2021 Question E - Digital-Analog Signal Hybrid Transmission Transceiver + All Guessed Correctly

 
1 System Solution
1.1 Overall System Design Solution
    The system consists of a transmitting device and a receiving device. After the transmitting end combines the digital signal and the analog signal, the modulation and transmission of the mixed signal are realized; the two-way signal is separated by using a single-channel to differential converter, and then the demodulation and display of the two-way signal are realized respectively. Both the transmitting device and the receiving device are powered by a single battery power supply, and the power supply requirements of each circuit module are met after the power conversion module.
    The transmitting device mainly includes a digital signal generator, a digital-analog signal synthesis circuit, a DDS module, an SSB modulation circuit and a power amplifier. The FSK digital signal is generated by using MCU and DDS, and the FSK signal is multiplied and combined with the analog signal. SSB modulation is used to reduce power consumption, and the power amplifier is designed to meet the antenna transmission power requirements. After
receiving the mixed signal, the receiving device uses low-noise small signal amplification, mixing and ceramic filtering to realize the demodulation function of the combined signal, and uses a single-ended to differential converter to separate the analog part from the digital part. The analog part adopts envelope detection demodulation. The digital part converts FSK into ASK through a zero intermediate frequency mixer circuit, and then realizes digital signal demodulation through envelope detection and overvoltage comparator, and finally displays it through a digital tube.
    Test ports are reserved at the output end of the combiner circuit, the front end of the transmitting antenna, and the battery single power supply output point at the transmitting end to observe the waveform of the combined signal, observe the bandwidth of the modulated signal, and measure the power consumption of the transmitting end. The overall design block diagram of the digital-analog signal hybrid transmission transceiver is shown in Figure 1-1.
 Figure 1-1 Overall design block diagram of the digital-analog signal hybrid transmission transceiver system
1.2 Scheme demonstration and selection
1.2.1 Digital-analog signal combining circuit
Scheme 1: Digital-analog signals are combined through an adder. After combining, the signals are directly added in the time domain. The digital and analog signals in the frequency domain may be aliased, which is not conducive to the modulation and demodulation of the subsequent circuit.
Scheme 2: Digital-analog signals are combined through the multiplier AD835. The subsequent modulation and demodulation are more convenient, and the peripheral circuit is relatively simple.
Considering the convenience and accuracy of modulation and demodulation, the second option is selected.
1.2.2 Demodulation mixer
Option 1: Passive mixer circuit based on diode. It is difficult for this mixer to generate stable and frequency-adjustable signals, and the circuit complexity is high.
Option 2: Use AD831 multiplier for mixing. This multiplier can work under DC conditions, and the maximum working frequency is 500MHz, which far exceeds the design requirements, and the peripheral circuit is simple.
In summary, considering the circuit complexity and system stability, the second option is selected.
1.2.3 Digital signal encoding and decoding
Option 1: Using the existing serial communication protocol, adjust the baud rate to achieve the frequency output of the data frame, the output data and decoding are convenient, and the transmission rate is fast. Option 2:
The IO port of the single-chip microcomputer outputs high and low level delays, and the input digital signal is sent through the Manchester encoding and decoding method, but the Manchester encoding and decoding process is relatively complicated, and it occupies more data bits, which is difficult to implement.
In summary, considering the signal transmission rate and decoding rate, the first option is selected.
2 Theoretical Analysis and Calculation
2.1 Theoretical Analysis and Calculation of Digital-Analog Signal Combination
In order to complete the mixed transmission of digital-analog signals within a narrow bandwidth, the 0 and 1 digital signals generated by the MCU are first converted into FSK signals, and then the multiplication combination of the analog signal and the FSK digital signal is realized using AD835. The specific formula is as follows:
When the sinusoidal signal expression and the FSK signal expression
 are , the signal expression after multiplication combination is
2.2 Theoretical Analysis and Calculation of Mixed Signal Modulation Transmission
    The current mainstream modulation methods are: FM, AM, DSB, SSB and other methods. FM modulation has strong anti-interference ability and is the current mainstream modulation transmission method, but the frequency deviation setting requires a larger channel bandwidth, which is difficult to meet the design requirements. AM and DSB modulation also have the problem of excessive channel bandwidth occupancy. Therefore, considering the overall power consumption, channel utilization and circuit complexity of the system, SSB modulation is used in this system design.
    The SSB signal can be generated by first generating a DSB signal, then passing it through a sideband filter, and filtering out the unwanted sideband to obtain a single-sideband SSB signal. Let the time domain expression of the DSB signal be:
Then the time domain expression corresponding to the lower sideband LSB signal is:
2.3 Theoretical analysis and calculation of channel bandwidth design
    In order to meet the requirements of the performance part, the analog signal frequency range is set to 50Hz-10kHz, and the occupied channel width can be approximately regarded as 10kHz. When the analog signal and the digital signal are combined and multiplied, the analog signal in the combined signal undergoes up-down conversion, and the occupied channel bandwidth increases to 20kHz. In order to meet the design requirement that the channel width does not exceed 25kHz, the frequency hopping frequency of the FSK digital signal is set to 5kHz. In addition, when the combined signal and the carrier are modulated, up-down conversion occurs, causing the channel occupied width to rise to 50kHz. Therefore, the USB lower sideband modulation method is finally adopted, and only a channel width of 25kHz is required to realize the combined modulation and demodulation of the digital-analog signal.
3 Circuit and program design
3.1 Design of hardware circuit of wireless transceiver system for mixed transmission of digital-analog signals
3.1.1 Design of digital-analog signal combining circuit
    The combination of digital-analog signals is completed by AD835. AD835 is a four-quadrant voltage output analog multiplier that can realize linear multiplication of input signals X and Y. The peripheral circuit is simple. The schematic diagram of the digital-analog signal combining circuit based on AD835 is shown in Figure 3-1 below.
3.1.2 Modulation and transmission circuit design
    The modulation of the combined signal is completed by AD831. After calculation, the value of the external capacitors C124 and C125 of AD831 is changed to 560p, which is more in line with the circuit design requirements. The schematic diagram of the modulation circuit based on AD831 is shown in Figure 3-2 below.
Figure 3-1 Schematic diagram of the combining circuit based on AD835 Figure 3-2 Schematic diagram of the modulation module circuit based on AD831
3.1.3 Receiving and demodulating circuit design
    The demodulated signal first enters the demodulation mixing module through a small signal amplifier ERA-8SM+. The demodulation mixing module is completed by a multiplier based on AD831. It can demodulate the modulated signal to a mixed baseband signal, which is convenient for the subsequent separation of digital-analog signals. The schematic diagram of the receiving and demodulating circuit based on ERA-8SM+ and AD831 is shown in Figure 3-3.
Figure 3-3 Schematic diagram of the receiving and demodulating circuit based on ERA-8SM+ and AD831
3.1.4 Transceiver separation circuit design
    The combining circuit is divided into two paths through a single-ended to differential circuit, and then processed later. The single-ended to differential circuit is designed using AD8138, and the gain of AD8138 is set to 1 to avoid signal distortion due to excessive gain. The schematic diagram and PCB diagram of the single-ended to differential circuit based on AD8138 are shown in Figure 3-4.
Figure 3-4 Schematic diagram and PCB diagram of the single-ended to differential circuit based on AD8138
    The analog signal processing circuit uses Schottky diodes for envelope detection to obtain analog signals, and then uses a bandpass filter to filter out irrelevant clutter to obtain a stable analog signal. The schematic diagram of the bandpass filter circuit based on NE5532 is shown in Figure 3-5.
    The digital signal processing circuit uses the ADL5801 zero intermediate frequency mixer to convert the FSK signal into an ASK signal, and then performs subsequent operations such as envelope detection to demodulate the digital signal. The schematic diagram of the zero intermediate frequency mixer circuit based on ADL5801 is shown in Figure 3-6.
Figure 3-5 Schematic diagram of bandpass filtering based on NE5532 Figure 3-6 Schematic diagram of zero intermediate frequency mixer based on ADL5801
3.2 Program design of wireless transceiver system for mixed digital-analog signal transmission
    The program design part of the wireless transceiver system for mixed digital-analog signal transmission consists of two parts: the transmitter program and the receiver program.
3.2.1 Program design of transmitter for mixed digital-analog signal transmission
    The transmitter MCU program mainly consists of two modules: DDS carrier frequency control and digital signal transmission. After the MCU is turned on, it will control the DDS to output a fixed carrier frequency. The output carrier frequency can be changed by pressing different key values. The digital signal is input and processed through the serial port screen, and the processed digital signal can be sent by pressing the up key. The transmitter program flow chart is shown in Figure 3-7.
3.2.2 Program Design of Digital-Analog Signal Mixed Transmission Receiver
    The receiver MCU program mainly consists of two parts: DDS local oscillator frequency control and digital tube light-up control. After the MCU is turned on, it will control the DDS to output a fixed local oscillator frequency. The output local oscillator frequency can be changed by pressing different key values. The digital tube control circuit determines whether the digital tube is on or off by the input digital signal. The receiver program flow chart is shown in Figure 3-8.
Figure 3-7 Transmitter program flow chart Figure 3-8 Receiver program flow chart
4 Test plan and test results
4.1 Test conditions and test plan
    Three test ports are reserved at the output end of the transmitter's combiner circuit, the front end of the transmitting antenna, and the battery power supply output, which are marked as TP1, TP2, and TP3 respectively. A signal source is used to generate an arbitrary waveform of 50Hz-10KHz as the transmitter analog signal. By selecting whether to connect an analog signal or send a digital signal to modulate and demodulate a single signal or a combined signal, an oscilloscope, a spectrum analyzer, and a multimeter are used to observe TP1, TP2, and TP3 respectively, which are used to observe the mixed signal time domain waveform, the modulated signal bandwidth, and the transmitter power consumption. The measuring instruments are shown in Table 4-1.
Table 4-1 Test Instruments Equipment

Name


Brand


Model


Quantity


Arbitrary Waveform Generator


Puyuan


DG4202


1


Digital Oscilloscope


Puyuan


MSO5354


1


Digital Multimeter


Puyuan


DM3058


2


Spectrum Analyzer


Puyuan


DM3058


1

4.2 System Test Data
Modulation and demodulation of single-channel signal: select the single-channel receiving and sending test of analog signal or digital signal by whether the analog signal is connected or the digital signal sending key is pressed. The test found that when a sine wave with a frequency range of 50Hz-10KHz is input as an analog voice signal, the analog signal can be demodulated, the waveform is not distorted, and the digital tube at the receiving end is off; when the analog signal is not connected, by typing a group of 4 numbers from 0 to 9 and pressing the send key, the 4 digital tubes at the receiving end can be displayed correctly, and the response time is within 1s. When the stop key is pressed, the display of the transmitted numbers is cleared at the sending end, and the display at the receiving end is automatically turned off after a delay of 5s;
Modulation and demodulation of digital-analog mixed signals: when a sine wave with a frequency range of 50Hz-10KHz is input as an analog voice signal, a group of numbers are typed at the same time for mixed signal modulation and demodulation. After testing, the digital display is correct and the analog signal waveform has no obvious distortion.
The performance of digital-analog signal mixed transmission: measured with a spectrum tester, it is found that when the analog signal frequency range is 100Hz-5KHz, the channel bandwidth of the transceiver is 10.2KHz-20KHz, which is not greater than the design requirement of 25KHz, and the carrier frequency can be selected in multiple gears between 20-30MHz. The voltage and current of the power supply circuit at the transmitting end are measured with a digital multimeter, and it is found that the system power consumption is about 5.916W. The system measurement results are shown in Figures 4-1 to 4-3.
Figure 4-1 Analog signal demodulation waveform Figure 4-2 Digital signal digital tube display Figure
4-3 Channel bandwidth measurement Figure
4.3 Test result analysis
(1) The system can realize the modulation and demodulation of single-channel analog signals 50Hz-10KHz, single-channel digital signals and digital-analog mixed signals. The transmission distance is greater than 100cm. The analog signal demodulation has no obvious distortion; the digital signal is displayed correctly, the response time is less than 1s, and it automatically turns off after 5s delay after stopping transmission;
(2) The carrier frequency of the transceiver is adjustable, and the channel bandwidth is not greater than 20KHz, which meets the design requirements. The main reason is that the system adopts SSB modulation, which saves bandwidth;
(3) Use two multimeters to connect to the power supply circuit in series and parallel respectively to measure the input current and voltage of the entire system, and then calculate the power consumption of the transmission system. After testing, the system power consumption is 5.916W.
5 Summary
The biggest difficulty of this problem lies in the limitation of bandwidth. The baseband signal is required to be between 50hz and 10khz, while the total channel bandwidth is required to be within 25khz. We first tried to use ASK to convert the digital signal, add it directly to the analog signal and then send it through AM modulation. The receiver first demodulates the digital and analog mixed signal, and then designs a filter to separate them. Ideals are always beautiful, but reality is cruel. At that time, because the modules were ready, I built a set directly. ASK chose a 20khz carrier. In fact, if the 25khz channel bandwidth is to be met, the ASK carrier must be lower, and the AM must be filtered to become similar to SSB when transmitting, that is, one of the sidebands must be attenuated to below -40dB. However, even if I take 20Khz analog signal from ASK and take 1Khz, the Butterworth low-pass filter with a cutoff frequency of 1khz (which I have on hand) will not distort the eighth-order analog signal, but the waveform will shake when each code element comes. Later, I spent some time to write the FPGA program, that is, collect the signal for FFT and select the maximum main frequency component, calculate the frequency and output it through DA. The result is perfect, but there is a fatal flaw that it is limited by the sampling frequency. Its minimum frequency resolution is 50Hz, which means that it can only output signals with 5 times the frequency... I also thought that I would have to make a high-pass filter to filter out the digital signal and filter AM into SSB. When the analog signal is 50Hz, the interval between the upper and lower sidebands is 100Hz, and the frequency is 20-30Mhz. I can only say that I dare not think about it. So I gave up this plan on the second day of the competition. Later, I thought of using FSK to transmit digital signals and load the information of the analog signal into the amplitude of FSK. In this way, demodulation is very convenient and there is no need to worry about the filter all day. As long as the frequency hopping of FSK is set small enough, the channel bandwidth requirement can be met. By chance, I thought of a way to demodulate FSK. First, find a better mixer and mix the FSK frequency down to 0 to turn it into a signal similar to ASK. At this time, just pass an LC filter to filter out the upper sideband, harmonic components and noise, and then pass an envelope detector and a comparator to perfectly restore the original digital signal. The modules are all there, so I just adjusted them casually and didn’t expect it to work. . . As for digital encoding, I used serial communication without thinking, which is simple and reliable. Unfortunately, during debugging, the first-stage small signal amplifier input high-frequency head from the receiving end had a bad direct contact. . . . As a result, the received analog and digital signals were unstable, and this only worked when I placed the antenna high-frequency head and the broken high-frequency head at a perfect angle. I couldn't receive anything at first, and almost lost it. I thought I was well prepared for the competition, but unexpected problems still occurred. I can only say that getting the second place in the national exam was a blessing in disguise. Finally, the overall installation diagram is attached.