“Design and manufacture a wireless transceiver that performs mixed digital-analog signal transmission on the same channel. Among them, the digital signal is composed of a group of four numbers from 0 to 9; the analog signal is a voice signal, and the frequency range is 100Hz~5kHz. Using wireless transmission, the carrier frequency range is 20~30MHz, the channel bandwidth is not more than 25kHz, and the shortest transmission distance between transceivers is not less than 100cm.
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This year’s Electronic competition for college students has ended. The following is an analysis of the E question.
1. Task
Design and manufacture a wireless transceiver that performs mixed digital-analog signal transmission on the same channel. Among them, the digital signal is composed of a group of four numbers from 0 to 9; the analog signal is a voice signal, and the frequency range is 100Hz~5kHz. Using wireless transmission, the carrier frequency range is 20~30MHz, the channel bandwidth is not more than 25kHz, and the shortest transmission distance between transceivers is not less than 100cm.
The sending end of the transceiver completes the digital signal and the analog signal combined processing, and modulates and transmits on the same channel.
The receiving end of the transceiver completes the receiving and demodulation, and separates the digital signal and the analog signal. The digital signal is displayed by a digital tube, and the analog signal is observed by an oscilloscope.
2. Requirements
1. Basic requirements
(1) Realize analog signal transmission. The analog signal is a voice signal of 100Hz~5kHz, and it is required that the waveform of the analog signal demodulated by the receiving end has no obvious distortion. When only analog signals are transmitted, the digital Display on the receiving end is off.
(2) Realize digital signal transmission. First, enter a group of 4 numbers from 0 to 9, store and Display it at the sending end, and then press the send key to transmit the digital signal continuously and cyclically. The digital signal is demodulated at the receiving end and displayed through 4 digital tubes. The response time required to start sending to the nixie tube display is no more than 2 seconds. When the sender presses the stop key, the digital signal transmission is ended, and at the same time, the display of the transmitted numbers is cleared at the sender, waiting for a new number to be entered.
(3) Realize the mixed transmission of digital-analog signal. Arbitrarily enter a group of numbers, mix and modulate with the analog signal for transmission. It is required that the receiving end can correctly demodulate the digital signal and the analog signal, the digital display is correct, and the analog signal waveform has no obvious distortion.
(4) The channel bandwidth of the transceiver is not more than 25kHz, and the carrier frequency range is 20~30MHz. It is required that the transceiver can be selected from no less than 3 carrier frequencies, and the specific carrier frequency can be determined by itself.
2. Play the part
(1) After the sending end stops digital signal transmission, the digital display at the receiving end will automatically turn off after a delay of 5 seconds.
(2) On the premise of meeting the basic requirements, the lower the power consumption of the transmitter of the transceiver, the better.
(3) Under the premise of meeting the basic requirements, the frequency range of the analog signal transmitted by the transceiver is extended to 50Hz~10kHz.
(4) Others.
3. Description
(1) The digital and analog signals must be processed by the combined circuit first, and then modulated and transmitted on the same channel. The modulation mode and modulation degree are determined by themselves. An observation port should be reserved at the output end of the combined circuit for the oscilloscope to observe the waveform change of the combined signal.
(2) There shall be no connection between the transmitter and receiver of the transceiver.
(3) The connection between the transmitting end of the transceiver and the antenna adopts an SMA connector, the transmitting end is an F (female) head, and the antenna end is an M (male) head. The length of the antenna should not exceed 1 meter.
(4) The transmitter and receiver of the transceiver are powered by a single battery, and the power supply circuit of the transmitter should have a test port for power supply Voltage and current.
(5) The selection of the carrier frequency of the transceiver should try to avoid the interference of environmental radio waves.
(6) In this question, the channel bandwidth is agreed to be the -40dB bandwidth of the modulated signal, which is measured by a spectrum analyzer. The details are shown in the figure below.
Topic analysis and program design
This topic is a relatively complete wireless transceiver system, which requires simultaneous interpretation of analog signals and digital signals.
According to the description of the requirements for analog signal transmission in the title, the analog signal adopts the analog transmission method. The frequency range of the analog signal to be transmitted is 50Hz~10kHz. If AM modulation is used, the required channel bandwidth is 20kHz.
The digital signal to be transmitted is a 4-digit decimal number, plus redundant information such as the synchronization header and check code of the data frame, the number of bits to be transmitted should not exceed 40. The title also stipulates that the time from the start of transmission to the terminal display does not exceed 2 seconds, so the actual transmission bit rate can be as low as 20bps. According to Shannon’s theorem, as long as the signal-to-noise ratio of the channel is not extremely poor, the channel bandwidth required for data transmission is extremely narrow, which is almost negligible compared with analog signals.
The channel bandwidth specified by the title is 25kHz, the analog signal occupies the middle 20kHz, and the bandwidth of 2.5kHz is left on each side. It is more than enough to place the above digital channels. Therefore, a subcarrier frequency can be selected between 10kHz and 12.5kHz (for example, 11.2kHz, which is the geometric mean of 10kHz and 12.5kHz), and the subcarrier can be modulated with a digital signal to obtain a digital modulated signal (the simplest modulation method is OOK modulation).
Finally, the digital modulated signal and the analog signal are superimposed, and the superimposed mixed signal modulates the carrier wave by AM modulation, and then is amplified and sent to the antenna for transmission. The overall spectral structure is as follows:
The circuit structure using this scheme is shown in the figure below.
In this scheme, the filter that distinguishes the two signals is the key.
The requirements of the low-pass filter of the analog signal are: there is a large enough attenuation near the sub-carrier frequency of the digital signal, and the attenuation at 12.5kHz should be greater than 40dB. According to these two requirements, it is a better choice to use a high-order elliptic filter and design the first zero frequency of the filter near the sub-carrier frequency of the digital signal. For example, using a 7th-order elliptical low-pass filter with an in-band fluctuation of 0.5dB can achieve attenuation greater than 45dB at 11.2kHz (digital subcarrier frequency) and after greater than 12.5kHz.
The requirement of the band-pass filter of digital signal is: There should be enough attenuation at 10kHz and 12.5kHz. Due to the extremely low bandwidth of digital signals, it is easiest to cascade multiple point-pass filters. For example, if three point-pass filters with a center frequency of 11.2kHz and a Q value of 25 are cascaded, their total bandwidth is about 20Hz, and the attenuation at 10kHz and 12.5kHz is about 45dB.
The receiving part is a typical superheterodyne receiver structure, in which the analog signal reception is basically the same as that of the ordinary AM receiver.
The intermediate amplifier circuit should have the AGC function to ensure that the amplitude of the output signal does not change much when the receiving conditions change, so as to facilitate the subsequent digital signal demodulation.
The decoder structure of the digital receiving part varies with the modulation method. If it is OOK modulation, the amplitude of the signal after passing through the band-pass filter is 1 and 0 of the baseband signal, which can be sent to the microprocessor for decoding as long as it is shaped by the comparator. If it is other modulation methods, it may be necessary to restore the subcarrier as a synchronization signal and then perform synchronization decoding.
Obviously, for the case where the bandwidths of the two signals in this topic are one wide and the other narrow, as long as the spectrum is properly arranged, the above hybrid transmission scheme is effective and the circuit is relatively simple. From the parameters given by the question and the basic requirements 1, description 1 and other conditions, it can be guessed that the proposer also seems to have asked the question according to this mixed transmission scheme.
The Links: SP14N02L6ALCZ PS11034-Y1
