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

0 Introduction

Frequency characteristics are important characteristics of circuit networks. In the past, manual measurement methods were often used to stimulate circuit networks by outputting sinusoidal signals at different frequencies, and then measuring the response of the circuit network. A test often takes a long time to complete. Although it only takes a few minutes to measure the frequency characteristics of circuit networks using dedicated sweep frequency analyzers, network analyzers, etc., they are rarely equipped in ordinary teaching laboratories due to the high price of the equipment. The method of using a microprocessor to control a direct digital synthesis (DDS) sweep frequency source can better realize the test of frequency characteristics, but the design and production of the sweep frequency signal source, amplitude and phase detection circuits are difficult, and the implemented devices often have shortcomings such as simplicity and unstable performance.

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 a variety of analysis functions, and can even save and process waveforms. In particular, most digital oscilloscopes provide built-in waveform amplitude measurement and waveform delay measurement. These instruments combined with virtual instrument design platforms can build automatic test systems at low cost and conveniently. This paper 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 frequency characteristic test of the circuit network. The system combines the advantages of the point frequency method and the sweep frequency method. The computer controls the function signal source through the RS 232 serial port to generate a signal with constant amplitude and frequency that changes continuously with time as the sweep frequency signal of the network under test. The digital oscilloscope samples and processes the output and input signals of the network under test. The computer obtains the signal amplitude measured by the digital oscilloscope through the USB interface, and analyzes and displays the amplitude-frequency characteristics of the circuit network through the friendly user interface of LabVIEW8.6 software.

1 System composition The frequency

response characteristics of the circuit network under test can be obtained by dynamically measuring the circuit network under test with the sweep frequency signal. The ratio of the signal amplitude at the input and output ends of the network under test 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 synthetic signal source through the serial port to generate a sweep frequency signal to act on the circuit to be tested. The computer reads the RMS value collected by the digital oscilloscope through the USB interface, and uses LabVIEW8.6 software to process the data and display the amplitude-frequency characteristic curve.

 

The TDS1000C-SC series digital storage oscilloscope comes standard with USB connection, 16 automatic measurements, limit test, data logging and context-sensitive help. It has a bandwidth of up to 100 MHz and a maximum 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, the sweep speed gear is best placed at a faster sweep speed. This article uses the automatic setting (AUTO SET) method to adjust the sampling rate of the digital oscilloscope in a timely manner so that it can match the output frequency of the current function generator and complete accurate sampling.

2 Software Design

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

 

After the program is run, the user interface is initialized first, allowing the user to select the communication interface connected to the instrument. Then enter the required frequency sweep control quantity, such as the start frequency (minimum 20 Hz), end frequency (no more than 40 MHz) and sweep amplitude, and select continuous or logarithmic sweep mode. The response frequency interval is automatically calculated based on the start and end frequencies entered by the user, and the calculated frequency points are saved in the frequency array. The frequency array data is obtained as shown in Figure 3.

 

After calculating each frequency point, the amplitude control word and the frequency control word are first sent to the function signal source according to the serial port selected by the user to generate sweep signals of different frequencies. The program for sending the amplitude and frequency control words is shown in Figure 4.

 

In order to ensure the accuracy of the read values, the system selects several frequency points for waveform correction operations. The method is to control the digital oscilloscope through the USB interface to perform an "AUTO SET" operation. When the sending frequency is 10 Hz~1 kHz, 1~100 kHz or 100 kHz~10 MHz, the digital oscilloscope is respectively subjected to a waveform correction operation. The correction procedure is shown in Figure 5. [page]

Then, the effective value (RMS) measured by the digital oscilloscope channel 1 and channel 2 is read through the USB interface [10], the gain is calculated and filled into the gain array, the unit is dB, see Figure 6. Finally, the graphic display control "expressXY graph" function on the express panel is used to realize the XY graph display (see Figure 7).

3 System test

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

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

4 Conclusion

This paper 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 scheme tests the amplitude-frequency characteristics of circuits such as Butterworth low-pass filter, band-pass filter and tuned amplifier. The experimental results prove the effectiveness and practicality of this scheme in the 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 still needs to be improved, its programming and control are simple, and it only needs to use the existing equipment in the laboratory. It is a feasible solution to improve the utilization rate of equipment resources in university teaching laboratories.