 1 ) Direct frequency measurement by counting : In the direct frequency measurement by counting method, the main gate has the logic function of "AND gate". A narrow pulse with a frequency of X is sent to one input of the main gate, which is obtained by amplifying and shaping the measured signal through channel A. The gate time signal Ts from the gated bistable is sent to the other input of the main gate. Because the gated bistable is controlled by the time base (standard frequency) signal, Ts is both accurate and stable. During the design, the gate time of 10 s, 1 s, 0.1 s, etc. can be obtained by the cooperation of the crystal oscillator and the frequency divider. Due to the "AND" function of the main gate, its output end only outputs a narrow pulse with a frequency of X during the effective period of the gate signal Ts, and sends it to the counter for counting. The count value is N=TS/TX=TS*X, which is proportional to the frequency X of the measured signal. Therefore, X=N/TS can be obtained.

( 2 ) Counting type direct period measurement : Compared with the counting type direct frequency measurement method, the gated bistable is controlled by the pulse after the input signal is amplified, shaped and divided, so the width of the gate time is equal to k times the period kTX of the measured signal; and the other input end of the main gate is fed with a time-marker pulse signal with a period of T0 generated by a crystal oscillator and a frequency divider. Due to the "AND" function of the main gate, it only outputs a time-marker pulse signal during the kTX period, and is counted by the counter, and its value is N. It is not difficult to see that the period of the measured signal is: TX=NT0/k.

( 3 ) Counting type measurement of time interval : The counting type measurement of time interval is based on the block diagram of the measurement period, and the gated bistable is changed to be controlled by pulse signals output by two timing channels respectively. One pulse corresponds to the starting point of the measured time interval, called the start signal, which sets the gated bistable and opens the main gate; the other pulse corresponds to the end point of the measured time interval, called the stop signal, which resets the gated bistable and closes the main gate. Therefore, the width of the gate signal and the time the main gate is open are equal to the measured time interval ΔTX. During this period of time, the number of time-stamp pulses To counted by the counter is N, so the measured time interval is: ΔTX=NT0.

( 4 ) Equal-precision frequency and period measurement : The reciprocal counter uses a multi-period synchronous measurement method, that is, it measures multiple (integer) period values of the input signal, and then performs a reciprocal operation to obtain the frequency. Compared with the direct measurement method, its advantage is that it can obtain the same high test accuracy and resolution in the entire frequency measurement range.

( 5 ) Measuring time intervals with equal precision : To measure the time interval between two pulse signals with equal precision, a synchronization circuit 2 (D flip-flop) and a B input channel can be added on the basis of the equal precision frequency and period measurement methods, and its output is inverted and sent to the reset terminal of synchronization circuit 2. The trigger clock of the synchronization circuit is obtained by the output of input channel A after being delayed by two stages of inverters. The output UQ2 of the synchronization circuit is directly counted by counter A and also serves as the opening signal of gate B. Counter B records the number of clock pulses passing through gate B. Finally, the numbers counted by the two counters are sent to the operation circuit for processing to obtain the value of the measured time interval.

If the input ends of the two input channels are connected together, and the trigger polarity and trigger level of the two channels are selected respectively, a pulse corresponding to A is generated at the leading edge of the pulse, and a pulse corresponding to B is generated at the trailing edge of the measured pulse, the pulse width can be measured.

Based on the measured signal pulse width and period, the duty cycle can be obtained by calculation.

2. Design

(1) The use of programmable devices such as FPGA can easily complete the design of frequency meters with different measurement principles.

(2) The frequency meter is constructed with the AT89S52 single-chip microcomputer as the core, and adopts high-impedance, high-gain front-end amplifiers and dividers. It adopts a variety of software anti-interference measures such as shielding and watchdog, software traps, and software fault tolerance. The input signal to be measured (0.1 Hz~30 MHz) is divided into four frequency bands: 0.1 Hz~1 Hz, 1Hz~50 kHz, 50 kHz~1 MHz, and 1 MHz~30 MHz. Two pre-processing amplifiers are used for signals less than 1 MHz and greater than 1 MHz, respectively, to amplify them, and then the output signals of the amplifiers are shaped and divided. The single-chip microcomputer is used to measure and calculate the frequency, period, pulse width, and duty cycle, and the calculation results are serially output to the digital display.

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Previous " Instrumentation " competition requirements and design solutions 2

Simple digital frequency meter ( 1997 B question )

Design requirements for a simple digital frequency meter: Design and make a simple frequency meter with digital display.

1. Design requirements

( 1 ) Basic requirements
　　① Frequency measurement
　　a. Measurement range Signal: square wave, sine wave; Amplitude: 0.5V～5V; Frequency: 1Hz～1MHz;
　　b. Measurement error ≤0.1%.
　　② Period measurement
　　a. Measurement range Signal: square wave, sine wave; Amplitude: 0.5V～5V; Frequency: 1Hz～1MHz;
　　b. Measurement error ≤0.1%.
　　③ Pulse width measurement
　　a. Measurement range Signal: pulse wave; Amplitude: 0.5V～5V; Pulse width ≥100μs;
　　b. Measurement error ≤1%.
　　④ Display
　　Decimal digital display, display refresh time 1～10 seconds continuously adjustable, different color LEDs are used to indicate the above three measurement functions.
　　⑤ With self-calibration function, the time scale signal frequency is 1MHz.
　　⑥ Design and make a voltage-stabilized power supply that meets the requirements of this design task.

( 2 ) Development part
　　① Extend the frequency measurement range to 0.1Hz～10MHz (signal amplitude 0.5V～5V), and reduce the measurement error to 0.01% (maximum gate time ≤10s).
　　② Measure and display the duty cycle of periodic pulse signals (amplitude 0.5V～5V, frequency 1Hz～1kHz), the duty cycle range is 10%～90%, and the measurement error is ≤1%.
　　③ Perform frequency measurement of small signals within the range of 1Hz～1MHz and under the condition that the measurement error is ≤1%, and propose and implement anti-interference measures.

Design of Simple Digital Frequency Meter System

1. Frequency measurement method

( 1 ) Direct frequency measurement by counting : In the direct frequency measurement by counting method, the main gate has the logic function of "AND gate". A narrow pulse with a frequency of X is sent to one input of the main gate, which is obtained by amplifying and shaping the measured signal through channel A. The gate time signal Ts from the gated bistable is sent to the other input of the main gate. Because the gated bistable is controlled by the time base (standard frequency) signal, Ts is both accurate and stable. During the design, the gate time of 10 s, 1 s, 0.1 s, etc. can be obtained by the cooperation of the crystal oscillator and the frequency divider. Due to the "AND" function of the main gate, its output end only outputs a narrow pulse with a frequency of X during the effective period of the gate signal Ts, and sends it to the counter for counting. The count value is N=TS/TX=TS*X, which is proportional to the frequency X of the measured signal. Therefore, X=N/TS can be obtained.

( 2 ) Counting type direct period measurement : Compared with the counting type direct frequency measurement method, the gated bistable is controlled by the pulse after the input signal is amplified, shaped and divided, so the width of the gate time is equal to k times the period kTX of the measured signal; and the other input end of the main gate is fed with a time-marker pulse signal with a period of T0 generated by a crystal oscillator and a frequency divider. Due to the "AND" function of the main gate, it only outputs a time-marker pulse signal during the kTX period, and is counted by the counter, and its value is N. It is not difficult to see that the period of the measured signal is: TX=NT0/k.

( 3 ) Counting type measurement of time interval : The counting type measurement of time interval is based on the block diagram of the measurement period, and the gated bistable is changed to be controlled by pulse signals output by two timing channels respectively. One pulse corresponds to the starting point of the measured time interval, called the start signal, which sets the gated bistable and opens the main gate; the other pulse corresponds to the end point of the measured time interval, called the stop signal, which resets the gated bistable and closes the main gate. Therefore, the width of the gate signal and the time the main gate is open are equal to the measured time interval ΔTX. During this period of time, the number of time-stamp pulses To counted by the counter is N, so the measured time interval is: ΔTX=NT0.

( 4 ) Equal-precision frequency and period measurement : The reciprocal counter uses a multi-period synchronous measurement method, that is, it measures multiple (integer) period values of the input signal, and then performs a reciprocal operation to obtain the frequency. Compared with the direct measurement method, its advantage is that it can obtain the same high test accuracy and resolution in the entire frequency measurement range.

( 5 ) Measuring time intervals with equal precision : To measure the time interval between two pulse signals with equal precision, a synchronization circuit 2 (D flip-flop) and a B input channel can be added on the basis of the equal precision frequency and period measurement methods, and its output is inverted and sent to the reset terminal of synchronization circuit 2. The trigger clock of the synchronization circuit is obtained by the output of input channel A after being delayed by two stages of inverters. The output UQ2 of the synchronization circuit is directly counted by counter A and also serves as the opening signal of gate B. Counter B records the number of clock pulses passing through gate B. Finally, the numbers counted by the two counters are sent to the operation circuit for processing to obtain the value of the measured time interval.

If the input ends of the two input channels are connected together, and the trigger polarity and trigger level of the two channels are selected respectively, a pulse corresponding to A is generated at the leading edge of the pulse, and a pulse corresponding to B is generated at the trailing edge of the measured pulse, the pulse width can be measured.

Based on the measured signal pulse width and period, the duty cycle can be obtained by calculation.

2. Design

(1) The use of programmable devices such as FPGA can easily complete the design of frequency meters with different measurement principles.

(2) The frequency meter is constructed with the AT89S52 single-chip microcomputer as the core, and adopts high-impedance, high-gain front-end amplifiers and dividers. It adopts a variety of software anti-interference measures such as shielding and watchdog, software traps, and software fault tolerance. The input signal to be measured (0.1 Hz~30 MHz) is divided into four frequency bands: 0.1 Hz~1 Hz, 1Hz~50 kHz, 50 kHz~1 MHz, and 1 MHz~30 MHz. Two pre-processing amplifiers are used for signals less than 1 MHz and greater than 1 MHz, respectively, to amplify them, and then the output signals of the amplifiers are shaped and divided. The single-chip microcomputer is used to measure and calculate the frequency, period, pulse width, and duty cycle, and the calculation results are serially output to the digital display.

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Previous "Instrumentation" competition requirements and design solutions 3

Digital Power Frequency Effective Value Multimeter (1999 Question B)

Design requirements for digital power frequency effective value multimeters[1]

Design and manufacture a digital multimeter that can simultaneously measure the effective value of voltage, effective value of current, active power, reactive power, and power factor of one industrial frequency alternating current (frequency fluctuation range is 50 ± 1Hz, distorted sine wave).

(1) Basic requirements
　　① Measurement function and range
　　a. AC voltage: 0~500V;
　　c. Active power: 0~25kW;
　　d. Reactive power: 0~25kvar;
　　e. Power factor (active power/apparent power): 0~1.
　　② To facilitate the design and production of this test question, it is assumed that the AC voltage of 0~500V and the AC current of 0~50A to be measured have been converted into an AC voltage of 0~5V by the corresponding converter.
　　③ Accuracy
　　a. Displayed as digits (0.000～4.999), with over-range indication;
　　b. AC voltage and AC current: ±(0.8% reading + 5 characters), for example: when the measured voltage is 300V, the reading error should be less than ±(0.8%×300V＋0.5V)＝±2.9V;
　　c. Active power and reactive power: ±(1.5% reading + 8 characters);
　　d. Power factor: ±0.01.
　　④ Function selection: Use the key to select the measurement and display of AC voltage, AC current, active power, reactive power and power factor.

(2) Development part
　　① Use the key to select the effective value measurement and display of the voltage fundamental wave and total harmonics.
　　② It has the function of automatic range conversion. When the voltage value output by the converter is less than 0.5V, it can automatically increase the resolution to 0.01V.
　　③ Use the key control to realize the maximum and minimum value measurement of AC voltage, AC current, active power and reactive power during the test process.
　　④ Others (such as expanding functions and improving performance).

Design of Digital Power Frequency Effective Value Multimeter System

The design requires the measurement of electrical parameters of single-phase electricity, among which voltage and current are basic quantities, and other parameters are derived quantities. Since the power factor needs to be calculated, the current and voltage signals need to be sampled simultaneously. If a non-simultaneous sampling method is used, a software correction method is required to eliminate or reduce the introduced fixed phase error. Common methods for true RMS AC/DC conversion include: thermoelectric conversion method, sampling calculation method, analog direct operation conversion method, and monolithic integrated RMS conversion component (logarithmic amplifier) method.

The sampling calculation method can quickly sample periodic signals and obtain multiple discrete values. Then, the calculation function of the single-chip microcomputer is used to perform related calculations with high conversion accuracy, and phase information can be calculated. The system is based on the AT89S52 single-chip microcomputer and includes three modules: data acquisition, data processing (single-chip microcomputer system) and input/output module (keyboard/display module). The input voltage signal and current signal are amplified and maintained by the programmable amplifier PGA 103, and then sampled by entering the A/D converter through a multiplexer.

In order to keep the sampling interval changing with the fluctuation of signal frequency, that is, to change the sampling of equal time interval in one cycle into equal phase sampling, a phase-locked loop circuit is used in the design, which multiplies the frequency of the signal by 64 times through the counter, thereby generating 64 pulses in one cycle of the signal to be collected, and using this pulse signal as the external interrupt signal of the single-chip microcomputer to quickly start the AD converter for conversion, so as to achieve high-speed data sampling.

(1) Input amplifier circuit design: In order to process current and voltage signals of different sizes, the input amplifier circuit adopts a programmable form and is composed of a programmable operational amplifier chip PGA103. By controlling pin 1 and pin 2 of PGA103 through a single-chip microcomputer, different amplification factors can be obtained.

(2) Signal sampling and holding circuit design

When measuring power, the voltage and current signals need to be measured simultaneously, but the A/D conversion of the voltage and current signals can only be performed sequentially by the single-chip microcomputer. Therefore, a sampling and holding circuit is required to hold the two signals separately. The sampling and holding circuit can be controlled by the control signal sent by P1.4 of the single-chip microcomputer P1 port. When measuring, the single-chip microcomputer first collects and A/D converts the voltage signal, and the current signal at this time is sent to the sampling and holding circuit for holding. After the voltage signal is processed, the held current signal is converted. The signal sampling and holding circuit can use sampling and holding chips such as AD585 or LF398.

(3) A/D sampling circuit design: The A/D sampling circuit can be composed of a multi-way switch CD4051 and an A/D converter chip AD754, as shown in Figure 5.4.6. The single-chip microcomputer controls the conduction of the CD4051 switch, and respectively connects the output signals of the voltage and current signal amplifiers. After A/D conversion in AD754, the 12-bit digital signal is output to the single-chip microcomputer. The A/D sampling circuit can also use chips for industrial power metering or multi-channel analog quantity acquisition, such as THS1206, AD73360, etc.

(4) Signal frequency sampling and frequency multiplication circuit design: The signal is sampled using the equal phase interval method, and the sampling interval changes accordingly with the fluctuation of the signal frequency. The implementation scheme is to use a phase-locked loop plus counter to multiply the signal frequency by 64, thereby generating 64 pulses in one cycle of the signal to be collected. This pulse signal is used as the external interrupt signal of the microcontroller to quickly start the A/D converter for conversion and achieve high-speed data collection. In the signal frequency sampling and frequency multiplication circuit using the CD4046 phase-locked loop, TL082 constitutes the zero-crossing detection circuit, and CD4046 completes phase locking and frequency multiplication.

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Previous "Instrumentation" competition requirements and design solutions 4

Frequency characteristics tester (1999 C) [1]

Frequency characteristics tester design requirements: Design and produce a frequency characteristics test system, including three parts: test signal source, tested network, detection and display.

(1) Basic requirements

① Make amplitude-frequency characteristic test

a. Frequency range: 100Hz～100kHz;
b. Frequency step: 10Hz;
c. Frequency stability: 10-4;
d. Measurement accuracy: 5%;
e. Automatic step measurement in the full frequency range and specific frequency range, and manual preset measurement range and step frequency value;
f. LED display, frequency display is 5 digits, voltage display is 3 digits, and can be printed out.
② Make a network to be tested
a. Circuit type: RC double T network;
b. Center frequency: 5kHz;
c. Bandwidth: ±50Hz;
d. Calculate the amplitude-frequency and phase-frequency characteristics of the network, and draw the phase curve;
e. Use the amplitude-frequency characteristic tester to test the amplitude-frequency characteristics of the self-made network to be tested.

(2) Development part

① Make a phase-frequency characteristic tester
　　a. Frequency range: 500Hz~10kHz;
　　b. Phase degree display: the phase value is displayed in three digits and another digit is used as a symbol display;
　　c. Measurement accuracy: 3°.

② Use an oscilloscope to display the amplitude-frequency characteristics.

③ Display the amplitude-frequency and phase-frequency characteristics simultaneously on the oscilloscope.

④ Others.

Frequency characteristics tester system design

Frequency characteristics test can be done by impulse response test method and frequency sweep test method. The frequency range required by the design is 100 Hz ~ 100 kHz, which belongs to the frequency range of low-frequency frequency characteristics tester. Both impulse response test method and frequency sweep test method can be used. The design requires the frequency to step by 10 Hz, and the frequency sweep test method with frequency stepping sweep is more convenient to operate.

The frequency sweep test method can use the method of frequency stepping point by point or frequency continuous change to complete the measurement of the entire frequency characteristic. This method does not require the time domain and frequency conversion calculation of the signal, and can be completed by measuring and calculating the analog quantity. In the frequency characteristic tester using the frequency sweep test method, the scanning synchronization control part generates a sawtooth or step-type scanning voltage, synchronously controls the operation of the voltage-controlled oscillator (VCO) and the display part, and synchronously compensates the performance of other parts of the whole machine, such as compensating the amplitude flatness of the frequency sweep signal source.

The frequency sweep signal source generates a sine wave oscillation signal with a frequency change from low to high or from high to low. There are many ways to generate the frequency sweep signal, which can be made into point frequency (continuous wave CW), automatic frequency step (STEP), continuous frequency change (sweep frequency SWEEP) and other forms as needed. The frequency sweep signal can be generated by VCO, and the control amount of VCO uses a ramp voltage or a step voltage. At the same time, the ramp voltage or the step voltage is used as the X-axis scanning voltage of the display to achieve synchronization of the frequency sweep and the curve display.

The measurement and calculation part measures the amplitude and phase of the input and output signals. The amplitude ratio of the output signal to the input signal is calculated to obtain the amplitude-frequency characteristic; the phase difference between the output and the input is calculated to obtain the phase-frequency characteristic. Analyzing only the amplitude-frequency characteristic of the circuit is called scalar analysis, while giving both the amplitude-frequency characteristic and the phase-frequency characteristic is called vector analysis.

There are various forms of displaying the frequency characteristics of a system, such as using graphics and text information display, the most commonly used are amplitude-frequency characteristic curves and phase-frequency characteristic curves. For frequency characteristics, Bode diagrams can also be used, that is, the frequency axis is scaled logarithmically, and the corresponding frequency step (sweep) is taken in geometric series.

The frequency marker generator circuit generates a frequency marker signal and puts a graphic mark on the displayed frequency characteristic curve to indicate the corresponding frequency value at that location.

The frequency characteristic tester can be implemented using a single-chip microcomputer or FPGA.

1. Sweep signal source

The frequency sweep test method includes the frequency sweep signal source, amplitude and phase detection, numerical calculation processing, frequency characteristic curve display, synchronous control and other parts. The circuit design considerations of each part are as follows:

(1) Performance indicators of swept frequency signal source generator: Sweep frequency test requires a sine wave signal. For sine wave signals, the main performance indicators are frequency stability, frequency accuracy, distortion and noise, signal source internal resistance, and output amplitude. When a sine wave signal is used for swept frequency measurement, in addition to the above-mentioned index requirements, other performance indicators to be considered include: swept frequency range, or frequency deviation, swept frequency speed, swept frequency mode, swept frequency linearity, flatness, output dynamic range, and attenuator accuracy. When measuring phase-frequency characteristics, the phase of the signal source should be controlled by preset and easy to measure.

(2) Implementation of frequency sweep signal generator:

a. Voltage controlled oscillator (VCO) form, which can be composed of a dedicated VCO chip or a function generator chip.

b. Phase-locked loop (PLL) frequency synthesizer form.

c. Direct digital synthesizer (DDS) form. DDS can not only synthesize sine waves, triangle waves, square waves and other functional waveforms, but also synthesize various modulation waveforms and waveforms of arbitrary shapes, as long as the required waveform is pre-calculated and stored in the waveform memory. This method can be used to make an arbitrary waveform generator (AWG). The phase of the DDS signal can be controlled very accurately, which is very important when measuring phase-frequency characteristics. At present, the highest clock frequency of dedicated DDS integrated circuit chips can reach more than 1 GHz, and the achievable signal source sine wave frequency can reach more than hundreds of MHz.

d. DDS + PLL frequency synthesizer: The DDS + PLL frequency synthesizer uses two DDSs. DDS1 is used as a frequency divider to directly change the frequency of the reference oscillator. DDS2 is used as a loop frequency divider of the frequency synthesizer to achieve small frequency steps. As a frequency divider, the upper limit of the operating frequency of DDS cannot be too high. Therefore, before the DDS frequency division, it first undergoes a 2N frequency division. PLL + DDS frequency synthesizer can use dedicated chips such as AD9858.

2. Amplitude measurement circuit design

The commonly used detection methods for amplitude measurement are peak detection and effective value detection.

(1) Effective value detection circuit: The effective value detection circuit can use a dedicated effective value detection circuit chip to achieve accurate RMS detection, such as the RMS-DC converter chip MX536A/MX636.

(2) Peak detection circuit: An active peak detection circuit is composed of an OP and a diode. The capacitor C used to maintain the peak voltage should take a corresponding value according to the bandwidth of the signal to be detected, and generally should not be too large. After completing a peak detection, the discharge switch tube is turned on to clear the charge on C, and then the next measurement is performed. Each measurement should be performed when the network reaches a steady-state output and should include at least one peak cycle. Therefore, the measurement speed varies with the network bandwidth and the excitation frequency.

The DC analog voltage obtained by the above two analog detection circuits needs to be converted into a digital quantity through an A/D converter for digital display.

3. Phase measurement circuit design

The measurement of phase-frequency characteristics is achieved by measuring the phase difference between the output and input signals of the network. It can also be divided into two types: analog circuit measurement method and digital measurement method.

(1) Analog measurement method: Use a zero-crossing voltage comparator to shape the input and output sine waves into square waves and send them to a phase detector for phase detection. The phase detector circuit consists of an XOR gate and a low-pass filter. The output of the XOR gate is a pulse square wave, and its duty cycle is proportional to the phase difference between the two signals. After passing through a low-pass filter, the duty cycle can be converted into a DC voltage, and after A/D conversion, the microcontroller reads the phase difference value. This value represents the relative phase difference between the two waveforms, but it cannot distinguish whether the phase relationship between the two is leading or lagging. For this purpose, a phase polarity discrimination circuit must be added.

(2) Digital method: The phase difference can be measured directly by using digital circuit technology to measure the output pulse width. The design requires that the phase measurement accuracy reaches 3° at 10 kHz, and the corresponding pulse width is about 1μs. General digital circuits can meet this counting speed requirement. The counting and frequency measurement functions in the microcontroller can also be used to complete this task. The specific method is to directly use the edges of the two square wave signals after shaping as the two interrupt sources of the microcontroller and measure the time interval between the two interrupts. This method requires the microcontroller clock frequency to be high enough.

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Previous "Instrumentation" competition requirements and design solutions

Simple Digital Storage Oscilloscope (2001 Question B)

Simple Digital Storage Oscilloscope Competition Requirements: Design and make a simple digital storage oscilloscope that uses an ordinary oscilloscope to display the measured waveform.

(1) Basic requirements (50 points)

① The instrument is required to have a single-shot trigger storage and display mode, that is, each time the "single-shot trigger" key is pressed, the instrument can collect and store the measured periodic signal or single non-periodic signal once when the trigger condition is met, and then display it continuously.

② The input impedance of the instrument is required to be greater than 100kΩ, the vertical resolution is 32 levels/div, and the horizontal resolution is 20 points/div; the horizontal scale of the oscilloscope display is set to 10div, and the vertical scale is 8div.

③ It is required to set three scanning speeds of 0.2s/div, 0.2ms/div, and 20μs/div. The frequency range of the instrument is DC~50kHz, and the error is ≤5%.

④ It is required to set the vertical sensitivity to two levels: 0.1V/div and 1V/div, with an error of ≤5%.

⑤ The trigger circuit of the instrument adopts internal triggering mode, which requires rising edge triggering and adjustable trigger level.

⑥ There is no obvious distortion in the observed waveform.

(2) Performance section (50 points)

① Add continuous trigger storage display mode. In this mode, the instrument can continuously collect, store and display signals in real time, and has a latch function (press the "latch" key to store the current waveform). (15 points)

② Add dual-trace oscilloscope function, which can display the waveforms of two measured signals at the same time. (8 points)

③ Add the horizontal movement extended display function, which requires the storage depth to be doubled, and any part of the stored signal waveform can be displayed by operating the "move" key. (5 points)

④ Increase the vertical sensitivity by 0.01V/div to improve the vertical sensitivity of the instrument and try to reduce the output noise voltage when the input is short-circuited. (10 points)

⑤ Others. (12 points)

(3) Description

During the test, ordinary oscilloscopes cannot be operated or adjusted.

Simple digital storage oscilloscope competition analysis and design solution Example: The competition requires the design and production of a simple digital storage oscilloscope.

A simple digital storage oscilloscope consists of signal conditioning, trigger circuit, A/D (ADC), D/A (DAC), Y output circuit, X output circuit, controller, etc. The measured signals A and B are analog signal inputs, and Y and X signals are output signals, which are added to the Y and X input terminals of the ordinary oscilloscope respectively.

The input signal (analog signal) to be measured is conditioned and quantized (A/D conversion) and then stored in the data memory. Then, under the control of the controller, the data is read out from the memory and restored (D/A conversion) to an analog signal, which is input to the Y channel of the general oscilloscope; at the same time, the system also needs to generate a corresponding scanning signal, which is added to the X channel of the general oscilloscope to display the input signal to be measured on the fluorescent screen of the general oscilloscope.

The controller is the core of the entire system. According to the design requirements, the controller needs to have the following functions:

① When the trigger conditions are met, it can start sampling (real-time sampling mode), storing and displaying the measured signal.

② Determine the corresponding sampling rate according to the frequency range of the signal being measured, and determine the corresponding sampling rate according to the requirements of different scanning rates.

③ When displaying the stored signal, it is possible to select a suitable rate to read out the stored signal data and restore it to analog quantity as the Y channel input signal of the general oscilloscope; at the same time, a scanning voltage adapted to the Y channel signal rate is provided as the X channel input signal.

④ The corresponding gain of the signal conditioning circuit should be selected according to the vertical sensitivity requirements so that the A/D converter can convert at the appropriate analog input signal amplitude.

⑤ It can realize the simultaneous acquisition and storage of two signals and realize the dual-trace display function.

The controller can use single-chip microcomputer, programmable logic device and other chips. According to the design requirements, single-chip microcomputer and programmable logic device can be selected. Programmable logic device (FPGA) is used to complete the acquisition and storage control of the signal, and undertakes the bottom-level control; single-chip microcomputer is used to realize the management of programmable logic device and the entire system, and undertakes the top-level control and data processing, such as selecting the sampling rate from the keyboard input, selecting the gain of the signal conditioning circuit, processing the stored digital signal and restoring it to analog signal for display, etc.

The design requires that two measured signals (A and B) be displayed simultaneously, so the two measured signals A and B must be sampled and stored at the same time. Usually, there are two methods for conditioning, sampling, and storing two signals, namely the alternating method and the dual-channel method. For electronic design competitions, the dual-channel method uses two A/D converters, has a relatively simple design idea, and eliminates the complex requirements for control signals, so it is more suitable.

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Previous "Instrumentation" Competition Requirements and Design Plans VI

Simple Logic Analyzer (2003 Question D)

Simple Logic Analyzer Competition Requirements
The competition requires the design and production of an 8-channel digital signal generator and a simple logic analyzer.

(1) Basic requirements (50 points)

Part 1: Making a Digital Signal Generator

According to the given logic signal sequence example, 8 presettable cyclic shift logic signal sequences are generated. The output signal is TTL level, the sequence clock frequency is 100Hz, and it can be output repeatedly.

Part 2: Building a Simple Logic Analyzer

① It has the function of collecting 8-channel logic signals and can set a single-level trigger word. The trigger condition for signal collection is that the level of each channel of the measured signal is the same as the logical state set by the trigger word. When the trigger condition is met, the measured signal can be collected and stored once;

② The analog oscilloscope can be used to clearly and stably display the waveforms of the 8-channel signals collected and display the trigger point position;

③ The input impedance of the 8-bit input circuit is greater than 50kΩ, and its logic signal threshold voltage can be changed in 16 levels within the range of 0.25~4V to adapt to the logic levels of various input signals;

④ The storage depth of each channel is 20 bits.

(2) Performance section (50 points)

① Be able to display a movable time mark line on the oscilloscope, and use LED or other methods to display the logical state of the 8-way input signal at the time corresponding to the time mark line; (18 points)

② The simple logic analyzer should have a 3-level logic state analysis trigger function, that is, when the set 3 trigger words are captured in sequence, the measured signal will be collected, stored and displayed, and the trigger point position will be displayed. The 3-level trigger word can be set arbitrarily (for example: specify that the continuous capture of two signals 11, 01, 00 in 8 signals is used as the 3-level trigger state word); (18 points)

③ The trigger position is adjustable (i.e. the number of logic state words saved before and after the trigger can be displayed); (5 points)

④ Others (such as increasing storage depth and displaying in pages, etc.). (9 points)

Simple Logic Analyzer Competition Question Analysis and Design Example
The competition question requires the design and production of an 8-channel digital signal generator and a simple logic analyzer.

A simple logic analyzer consists of three AT89S52 single-chip microcomputer systems. One AT89S52 single-chip microcomputer system A generates 8 preset cyclic shift logic signal sequences, one AT89S52 single-chip microcomputer system B realizes human-computer interaction, and another AT89S52 single-chip microcomputer system C is used to trigger and display signals. It uses dual-port RAM and has functions such as page display, movable time mark line, settable trigger bit, continuous and discontinuous trigger, and various trigger modes.

(1) AT89S52 microcontroller small system A

According to the preset cyclic shifter logic signal sequence (set by the 8-way switch), the cyclic shifter outputs this sequence with a clock frequency of 100 Hz, and at the same time outputs this clock signal to system C as the signal sampling clock.

(2) AT89S52 microcontroller small system B

Control a 64×128 dot matrix LCD and accept keyboard input. The menu function is detailed and easy to operate. The working mode of the logic analyzer can be set. The working mode is written into the dual-port RAM in a certain format and will be read by system C. At the same time, the threshold level value set by the user is DA-converted and compared with the circular shift element logic input signal. After the working mode is set, the logic state of the 8-way input signal at the time corresponding to the time mark line can be read from the dual-port RAM and displayed on the LCD screen.

(3) AT89S52 single-chip microcomputer small system C

The signal sequence output by the comparator is sampled according to the 100 Hz signal sampling clock sent by microcontroller A, the working mode setting of the dual-port RAM is read, the trigger point is determined, and the logic state of the input signal at the corresponding moment of the mark line is written to the dual-port RAM.

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Previous "Instrumentation" Competition Requirements and Design Plans VII

Low-frequency digital phase meter (2003 C)
Low-frequency digital phase meter competition requirements [1]
The competition requires the design and production of a low-frequency phase measurement system, including a phase meter, a digital phase-shifting signal generator and a phase-shifting network.

(1) Basic requirements (50 points)

① Design and build a phase measuring instrument

a. Frequency range: 20Hz～20kHz.

b. The input impedance of the phase meter is ≥100k.

c. Allow the peak-to-peak values of the two input sinusoidal signals to vary within the range of 1V to 5V.

d. The absolute error of phase measurement is ≤ 2°.

e. It has frequency measurement and digital display functions.

f. Phase difference digital display: Phase reading is 0°~359.9°, with a resolution of 0.1°.

② Refer to Figure 1.3.18 to make a phase shift network

a. Input signal frequency: 100Hz, 1kHz, 10kHz.

b. Continuous phase shift range: -45° to +45°.

c. The peak-to-peak value of the sinusoidal signals output by A' and B' can vary in the range of 0.3V to 5V respectively.

(2) Performance section (50 points)

① Design and manufacture a digital phase-shift signal generator to generate the input sinusoidal signal required by the phase measuring instrument. Requirements:

a. Frequency range: 20Hz~20kHz, frequency step is 20Hz, output frequency can be preset.

b. The peak-to-peak value of the sinusoidal signals output by A and B can vary in the range of 0.3V to 5V respectively.

c. The phase difference range is 0~359°, the phase difference step is 1°, and the phase difference value can be preset.

d. Digital display of preset frequency and phase difference. (22 points)

② Under the condition of keeping the measurement error and frequency range of the phase meter unchanged, expand the peak-to-peak value of the phase meter input sinusoidal voltage to the range of 0.3V to 5V. (6 points)

③ Use a digital phase-shift signal generator to calibrate the phase measuring instrument, and select several frequency points, phase differences and different amplitudes for calibration. (12 points)

④ Others. (10 points)

Analysis and design example of low-frequency digital phase measuring instrument competition question
According to the requirements of the competition question, it is necessary to design and produce a phase measuring instrument, a digital phase shift signal generator and a phase shift network.

There are two key parts of the phase meter. One is the phase difference detection circuit between the A (U1) and B (U2) input signals. This circuit needs to amplify and shape the input signal to extract the phase difference between the two input signals. The other is the phase difference digitization, frequency measurement and display circuit. This circuit part is not difficult to implement using a microcontroller.

The digital phase-shift signal generator is used to generate the input sinusoidal signal required by the phase measuring instrument. The sinusoidal signal waveform parameters can be digitized in advance and made into a data table and stored in the ROM, and then read out by the microcontroller and sent to the DAC output. The microcontroller can output a sinusoidal signal waveform with any phase.

The phase-shift network competition question provides a reference circuit, and the parameters of each component can be determined using circuit simulation software (such as multisim).

(1) Low frequency digital phase measuring instrument

Function of low frequency digital phase meter: measure and display the phase difference and frequency between A (U1) and B (U2) input signals.

First, the same frequency signals A (U1) and B (U2) are amplified by the operational amplifier and input into the zero-crossing comparator. The signal after the zero comparator is converted into a square wave signal and input into the FPGA chip. Through VHDL language programming, downloading to the FPGA chip and burning, the functions of frequency measurement, phase measurement and frequency and phase difference display are realized.

The devices required for the low-frequency digital phase measuring instrument include: operational amplifier TL082, comparator LM393, Xilinx's Spartan-ⅡE series xc2s100e-6pq208 FPGA chip and LED digital tube display.

(2) Digital phase-shift signal generator

The principle of digital phase shifting is briefly described as follows: first, the arbitrary waveform signal is digitized and a data table is formed and stored in the FPGA chip. After that, the data table can be continuously output in a loop through two D/A conversion chips under the control of the FPGA to obtain two arbitrary waveform signals. When the data sequences obtained by the two D/A conversion chips are exactly the same, the two arbitrary waveform signals obtained by conversion have no phase difference, which is called in-phase. When the data sequences obtained by the two D/A conversion chips are different, the two arbitrary waveform signals obtained by conversion have a phase difference. Since the total number of data in the data table is fixed, the value of the phase difference is only related to the offset of the data address. The essence of this processing method is to map the offset of the data address to the phase value between signals.

The digital phase-shift signal generator in this design can generate two sinusoidal signals of the same frequency. Since the sinusoidal function table has been edited and stored in ROM, the function of cyclic output of the data in the ROM address according to different data sequences can be realized through software programming, and two phase-shifted sinusoidal waves can be obtained after D/A conversion.

The FPGA of the digital phase-shift signal generator uses the Spartan-ⅡE series xc2s100e-6pq208. The AD7520 and op amp are used to convert the amplitude data from the FPGA into the reference voltage of the AD7524, thereby realizing digital control voltage regulation with a step of 10mv. The AD7524 and op amp are used to convert the waveform data from the FPGA into a sine wave A. The smoothing filter uses an RC filter network.

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Previous "Instrumentation" competition requirements and design solutions

Simple spectrum analyzer (2005 C question)

Simple Spectrum Analyzer Competition Requirements
Design and implement a spectrum analyzer using the heterodyne principle. The reference principle block diagram is shown in Figure 2.6.14 below.

(1) Basic requirements (50 points)

① The frequency measurement range is 10MHz~30MHz;

② The frequency resolution is 10kHz, the effective value of the input signal voltage is 20mV±5mV, and the input impedance is 50Ω;

③ The center frequency and sweep width can be set;

④ Use an oscilloscope to display the spectrum of the measured signal and mark the frequency marker with an interval of 1MHz on the oscilloscope.

(2) Performance section (50 points)

① The frequency measurement range is extended to 1MHz~30MHz; (20 points)

② It has the function of identifying amplitude modulation, frequency modulation and constant amplitude wave signals and determining their center frequencies. The amplitude modulation, frequency modulation and constant amplitude wave signals output by the signal generator are used as the input signals of the heterodyne spectrum analyzer. The carrier can select any frequency value within the frequency measurement range. The modulation index of the amplitude modulation wave is ma=30%, and the modulation signal frequency is 20kHz; the frequency deviation of the frequency modulation wave is 20kHz, and the modulation signal frequency is 1kHz; (20 points)

③ Others. (10 points)

Simple spectrum analyzer competition analysis and design example [5,8,9,22,23]
According to the requirements of the competition, a simple spectrum analyzer is designed and implemented using the heterodyne principle. The mixer needs to use an analog circuit (such as a multiplier circuit), the sweep signal generator can be implemented using a DDS circuit, and the filter can be implemented using an integrated circuit chip.

The design of a spectrum analyzer system is as follows:

The simple spectrum analysis uses the SPCE061A single-chip microcomputer as the main controller for signal processing and human-computer interaction control. The 32K-word flash memory (Flash) embedded in the SPCE061A is used to store the amplitude of the frequency points obtained by scanning, without the need for external memory expansion; the signal sampling is completed using the embedded 10-bit voltage analog/digital converter (ADC); and the universal programmable input/output port is used to connect to peripheral devices (LCD display, keyboard).

The local oscillator circuit is composed of AD9850 DDS chip and is connected to an external precision clock source.

The mixer circuit uses the AD835 multiplier chip. AD835 is a voltage output four-quadrant multiplier circuit that can complete the W=XY+Z function.

The programmable amplifier circuit uses the AD603 90MHz low-noise programmable amplifier chip to amplify the input signal and attenuate the output of the DDS local oscillator circuit before sending it to the multiplier circuit.

This design requires a spectrum resolution of 10KHz, so the interval of each frequency sweep point is 10KHz. With this frequency point as the center, the effective value is within the range of 5KHz on the left and right, so the filter needs a bandwidth of 5KHz. MAX297 is an eighth-order low-pass elliptical, switched capacitor filter produced by Maxim. It uses the input clock frequency to control the output cutoff frequency to filter analog and digital signals.

In order to improve the detection accuracy, the True RMS-to-DC converter chip MX636 is selected as the detection circuit.

This design uses an ordinary 4×4 keyboard with the following key function assignments: 11 ordinary numeric input keys from 0 to 9 and "."; a setting key for the frequency and bandwidth unit "MHz"; a "Start/Return" key for starting measurement and interface switching after a new signal is input; a center frequency setting key; a scan bandwidth setting key; and a frequency marker display setting key.

The display module uses the commonly used FM1602C liquid crystal display module, and the operation instructions are displayed on the LCD screen.

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Previous "Instrumentation" competition requirements and design solutions

Integrated Operational Amplifier Parameter Tester (2005 Question B)
Requirements for the Integrated Operational Amplifier Parameter Tester Competition [1]
The competition requires you to design and build a tester that can test the parameters of general-purpose integrated operational amplifiers.

(1) Basic requirements (50 points)

① It can test four basic parameters: VIO (input offset voltage), IIO (input offset current), AVD (AC differential mode open-loop voltage gain) and KCMR (AC common mode rejection ratio). The maximum display number is 3999.

② The measurement range and accuracy of each measured parameter are as follows (the operating voltage of the measured operational amplifier is ±15V):

VIO: The measurement range is 0-40mV (the range is 4mV and 40mV), and the absolute value of the error is less than 3% of the reading + 1 word;

IIO: The measurement range is 0~4μA (the range is 0.4μA and 4μA), and the absolute value of the error is less than 3% reading + 1 word;

AVD: The measurement range is 60dB to 120dB, and the absolute value of the test error is less than 3dB;

KCMR: The measurement range is 60dB to 120dB, and the absolute value of the test error is less than 3dB;

③ The signal source (self-made) in the tester is used to measure the AVD and KCMR parameters. The signal source is required to output a sine wave signal with a frequency of 5 Hz and an effective output voltage of 4 V. The absolute value of the frequency and voltage error is less than 1%;

④ According to the test schematic diagram in accordance with GB3442-82 provided in the appendix of this question, make a set of test circuits that meet the standard for testing VIO, IIO, AVD and KCMR parameters. Use the test results of this test circuit as the test standard to calibrate the manufactured op amp parameter tester.

(2) Performance section (50 points)

① Add the BWG (unit gain bandwidth) parameter measurement function of voltage mode amplifier, requiring the measurement frequency range to be 100kHz to 3.5MHz, the measurement time to be ≤10 seconds, and the frequency resolution to be 1kHz;

Design and produce a frequency sweep signal source, with an output frequency range of 40kHz to 4MHz, an absolute value of frequency error less than 1% and an effective value of output voltage of 2V±0.2V; (30 points)

② Add the automatic measurement (including automatic range conversion) function. After this function is activated, it can automatically measure, display and print the measurement results of the above five parameters in the order of VIO, IIO, AVD, KCMR and BWG; (15 points)

③ Others. (5 points)

Analysis and design example of integrated operational amplifier parameter tester competition question
This competition question requires the design and production of a tester that can test the parameters of general-purpose integrated operational amplifiers. The test method provided is also proposed according to relevant standards. However, a problem that the question setter did not expect occurred during the production process. The general-purpose operational amplifiers such as LM741, μA741, and AD741 available on the market now have greatly improved and changed their performance indicators due to the advancement of manufacturing technology. There are some difficulties and problems in testing some parameters.

A reference integrated operational amplifier comprehensive parameter tester system design scheme uses Lingyang SPCE061A single-chip microcomputer as the control core, and is composed of detection circuit, signal source, automatic test control circuit, keyboard and LED display. It can test and digitally display the basic parameters VIO, IIO, AVD, KCMR and BWG of LM741 and other integrated operational amplifiers (such as μA741, F007, F741) that are pin-compatible with it, and has automatic printing and voice broadcast functions. The signal source uses AD9835DDS dedicated chip to generate the 40kHz~4MHz sweep frequency signal required for the test and the 5Hz signal in the tester. The LED display adopts serial working mode, and 8 74HC595 chips work in static display mode.

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Previous "Instrumentation" competition requirements and design solutions

Integrating DC Digital Voltmeter (2007 G) [Higher Vocational and Technical College Group]
Integrating DC Digital Voltmeter Competition Requirements [1]
The competition requires the design and manufacture of an integrating DC digital voltmeter without using a dedicated A/D converter chip.

(1) Basic requirements (50 points)

① Measuring range: 10mV～2V.

② Range: 200mV, 2V.

③ Display range: decimal number 0 to 1999.

④ Measurement resolution: 1mV (2V range).

⑤ Measurement error: ≤±0.5%±5 digits.

⑥ Sampling rate: ≥ 2 times/second.

⑦ Input resistance: ≥1M.

⑧ It has the function of suppressing power frequency interference.

(2) Performance section (50 points)

① Measurement range: 1mV～2V. (4 points)

② Range: 200mV, 2V.

③ Display range: decimal number 0 to 19999. (3 points)

④ Measurement resolution: 0.1mV (2V range). (2 points)

⑤ Measurement error: ≤±0.05%±5 characters. (20 points)

⑥ It has automatic zero calibration function. (8 points)

⑦ It has the function of automatic range conversion. (8 points)

⑧ Others. (5 points)

Integrating DC digital voltmeter competition analysis and design example
The competition requires the design and production of an integrating DC digital voltmeter without using a dedicated A/D converter chip. In the performance part, it also requires the automatic switching of the range, and also puts forward high requirements for indicators such as accuracy, resolution, and input impedance. Ensuring measurement accuracy is the core requirement of this competition. The dual-slope integrating ADC is the best choice for integrating DC digital voltmeters. The dual-slope integrating ADC compares two integration processes (timed integration of the measured voltage and fixed value integration of the reference voltage) to obtain the measured voltage value. This competition requires the use of operational amplifiers and control circuits to realize an integrating DC digital voltmeter based on the working principle of the dual-slope integrating ADC.

The whole system can use a single chip microcomputer as the control core to control the dual slope integrating ADC composed of an operational amplifier and an electronic switch to complete the voltage measurement.

① The MCU controls the integrator to integrate the input voltage in the positive direction, and then connects the negative reference voltage to integrate the integral in the reverse direction. When the integrated voltage of the integrator is higher than the comparison point of the comparator, the comparator flips and triggers an interrupt, and the MCU counts the value of T2. By calculating the value of T2, the measured voltage value can be obtained.

② The single-chip microcomputer uses a range to roughly measure the voltage being measured, and determines whether the current measured voltage meets the measurement range of the current range. If not, it automatically switches the range.

③ For the integrator zero drift caused by system component aging, ambient temperature changes, etc., zero point and full-scale correction functions can be designed in the program to correct the errors generated by the system at any time.

The traditional 51 single-chip microcomputer has the characteristics of low price and easy use. Using the 51 single-chip microcomputer as the control core of the system can realize the basic functions required by the competition. You can also use an 8-bit RISC flash microcontroller (such as PIC16F628A) microcontroller as the control core of the system. Most of the instructions of the PIC16 series microcontroller are single-cycle instructions, which is beneficial to improving the timing accuracy of the software and thus improving the measurement accuracy. The built-in power-on reset and watchdog modules can improve the system reliability and simplify the external circuit. Another important feature is that its timer has an "automatic capture" function. When the external level jumps, the current value of the timer can be "captured" and recorded immediately without software intervention, which is beneficial to improving the measurement accuracy.

The input circuit can use a voltage follower composed of an operational amplifier to meet the input impedance requirement, and the input voltage of 0 to 200mV can be amplified by the operational amplifier. The operational amplifier can be selected from chips such as uA741, TL084, CA3140, etc.

The voltage comparator can be composed of chips such as LM393.

Electronic switches can choose conventional CD40xx series analog switches or chips such as 74HC4051.

The +2.5 V and -2.5 V reference voltage sources can be generated by two LM431s and then divided twice by two adjustable multi-turn precision potentiometers to achieve coarse and fine adjustment of the reference voltage.

According to the requirements of the competition, a six-digit display is required. The digital display module can be composed of a keyboard display circuit composed of an 8279 chip or a liquid crystal display module.

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Previous "Instrumentation" competition requirements and design solutions

Digital Oscilloscope (2007 C) [Undergraduate Group]
Digital Oscilloscope Competition Requirements
The competition requires you to design and build a digital oscilloscope with real-time sampling and equivalent sampling modes.

(1) Basic requirements (50 points)

① The frequency range of the measured periodic signal is 10Hz～10MHz, the input impedance of the instrument is 1M, the scale of the display screen is 8 div×10div, the vertical resolution is 8bits, and the horizontal display resolution is ≥20 dots/div;

② The vertical sensitivity requirement includes 1V/div and 0.1V/div. The voltage measurement error is ≤5%;

③ Real-time sampling rate ≤ 1MSa/s, equivalent sampling rate ≥ 200MSa/s; scanning speed requirements include 20ms/div,

2μs/div, 100 ns/div, three levels, waveform period measurement error ≤5%;

④ The trigger circuit of the instrument adopts internal triggering mode, requiring rising edge triggering, and the trigger level is adjustable;

⑤ The displayed waveform of the measured signal should have no obvious distortion.

(2) Performance section (50 points)

① To improve the vertical sensitivity of the instrument, it is required to add a 2mV/div level, with a voltage measurement error of ≤5% and an output noise peak-to-peak value of less than 2mV when the input is short-circuited; (22 points)

② Add the store/recall function, that is, by pressing the "store" button once, the instrument can store the current waveform and call out the stored waveform for display when needed; (7 points)

③ Add a single trigger function, that is, press the "single trigger" button once, and the instrument can collect and store the signal that meets the trigger condition once (the frequency range of the measured signal is limited to 10Hz~50kHz); (7 points)

④ Able to provide a square wave calibration signal with a frequency of 100kHz, with an amplitude of 0.3V±5% (when the load resistance is ≥1 M), and a frequency error of ≤5%; (6 points)

⑤ Others. (8 points)

Analysis and design example of digital oscilloscope competition question
According to the requirements of the competition question, design and make a digital oscilloscope with real-time sampling mode and equivalent sampling mode. After careful analysis of the question, it can be found that the question is very similar to the 2001 question B "Simple Digital Storage Oscilloscope". Comparing the technical indicators of the two questions, the main differences are: the frequency range and input impedance of the measured signal, scanning speed, sampling rate and dual-trace oscilloscope, etc. The concept of real-time sampling is relatively easy to understand, which is the same as the 2001 question B. The equivalent sampling method can obtain a higher bandwidth at a lower sampling rate. The concept of equivalent sampling needs to be found in relevant information. You can refer to the content of sampling oscilloscopes in the book "Electronic Measurement" compiled by Jiang Huanwen et al., or "Electronic Measurement and Instruments" compiled by Chen Shangsong et al. and other related materials for a correct understanding.

A digital oscilloscope design with real-time sampling mode and equivalent sampling mode is shown below:

The digital oscilloscope consists of signal conditioning, trigger circuit, acquisition storage, data processing, human-computer interaction and other modules. The system uses the single-chip microcomputer AT89S52 and EP1C6 FPGA as the control core. The signal conditioning module is composed of a high-speed, low-noise op amp (such as OPA690) to achieve high-impedance input and amplitude control of the signal; the high-speed op amp OPA690 is used to form a comparator to achieve internal triggering, and the trigger level is adjustable; the data acquisition module is composed of a sample-and-hold circuit (such as AD783) and an ADC (such as AD7822), and sampling is performed under strict FPGA timing control. AT89S52, as the system's master controller, is combined with the dual-port RAM and high-speed clock inside the FPGA to achieve real-time sampling, equivalent sampling, data exchange, storage and call-out, single trigger, square wave calibration, oscilloscope display and other functions.

According to the requirements of the competition, it is necessary to design a digital oscilloscope with real-time sampling mode and equivalent sampling mode.

Real-time sampling is sampling the signal within its existence period. Simply arranging the sampling points of each sampling period in time order can express a waveform. According to the Nyquist low-pass sampling theorem, the sampling frequency is at least twice the upper frequency limit of the measured signal. For a periodic sinusoidal signal, there should be at least 2 sampling points in one period. In order to restore the original measured signal without distortion, it is usually necessary to sample more than 8 points in one period. In order to restore the original measured signal without distortion, the sampling frequency of the real-time sampling digital oscilloscope is generally specified as 4 to 10 times the real-time bandwidth of the signal, and an appropriate interpolation algorithm is used. If the interpolation algorithm is not used, the sampling rate is required to be 10 to 20 times the real-time bandwidth of the signal. The higher the bandwidth, the higher the sampling rate requirement. The hardware design and software design of real-time sampling are relatively simple, and it can collect and restore any signal. The sampling time is short. The disadvantage is that it requires high speed and accuracy of the A/D converter. If you want to collect a 10 MHz signal, you need a high-speed ADC of at least 100MHz.

The equivalent sampling method can obtain a higher bandwidth at a lower sampling rate. The premise of using the equivalent sampling method is that the measured signal appears periodically. In order to reconstruct the original signal, a small number of samples can be extracted equivalently and at equal intervals in each cycle, and finally the samples extracted from multiple cycles are collected in the same cycle, so that the sampling effect can be equivalent to that in one cycle of the measured signal. The equivalent sampling method usually uses the MCU as the control core, and uses a precision clock generation circuit to control the low-speed ADC to perform cyclic sampling of high-frequency signals, and only one point is taken on each sampling waveform.

According to the competition question, choose a combination of real-time sampling and equivalent time sampling. The competition question requires the real-time sampling rate to be ≤1 MSa/s, which limits the rate of the A/D converter to ≤1 MSa/s; the competition question requires the horizontal resolution to be at least 20 points/div, so the real-time sampling method is used below 50 kHz, and the equivalent time sampling method is used from 50 kHz to 10 MHz, and the maximum equivalent sampling rate reaches 200 MSa/s. That is, when the system scan speed is 2μs/div and 100 ns/div, equivalent sampling is used. When the 2μs/div block is Δt, it is 100 ns, and when the 100 ns/div block is Δt, it is 5 ns. The highest clock of the system is 200 MHz.

sigma

Some knowledge required and mastered for the "Instrumentation" competition

Judging from the previous competition topics, students who focus on the "Instrumentation" competition topic need to understand and master:

(1) The existing competition questions can basically be seen and used in laboratories. The basic working principles, measurement and testing methods of the competition questions can be obtained from product specifications and user manuals.

(2) Note that the same type of instruments may have different working principles and measurement and test methods. For example, the methods for measuring low-frequency AC voltage and high-frequency AC voltage are completely different; for example, the measurement of frequency and period, etc. Choosing the right working principle and measurement and test method is the key to whether the competition question can be successfully prepared and whether good competition results can be obtained.

(3) Some basic circuits involved in the competition are:

Input circuit: voltage divider, preamplifier; filter circuit; ADC circuit; microcontroller (MCU, FPGA, ARM, DSP); DAC; keyboard and switch circuit, LED and LCD display; power supply circuit, etc.

(4) “Instrumentation” topics also appear in other competition topics. For example, the 2007 “Programmable Filter (Topic D)” required the creation of a “Simple Amplitude-Frequency Characteristics Tester”.

Some suggestions

(1) Select some existing competition questions and do some training; mainly train the common parts of this type of competition questions, such as microcontrollers, ADC/DAC, amplifiers, power supplies, etc.

(2) Students who are focusing on the "Instrumentation" topic can also use their imagination to consider:

① What other laboratory instruments have not appeared in the competition questions? For example, impedance analyzers, network analyzers, etc., you can practice them in advance during the training process.

② Consider some of the previous competition topics, which ones might appear in the amplifier, high frequency, and other competition topics?

③ Consider some of the previous contest questions, and consider which of them may have changes in indicators and functions, such as simple resistance, capacitance and inductance tester.

④ For some of the competition questions that have already appeared, consider which ones may have any changes in the production requirements?

gmchen

It seems to be the content of a book, and it is only collected up to 2011. The topics after 2011 (undergraduate group) are as follows:

Simple frequency characteristics tester (2013 E question)

80MHz-100MHz spectrum analyzer (2015 E question)

Digital frequency meter (2015 F question)

Remote amplitude-frequency characteristic test device (2017 H topic)

人去搂空

Thanks for sharing!