Analysis of College Students' Electronic Competition Questions - Analysis and Comparison of the 2019 and 2015 National Competition Retest Questions

gmchen

Analysis of College Students' Electronic Competition Questions - Analysis and Comparison of the 2019 and 2015 National Competition Retest Questions [Copy link]

 

This year's National College Student Electronics Competition has ended, and the winning team should have undergone a post-match retest. The author only found two retest questions on the Internet, one in 2015 and the other in 2019. After reading the retest questions of these two sessions, I found them quite interesting, so this article analyzes and compares them.

Overview of the tasks and requirements of the 2015 National Competition retest question "Multiple Waveform Generating Circuits"

The specified integrated test board has 555 chips, 74LS74 chips and a general-purpose quad op amp 324 chip. Design and make a frequency-variable circuit that can simultaneously output square wave I, square wave II, triangle wave, sine wave I, and sine wave II. Do not use any other devices or chips except the chips on the integrated test board (author's note: the word "device" should be deleted).

It is required to use the signal generated by the 555 time base circuit as the signal source; using this square wave I, output 4 waveforms on four channels: square wave II, triangle wave, sine wave I, sine wave II, and the load resistance of each channel output is 600 ohms.

1. Use the 555 time base circuit to generate a square wave I with a continuously adjustable frequency of 20kHz-50kHz and an output voltage amplitude of 1V .

2. Use the digital circuit 74LS74 to generate a square wave II with a continuously adjustable frequency of 5kHz-10kHz and an output voltage amplitude of 1V .

3. Use the digital circuit 74LS74 to generate a continuously adjustable triangle wave with a frequency of 5kHz-10kHz and an output voltage amplitude of 3V .

4. The output frequency is a continuously adjustable sine wave I of 20kHz-30kHz , and the peak-to-peak value of the output voltage amplitude is 3V .

5. The output frequency is 250kHz sine wave II, and the output voltage amplitude peak to peak value is 8V .

The waveforms of square waves, triangle waves and sine waves should have no obvious distortion (when measured with an oscilloscope). The frequency error should not exceed 5% ; the peak-to-peak error of the output voltage amplitude within the passband should not exceed 5% .

The power supply can only be a +10V single power supply, which must be supplied by a regulated power supply. No additional power supply may be used.

gmchen

Analysis of the 2015 National Competition Retest Question "Multiple Waveform Generating Circuits"

The question requires the generation of 5 waveforms, which tests 4 different knowledge points:

1. Use NE555 to generate square wave I.

If the contestant has used the NE555 chip, it should be easy to complete this. If not, as long as you are familiar with the principle of the RC relaxation oscillator, read the NE555 manual and follow the conventional connection method of NE555 , you can change the frequency by changing the timing resistor with a potentiometer.

2. Use 74LS74 to generate square wave II .

Square wave II is 4 times the frequency of square wave I. 74LS74 is a dual D flip-flop. Constructing a 4- frequency divider should be basic common sense after learning digital circuits, so there is no problem in principle.

However, this seemingly simple requirement contains a big "trap".

The first is the power supply voltage. The normal operating voltage of the 74LS series TTL digital circuit is 5V±0.25V , and the maximum allowable power supply voltage is about 7V . If it exceeds this value, it may be damaged by breakdown. However, the power supply given in the question is 10V , and it is very likely to burn the chip if it is powered directly (the original question has "deduction of points for burning"), so measures must be taken to reduce the power supply voltage of 74LS74 to below 7V . The question does not provide other chips including voltage regulators, so a compromise method is to use several silicon diodes in series in the power supply of 74LS74 . The voltage drop of each silicon diode is about 0.7V , and 6~7 diodes can reduce the voltage to a safe range. However, due to the increase in the internal resistance of the power supply, a power supply decoupling capacitor with a large enough capacity must be installed between the power pin and the ground pin of the 74LS74 chip.

The second is the level matching problem. The maximum input voltage of TTL cannot exceed 7V , and it must be controlled below 5V during actual operation . However, the source signal of this divider comes from NE555 , and the output high level of NE555 may reach more than 8V when powered by 10V , so it is necessary to use appropriate resistors to divide the voltage to below 5V .

gmchen

3. Generate a triangle wave with the same frequency as square wave II .

This question examines the conversion from square wave to triangle wave. The question "Use digital circuit 74LS74 to generate ... " has a grammatical error (because a triangle wave cannot be generated using digital circuits). It should be "Use digital circuit 74LS74 to generate a square wave II with a frequency of 5kHz-10kHz continuously adjustable as the signal source to generate ... "

The easiest way to think of is to convert the square wave into a triangle wave through an integrator. However, there are several issues that must be paid attention to when using LM324 to form an integrator:

First, a key to using the integrator correctly is that the average level of the input waveform (no matter what kind of waveform) should be absolutely the same as the reference level of the integrator! As long as there is a slight difference (for example, due to the input offset of the op amp), this unequal voltage will slowly accumulate on the integrating capacitor, and the final result must be that the op amp reaches saturation in a certain direction. However, the above requirements are almost impossible to achieve in actual circuits, so actual integrators often need to connect a large resistance in parallel to the integrating capacitor. Since this resistor will affect the output waveform of the integrator (the linearity of the integration is destroyed), its resistance value cannot be too small, and it should be as large as possible while ensuring that the integrated charge caused by the unbalanced input part can be released.

Second, in order to make the output dynamic range of LM324 meet the output amplitude (peak-to-peak value 3V ) requirement of the question, its reference level Vr should be at least 1.5V (half of the output peak-to-peak value). The high and low output levels of 74LS74 are roughly 3.5V and 0.2V , _and the average level is about 1.65V (the actual circuit may vary slightly depending on the manufacturer). According to the first description above, the reference level of LM324 should be designed to be equal to the average output level of 74LS74 .

Third, the question requires that the signal frequency be continuously adjustable from 5kHz to 10kHz and the output voltage amplitude be 3V . However, if the RC time constant of the integration circuit remains unchanged, it is impossible to generate a triangle wave by integration and keep the output voltage amplitude unchanged when the frequency changes. The solution is to use a double potentiometer to allow the RC time constant of the integration circuit to change synchronously with the input signal.

4. Generate sine wave I and sine wave II .

Since the question requires the use of the signal generated by the 555 time base circuit (square wave I ) as the signal source, the square wave contains the fundamental frequency and odd harmonics, so both signals can be resolved using filters.

The frequency of sine wave I is continuously adjustable from 20kHz to 30kHz , and the peak-to-peak value of the output voltage is 3V . The square wave I can be obtained by passing through a low-pass filter with a cut-off frequency higher than 30kHz to obtain sine wave I. What needs to be paid attention to in the design is that the filter must have sufficient attenuation at 60kHz (the third harmonic of 20kHz) .

The frequency of sine wave II is required to be 250kHz , and the peak-to-peak value of the output voltage is 8V . This frequency is 5 times the highest frequency of square wave I. Let square wave I pass through a bandpass filter with a center frequency equal to 250kHz, and its 5th harmonic sine wave II can be obtained . What needs to be paid attention to in the design is the Q value of the filter .

It should also be noted that under a single 10V power supply, the lower limit of the output dynamic range of the LM324 is about tens of millivolts, and the upper limit is about 8.5V . The reference level of the op amp needs to be set to the midpoint of this output dynamic range. In particular, the output amplitude of the sine wave II is close to the limit output dynamic range of the LM324 , so this should be paid more attention to.

gmchen

Overview of the tasks and requirements of the 2019 National Competition retest question "Multi-signal generator"

The question specifies that there is an LM324AD (quad op amp) and an SN74LS00D (quad NAND gate) on the comprehensive evaluation board. Using the comprehensive evaluation board and several resistors and capacitors, design and manufacture a multi-signal generator to generate the following four signals:

1. The frequency is a continuously adjustable square wave pulse signal of 19kHz~21kHz, and the amplitude is not less than 3.2V;

2. For a sine wave signal with the same frequency as the square wave, the output voltage distortion shall not exceed 5%, and the peak-to-peak value ( V pp) shall not be less than 1V;

3. A narrow pulse signal with the same frequency as the square wave and a duty cycle of 5% to 15% that is continuously adjustable, with an amplitude of not less than 3.2V;

4. The phase error of the cosine wave signal orthogonal to the sine wave is no more than 5°, and the output voltage peak-to-peak value ( V pp) is no less than 1V.

The signal load resistance of each channel is 1kΩ.

Only a single +5V power supply is allowed.

gmchen

Analysis of the 2019 National Competition Retest Question "Multi-Signal Generator"

The question requires using the above two chips to generate 4 different signals, which are analyzed one by one below.

1. Square wave

There are roughly the following solutions to generate square waves using op amps or digital circuits:

1. A multivibrator is composed of several NOT gates and RC networks. This structure can be realized by multiple specific circuits. As long as the time constant of the RC delay network is changed, the oscillation frequency can be changed. It is the simplest rectangular wave generator circuit. However, there is a small problem with this circuit: due to the influence of the input current of the digital circuit and other reasons, it cannot guarantee that the charging and discharging currents of the timing capacitor C are exactly the same, so the duty cycle of the rectangular wave output cannot be guaranteed to be exactly equal to 0.5 .

2. The oscillation circuit is composed of an integrator plus a hysteresis comparator. Its structure is shown in the figure below. The working principle is that the high level or low level output by the hysteresis comparator is converted into a falling or rising ramp voltage after integration by the integrator. After this voltage reaches the input threshold of the hysteresis comparator, the output of the comparator flips, and this repetition constitutes a square wave output. At the same time, a triangular wave is obtained at the output end of the integrator.

Suppose the comparator's output high and low levels are V _OH and V _OL respectively , and the integrator's reference level is _VR , then the charging and discharging currents of the integrator on the capacitor are ( V _OH - VR ₎ /R and ( VR - V _OL )/R respectively. As long as VR is adjusted to make these two _{currents equal, the charging and discharging time within one cycle must be the same, and} the duty cycle of the output square wave is 0.5 .

gmchen

2. Narrow pulse with adjustable duty cycle .

There are roughly two solutions that are suitable for this problem:

1. Delay the square wave through the RC network and perform a logical “AND” with the original square wave.

2. Shape the triangle wave through the comparator.

3. Sine wave .

There are usually two ways to generate a sine wave: 1. Directly construct a sinusoidal oscillation circuit using an operational amplifier, such as a Wien oscillator; 2. Pass a square wave or a triangle wave through a suitable filtering network to obtain a sine wave.

However, in this question, BJT , FET or diode is not allowed, and the passive devices are only resistors and capacitors. In this case, if the oscillator is directly constructed with an op amp, due to the lack of a suitable limiting device, the final result is either difficulty in starting oscillation, or the waveform after starting will have severe clipping distortion. Therefore, the only feasible solution is to obtain a sine wave through filtering.

Since the amplitude of the higher harmonics contained in the triangle wave is much smaller than that contained in the square wave, it is easier to obtain a sine wave by filtering the triangle wave.

4. Orthogonal cosine waves .

The cosine signal can be obtained by integrating the sine signal, so this part of the circuit is an integrator composed of an operational amplifier.

gmchen

Comparison of the 2019 and 2015 national competition retest questions

Since the competition retest must be completed within a few hours, the retest questions cannot be too complicated, but they must be able to test the contestants' knowledge reserves, analytical skills, and hands-on abilities. The retest questions in 2019 and 2015 are both about signal generators, and there are many similarities in their design requirements and design constraints. The questions themselves are not complicated, but the content of the test includes many basic application knowledge of digital circuits and analog circuits, which should be said to be a good entry point for the test.

However, the topics of the two sessions are very different in terms of rigor and flexibility.

The 2015 topic has many restrictions on which signal is generated by which device (mainly in terms of digital signals). These restrictions can provide convenience for the contestants, but also limit the contestants' ability to perform independently to a certain extent. In addition, there are many "inaccurate" or "traps" in terms of power supply and level matching. Those "inaccurate" or "traps" may be the negligence of the question setter, or they may be intentional. But for the participating college students, if they have not encountered these problems in their daily lives, or the instructor has provided special guidance in this regard, I am afraid that more than half of them will fail here.

Compared with the 2015 topic, the 2019 topic has been greatly improved in terms of rigor and flexibility. The topic only provides two chips, and does not specify which chip to use to generate which signal, which gives contestants more options to choose from. At the same time, the content of the assessment has not been reduced, including oscillators, comparators, integrators, filters, logic gates, etc., requiring contestants to have a solid basic knowledge and be able to adapt to changes.

freebsder

thorough!