Design Scheme of Amplitude-Frequency Characteristic Measuring Instrument Based on Instrument Integration

This article uses DDS function signal source, digital oscilloscope and ordinary computer as the hardware platform, and configures LabVIEW 8.6 program on the computer to control the function signal source to generate the sweep signal required for the test, and the digital oscilloscope collects the sweep signal and the response signal of the test network. Through computer analysis, calculation and display, the amplitude-frequency characteristic measurement is well realized.

  0 Preface

The frequency characteristic is an important characteristic of the circuit network. In the past, manual measurement was used to stimulate the circuit network by outputting sinusoidal signals at different frequencies, and then to measure the response of the circuit network. A test often took a long time to complete. Although it only takes a few minutes to realize the frequency characteristic measurement of the circuit network by using a dedicated sweeper, network analyzer, etc., due to the expensive equipment, ordinary teaching laboratories are rarely equipped. The method of using a microprocessor to control a direct digital synthesis (DDS) frequency sweep source can better realize the frequency characteristics test, but the design and manufacture of the frequency sweep signal source, amplitude and phase detection circuit are difficult, and the implemented devices often exist Simple, unstable performance and other shortcomings.

Direct digital synthesis (DDS) function generators and digital oscilloscopes with digital interfaces have been widely used in laboratories. The former can achieve high-precision amplitude and frequency switching, while the latter integrates data acquisition, software programming and other functions, can provide users with multiple analysis functions, and can even save and process waveforms. Especially most digital oscilloscopes provide built-in waveform amplitude measurement and waveform delay measurement. The combination of these instruments and the virtual instrument design platform can build an automatic test system at low cost and conveniently. This article uses LabVIEW8.6 as the design platform, uses the laboratory computer, Shengpu F40 digital synthesis function signal source with digital control interface and Tektronix TDS1012C digital storage oscilloscope to realize the frequency characteristic test of the circuit network. The realization of the system combines the advantages of the spot frequency method and the frequency sweep method. The computer uses the RS 232 serial port control function signal source to generate a signal with constant amplitude and continuous frequency change with time as the frequency sweep signal of the network under test, which acts on the network under test. The digital oscilloscope samples and processes the output signal and input signal of the tested network. The computer obtains the signal amplitude measured by the digital oscilloscope through the USB interface, and analyzes the amplitude-frequency characteristics of the circuit network through the friendly user interface of the LabVIEW8.6 software And show it.

1 System configuration

Using the frequency sweep signal to dynamically measure the circuit network under test, the frequency response characteristics of the circuit network under test can be obtained. The ratio of the signal amplitude between the input end and the output end of the tested network is the gain of the circuit. The overall block diagram of the system is shown in Figure 1. The computer controls the Shengpu F40 digital synthesis signal source through the serial port to generate a frequency sweep signal and applies it to the circuit under test. The computer reads the RMS value collected by the digital oscilloscope through the USB interface, uses LabVIEW8.6 software for data processing and displays the amplitude-frequency characteristic curve.



TDS1000C-SC series digital storage oscilloscope comes standard with USB connection, 16 kinds of automatic measurement, limit test, data recording and context-sensitive help. It has a bandwidth of up to 100 MHz and a sampling rate of 1 GS/s, which fully meets the design requirements of this article. When using a digital oscilloscope, in order to avoid aliasing, place the sweep speed in a position where the sweep speed is faster. This article uses AUTO SET to adjust the sampling rate of the digital oscilloscope to match the output frequency of the current function generator. , Complete sampling.

2 Software design

This article uses VISA interface to realize the communication between LabVIEW and digital digital oscilloscope. Its purpose is to control the DDS signal source to produce a given range of frequency sweep signal, use a digital oscilloscope to measure and calculate the effective value, and make a frequency characteristic curve after obtaining the calculation result. The main function process is shown in Figure 2.



After the program runs, first initialize the user interface and let the user select the communication interface connected to the instrument. Then enter the required sweep control quantity, such as start frequency (20 Hz), end frequency (not more than 40 MHz) and sweep amplitude, and select continuous or logarithmic sweep mode. The response frequency interval is automatically calculated according to the start and end frequencies entered by the user, and the calculated frequency points are stored in the frequency array, and the frequency array data obtained is shown in Figure 3.



After calculating each frequency point, first send the amplitude control word and frequency control word to the function signal source according to the serial port selected by the user to the function signal source according to the serial port selected by the user. After calculating each frequency point, first send the amplitude control word and frequency control word to the function signal source according to the serial port selected by the user to generate different frequencies The frequency sweep signal, the procedure of sending the amplitude and frequency control word is shown in Fig. 4.



In order to ensure the accuracy of the read value, the system selects several frequency points to perform the waveform correction operation. The method is to control the digital oscilloscope through the USB interface to perform the "AUTO SET" operation. When the sending frequency is 10 Hz~1 kHz, 1~100 kHz or Perform waveform correction operations on the digital oscilloscope from 100 kHz to 10 MHz. The correction procedure is shown in Figure 5.



Then read the effective value (RMS) measured by channel 1 and channel 2 of the digital oscilloscope through the USB interface [10], calculate the gain and fill it into the gain array, the unit is dB, as shown in Figure 6. Use the graphic display control on the express panel " expressXY chart” function to realize XY chart display (see Figure 7).

3 System test

Connect the computer, Shengpu F40 DDS signal source and TDS1000C-SC series digital storage oscilloscope, connect the function signal source output terminal to the input terminal of the circuit to be tested, channel I of the digital oscilloscope to the input terminal of the circuit to be tested, and channel II to the circuit to be tested The output terminal. In the user interface, select the serial port corresponding to the DDS signal source (such as COM1) and the USB port corresponding to the digital storage oscilloscope, enter the required start frequency, end frequency and amplitude, and select the sweep mode. After setting, click the start button to start measurement. Figure 8 shows the measured amplitude-frequency characteristics of a band-pass filter with a center frequency of about 16 kHz.





The center frequency of the tested band-pass filter is about 16 kHz. In the actual measurement, the frequency sweep range is from 1 to 60 kHz, and it takes about 2 minutes and 30 seconds to sweep 60 frequency points. If you need to improve the measurement accuracy of the amplitude-frequency characteristic curve, you can increase the frequency sweep point.

4 Conclusion

This article uses LabVIEW8.6 as the design platform, and uses the laboratory computer, Shengpu F40 digital synthesis function signal source with digital control interface and Tektronix TDS1012C digital storage oscilloscope to realize the amplitude-frequency characteristic test of the circuit network. The method used in this program tested the amplitude-frequency characteristics of Butterworth low-pass filters, band-pass filters, and tuned amplifiers. The experimental results prove the effectiveness and practicability of the scheme in application. On this basis, the phase-frequency characteristics can be further obtained. Compared with commercial equipment, although the response time of this system is slower and the user interface needs to be improved, its programming and control are simple. It only needs to use the existing equipment in the laboratory, which is a way to improve the resource utilization of university teaching laboratory equipment. A feasible solution.