Design of portable signal generation and detection device

0 Introduction
At present, in many fields, electronic instruments are required to work outdoors for a long time, even in many harsh environments. Although there are many targeted designs in hardware for different environments in which the instruments are suitable, instrument damage caused by natural or human factors in outdoor environments will occur from time to time. Due to the lack of commonly used electronic instrument detection equipment such as oscilloscopes and signal generators, it is not convenient to detect them in real time. They can only stop working and take them back to laboratories with conditions for testing. If there is a set of portable signal generation and detection devices, the instrument can be tested at the scene of the accident in the first time, and the damage can be judged according to the test results to decide whether to repair it on the spot or take the instrument back for further testing.
In response to the above requirements, this paper designs a portable signal generation and measurement device using widely used embedded technology. Considering the different situations of the two, the signal generation and signal measurement devices are designed independently. They can be used alone or in combination. The signal generation device uses AT89S51 as the core to generate any adjustable function signal through the DDS chip AD9851. The signal measurement device is controlled by the popular STM32F106ZE core, equipped with necessary analog signal modulation, shaping circuits, digital-to-analog circuits and display circuits to realize the detection of the peak-to-peak value, effective value, frequency, etc. of the analog signal parameter package.

1 Portable signal generation device
In the signal generation device designed this time, the advanced direct digital frequency synthesis technology (Direct Digital Synthesis, DDS) is used as the core of the signal generator to generate any waveform signal. The system block diagram of the signal generation device is shown in Figure 1. The workflow of the entire system is as follows: the keyboard controls the microcontroller to generate control signals such as the corresponding waveform, frequency, phase, amplitude, etc., among which the waveform, frequency and phase control words enter the AD9851 module, and AD9851 selects the corresponding waveform, phase and frequency in its waveform library according to the relative control signal. The amplitude control word enters the program-controlled amplifier circuit. The single-chip microcomputer adjusts the amplification factor of the program-controlled amplifier according to the output voltage amplitude of the peak-to-peak detection and A/D conversion. Finally, the power amplifier circuit amplifies the signal to obtain the final signal output.
In the entire signal generating device, the core module is the DDS module composed of the AD9851 chip. Compared with the traditional phase-locked frequency synthesizer, the signal generator built with pure analog circuits, and the signal generator composed of the single-chip microcomputer and the analog-to-digital conversion module, the signal generating device composed of the single-chip microcomputer and the analog-to-digital conversion module has the advantages of being able to generate arbitrary waveforms, simple software and hardware design, low power consumption, convenient debugging, and stable output frequency.

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It is worth noting that the AD9851 chip does not have a low-pass filter inside. In order to obtain a purer signal, a low-pass filter circuit needs to be designed after the AD9851 module. There are two design schemes in the design of the filter circuit. One is an active filter composed of an operational amplifier, and the other is a passive filter composed of an inductor and a capacitor. Since the maximum stable value of the signal frequency output by the AD9851 is 70 MHz, the cutoff frequency of the filter should be above 70 MHz. Therefore, due to the bandwidth limitation of the operational amplifier, the LC filter circuit will be used in the high-frequency field. Relatively speaking,
the parameter calculation of the LC filter circuit is difficult and difficult to debug. During the design, the auxiliary design software Filter Solutions 10.0 for LC circuits was used to help the design, saving design time and workload.

2 Portable signal detection device
The signal measurement device can essentially be regarded as a simplified design of the oscilloscope. Although its functions cannot be compared with those of a laboratory oscilloscope, it still has the basic functions commonly used in the measurement of the oscilloscope, such as waveform display, frequency value, amplitude value, etc. The system uses the currently popular STM32 core processor.

The signal detection working circuit is shown in Figure 2. The measured signal enters the detection device through the probe. In order to ensure that the system has a wide measurement range, the signal must first be attenuated by a fixed multiple, and then the impedance matching is performed to enable the system to have a good signal absorption capability and to maximize the signal distortion. Then the filter circuit is used. The low-pass filter circuit composed of LC is used here. Its design process is similar to that of the signal generator. The signal is divided into two paths. One path enters the programmable amplifier, so that the signal is amplified before entering the A/D conversion circuit, so that the signal is within the optimal input range of the A/D conversion circuit, and the full-scale error of the A/D conversion circuit is minimized. The analog signal is converted into a digital signal through the A/D conversion circuit and sampled by the STM32 processor. The other path enters the waveform conversion circuit, whose function is to convert sine waves into regular square wave signals and enter the STM32 processor to facilitate the operation of the STM32 internal counter and calculate the frequency of the signal. The following is an introduction to the main circuit modules:
In analog signal detection, especially for small or even weak signal detection, programmable amplifiers play an important role. Due to the limited conversion accuracy of the A/D converter, when the amplitude of the measured signal is so small that it cannot be recognized by the A/D converter, it is necessary to amplify the signal before inputting it into the A/D conversion module for sampling. However, the amplitude of the measured signal has a certain range of variation. Taking this system as an example, the normal signal amplitude that can be detected is between 10 mV and 8 V. Considering that it will experience 2 times attenuation at the front end, the voltage range entering the A/D is 5 mV to 4 V, which has a certain dynamic range. The amplification factor of the required amplifier must also be variable to adapt to different measured signal frequencies. Here, a programmable amplifier is used. The programmable amplifier circuit is shown in Figure 3. In this design, the design scheme of the four-chip combination of PGA207 and PGA206 is selected, and its theoretical amplification factor can reach up to 6400 times.

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The A/D conversion circuit (see Figure 4) is mainly composed of two chips, LT6350 and LTC2393-16. LTC2393-16 is the A/D conversion chip and LT6350 is the driver chip of LTC2393-16. The single-ended sine wave signal from the programmable amplifier is connected to the input of LT6350 through the adapter.

The shaping circuit can also be understood as a waveform conversion circuit. Its main function is to compare the measured sine wave signal with the ground through the comparator, generate a square wave signal and enter the processor. The processor counts the rising or falling edge of the input square wave signal to achieve the purpose of frequency detection. As shown in Figure 5.

3 Conclusion
The software design of the signal generation and detection device uses C language as the design language to complete all the control and algorithm design. The signal generation device with AD9851 and AT89S51 as the core and the signal detection device with STM32 as the core not only reduce the numerical calculation of complex analog circuits, but also are not inferior to traditional design schemes in terms of performance and function. After testing, the signal generation and signal detection devices basically achieved the expected design goals (signal generator output and signal detection device range amplitude: 1 mV~10 V frequency; 0 Hz~30 MHz), and realized the generation and detection of signals including sine waves, square waves, triangle waves, etc. At the same time, the waveform, amplitude and other information of the current output/input signal can be displayed on their respective LCD screens, and the signal detection device can display the original waveform of the current measured signal in real time by relying on the various algorithm processing of its built-in chip STM32. Both use touch screen operation and have a good human-computer interface.