Design of Digital Storage Oscilloscope

1. Master the basic principles of digital oscilloscope measurement.
2. Be familiar with the hardware structure of digital storage oscilloscope.
3. Master the CVI software design of virtual digital storage oscilloscope.

  Design a virtual digital storage oscilloscope
  (1) Design a virtual digital storage oscilloscope interface that includes basic functions such as coupling mode selection, volt/div adjustment, trigger source selection, and time base selection. The horizontal scale of the display screen is required to be 10div, and the vertical scale is required to be 10div. 
  (2) Based on the completion of content (1), realize a virtual dual-trace digital storage oscilloscope with adjustable amplitude and time base.
   Requirements: ① The vertical sensitivity includes at least four levels: 50mV/div, 0.1V/div, 0.5V/div, and 1V/div; ② The scanning speed includes at least eight levels: 0.1u/div, 1u/div, 10u/div, 100u/div, 500u/div, 1m/div, 10m/div, and 0.1s/div; ③ Add a dual-trace oscilloscope function that can simultaneously display the waveforms of two measured signals.
  (3) Data processing design
   requirements: ① Display the amplitude of the measured signal including: effective value, peak-to-peak value, and average value; ② Display the frequency value of the measured signal.
  (4) Design waveform storage and playback functions based on the 32K storage depth provided by the oscilloscope hardware in the electronic measurement experiment box.

3. Experimental Equipment

1. SJ-8002B electronic measurement experiment box 1 set
2. Computer 1 set
3. Signal source (DDS signal source of the platform can also be used) 2 sets
4. Q9 line 1 set
5. Oscilloscope 1 set

  4.1 Digital Oscilloscope Principle

  The digital storage oscilloscope uses an A/D converter to convert analog signals into digital signals, and then stores the data in a semiconductor memory RAM. When necessary, the contents stored in the RAM are called out, and the waveform is reproduced through the LCD in a dot matrix or wired manner. The principle block diagram can be referred to Figure 1. In this oscilloscope, the signal processing and signal display functions are separated, and its performance mainly depends on the performance of the AD, RAM and microprocessor for signal processing. Because RAM memory is used, it can write data quickly and read data slowly, so that there will be no flickering even when observing slow signals.

  4.2 Composition of Virtual Digital Storage Oscilloscope

                              Figure 1 Virtual digital storage oscilloscope

    The virtual oscilloscope integrates the computer and the measurement system, replaces some hardware functions of traditional instruments with computer software, and replaces the physical panel of traditional instruments with computer monitors. The instrument panel that is easy to operate and lifelike can be designed through relevant software. It can not only realize the functions of traditional oscilloscopes, but also has the characteristics of storing, reproducing, analyzing, and processing waveforms. It can also process, process and analyze various signals and complete measurement tasks of various scales. Moreover, the instrument is small in size, consumes less power, is easy to carry, and can be used on different computers.
   Therefore, in SJ-8002B, the principle of virtual digital storage oscilloscope is also cited to realize data acquisition. The signal conditioning, AD conversion, SRAM for storing data, and control logic are all in the experimental platform, and the computer mainly plays the role of data processing and display.

   4.3 SJ-8002B Electronic Measurement Experiment Box Oscilloscope Hardware Structure

   4.3.1 Test range and acquisition parameter adjustment range
   Test voltage amplitude range: -20V~+20V (peak-to-peak value)
   Measurement frequency range: 1Hz~1MHz
   Sampling clock:

   Programmable gain:

   Data cache depth: 64KB
   Analyze the collected data and display the peak value, average value, effective value, frequency, period and other parameters of the waveform.

   4.3.2 Hardware Schematic Diagram

                                   Figure 2 SJ8002B oscilloscope hardware schematic

   Figure 2 is a block diagram of the oscilloscope module. As can be seen from the figure, the dual channels of high-speed acquisition are completely independent, so a variety of different test tasks can be completed, realizing the various functions of the virtual dual-trace digital storage oscilloscope.

   4.3.3 Control Logic
   The hardware control of the oscilloscope is mainly divided into two parts: data writing (acquisition) and data reading (display). All control logic is stored inside the CPLD, as shown in Figure 3:

                                 Figure 3 Oscilloscope hardware control logic

    Ain1 and Ain2 channels are connected to the same sampling clock and converted at the same time. The converted data is sent to SRAM for latching through the buffer. When one acquisition is completed, the host reads back the data for further processing, such as filtering, display, etc.
   Data writing: Under the control of the sampling clock CLK, AD9288 digitally discretizes the two input analog signals into 8-bit digital signals and sends them to SRAM through the data buffer. The address is provided by the same address counter, which is an up/down counter (incrementing when acquiring data and decrementing when reading). In this way, the data acquired each time will be smoothly stored in SRAM.

    4.3.4 Acquisition Part
   The key device of the acquisition part is ADI's AD9288, which is an 8-bit dual-channel monolithic integrated analog/digital converter with a sample-and-hold circuit, with low power consumption, small size, good dynamic characteristics, and easy to use. Dual 8bits, 40MSPS, low power consumption (90mw per channel), SNR=47DB (at 41MHz), analog input range of each channel 1.024Vp-p, center level Vin0=1/3 Vdd, 3.0V analog power supply (2.7V-3.6V), two data output modes (complementary code or original code), level compatible with TTL/CMOS.

    4.3.5 Conditioning Circuit
    Since AD9288 uses differential input, it is necessary to convert the DC-coupled unilateral signal of the analog input of the experimental board into a differential signal without distortion, and shift the signal amplitude to meet the 0.5-1.5V input requirement of AD9288. The following channel circuit is used to achieve signal conversion.

                             Figure 4 Differential signal conversion circuit

   In the circuit, U1 is a follower, which is used for impedance matching. Two operational amplifiers are used to generate differential signals, of which U2 is a common-phase amplifier as input, and U3 is an inverting amplifier as input supply. Two diodes and U4 form the signal shifting part.

   4.3.6 Attenuation and gain control circuit
    Since the test range of the oscilloscope (-20V~+20V) is wider than the test range of AD9288 (0.5V~1.5V), a two-stage amplitude adjustment circuit of signal attenuation and signal gain is added to the front end of the acquisition circuit to ensure the correctness of the measurement and improve the accuracy of the measurement. [page]

                                     Figure 5 Attenuation circuit

   The first stage of the input channel is a high-resistance attenuation circuit as shown in Figure 5. This ensures that the digital oscilloscope has a higher input impedance and attenuates the larger input signal to meet the needs of the subsequent circuit. Here, the circuit is switched by a relay. The primary gain factor A1=0.5, 1.25, 2.5, 5 is set by setting the on-off of the attenuator; the coarse adjustment of the amplitude and the DC offset are controlled by the 12-bit DAC7512. The gain of the op amp is changed by an electronic switch to achieve amplitude adjustment. The secondary gain factor is A2=×5, so the total channel gain A=K1*A1*K2*A2

   4.3.7 Data processing
   Calculate the effective value, mean, peak value, and frequency of the measured signal.
   The mathematical expressions of the voltage average and peak value of the discrete signal are as follows:
   Voltage effective value: 　　  Voltage
   average value:　 　　 
   Voltage peak-peak value:      
   Signal frequency calculation: According to the definition of frequency, the number of changes per second. The
   above parameters of the measured signal can be obtained by calling the function processing_data() provided by the experimental platform software. For a detailed description of this function, see the function list provided by the experimental platform in Section 5.2.

   5.1 Design of Virtual Digital Storage Oscilloscope Interface Design
   a virtual digital storage oscilloscope interface that includes basic functions such as coupling mode selection, volt/division adjustment, trigger source selection, and time base selection. The horizontal scale of the display screen is required to be 10 div and the vertical scale is 10 div, as shown in Figure 6.

                                Figure 6 Virtual digital storage oscilloscope panel

    The controls used include list control (Ring), command button (Command), graphic control (Picture), text control (Text), and numeric control (Numeric).

    5.2 On the basis of completing the interface design,
    the requirements for realizing a virtual dual-trace digital storage oscilloscope with adjustable amplitude and time base are as follows: ① Set the vertical sensitivity to at least 50mV/div, 0.1V/div, 0.5V/div, and 1V/div; ② Set the scanning speed to at least 0.1u/div, 1u/div, 10u/div, 100u/div, 500u/div, 1m/div, 10m/div, and 0.1s/div; ③ Add dual-trace oscilloscope function to display the waveforms of two measured signals at the same time; ④ Data processing design: Display the amplitude of the measured signal including: effective value, peak-to-peak value, and average value; Display the frequency value of the measured signal.
    When designing, the program flow and function call that can be referred to are shown in Figure 7. First, initialize the EPP interface and retrieve the acquisition parameters set by the user interface, such as coupling mode, trigger mode, vertical sensitivity, time base, etc., and then start the acquisition; when the preset acquisition points are acquired, the computer reads the data back; finally, process the read-back data, such as calculating the amplitude and frequency, filtering and displaying the waveform.
Before designing, please complete the calculation and selection of the gain and time base corresponding to each gear and fill in the results in Table 1 and Table 2 below. Calculation method example:
   ⑴ Given a vertical sensitivity of 20mv/div, the range = 20mV × 10 = 200mV = 0.2V; Since the input range of AD9288 is 0.5V----1.5V, in order to improve the resolution, a 5-fold channel total gain can be used, that is, 0.2V×5=1 V, corresponding to the test range and acquisition parameter adjustment range of the SJ8002B electronic measurement experiment box in Section 4.3.1, the Div number can be obtained as 2. [page]

                                                  Table 1

   ⑵ Given a scanning speed of 200u/div, the sampling time TS = 200u×10 = 2000u=2ms. Since the number of full-screen sampling points is fixed at 25,000 points, FS×TS <25,000 means FS < 12.5M. Since the provided clock is not 12.5M, the closest 10MHZ can be selected as the sampling clock.

                                                          Table 2

                                 Figure 7 Oscilloscope software flow chart

The main functions provided by the CVI software and experimental software platform that can be used as reference in the design are shown in the following table:

   5.3 Design waveform storage and playback functions based on the 32K storage depth provided by the oscilloscope hardware in the electronic measurement experiment box.
   There are two command button (Command) controls on the virtual storage oscilloscope panel designed in Section 5.2, as shown in Figure 6. Saving the waveform (Save button) calls the ArrayToFile() function, opening the waveform file (Open button) calls the FileToArray() function, and displaying the waveform calls the PlotWaveform() function. The above functions are all provided by the CVI software and can be used directly.

   6.1 Observation of dual-trace display waveform
   Two function signal sources generate two voltage signals respectively, one generates a sine wave with an amplitude of 5V and a frequency of 5KHz, and the other generates a triangle wave with an amplitude of 3V and a frequency of 5KHz. The designed virtual digital storage oscilloscope is used for dual-trace display and the waveform is drawn.
   Draw the waveform of signal one: Draw the waveform of signal two:

   6.2 A sine wave with a frequency of 5KHz and amplitude variation (effective value) as shown in the following table is generated by the function signal source, and the effective value is measured using the designed virtual digital oscilloscope.

    6.3 The function signal source generates a sine wave with an amplitude of 5V and a frequency variation as shown in the table below. The frequency is measured using the designed virtual digital oscilloscope.

7. Questions for reflection and practice

1. Can you use an oscilloscope with a bandwidth of 20MHz to observe sine waves and square waves with a repetition frequency of 15MHz? Why?
2. When using an oscilloscope to measure various waveform parameters, how do you reduce its measurement error?
3. What methods can be used to measure the rise and fall time of a square wave?