[F-Wireless Microphone Amplification System] Hubei Province_First Prize

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[F-Wireless Microphone Amplification System] Hubei Province_First Prize [Copy link]

The transmitter part of this system uses the integrated analog FM chip Si4713 to make the transmitter module, and is controlled by the Arduino single-chip microcomputer to synthesize the FM signal and transmit it to the antenna to achieve the purpose of analog FM. The receiver part uses the integrated tuning chip Si4703 to make the receiver, which is controlled by the STM32 single-chip microcomputer, and the carrier frequency is set manually to demodulate the audio signal.

In this competition, the three of us cooperated with each other and made full use of the four days and three nights of the competition. We applied the knowledge and experience accumulated in the center to practice, and our abilities were fully exercised during the competition. I would like to thank TI for its strong support and the school teachers for their guidance.

Work Report

Abstract: The wireless microphone sound amplification system implemented in this design can realize the function of voice transmission and sound amplification within a certain distance. The system consists of four parts: signal acquisition circuit, transmitter, receiver and sound amplification system. After the audio signal is processed by signal acquisition, the transmitter receives and generates the transmission signal in analog FM mode. The baseband audio signal can be obtained through demodulation of an ordinary radio or a homemade receiver. The sound amplification system can manually select to amplify or mix the two microphones separately, and the output is output by the speaker after amplification by the power amplifier. When the wireless microphone is turned on, it will automatically detect the current channel occupancy. If the transmitter is interfered by other signals, the wireless microphone can automatically switch to other carrier frequencies to avoid interference signals, and it has an LED prompt.

Keywords: analog FM; wireless transmission; interference avoidance

I. System Solution

1. Comparison and Selection

1.1 Analog FM Design Solution

Solution 1: Use a phase-locked loop modulator to generate analog modulation signals, use analog composite audio signals as the control signal of the voltage-controlled oscillator VCO, and use the phase-locked loop to stabilize the frequency, which can generate frequency modulation signals within the range required by the question. Solution 2: Use the integrated analog FM chip Si4713 that can meet the requirements of the question to make a transmitter module, and control it through the Arduino microcontroller, synthesize the FM signal and transmit it to the antenna to achieve the purpose of analog FM.

Solution selection: Solution 1, the carrier frequency stability is very high, but the low-frequency modulation characteristics are poor; Solution 2, Si4713 is a frequency synthesizer with integrated VCO, the circuit is simple, and the carrier frequency can be easily adjusted using a single-chip microcomputer. After comprehensive consideration, use Solution 2.

1.2 Receiver Design Solution

Solution 1: Use a frequency detector to transform the constant amplitude FM wave into an AM FM wave whose amplitude is proportional to the instantaneous frequency of the FM wave, and then use an amplitude detector to perform amplitude detection to extract the audio signal. Solution 2: Use the integrated tuning chip Si4703 to make a receiver, control it through the STM32 microcontroller, manually set the carrier frequency, and demodulate to obtain the audio signal.

Solution selection: Solution 1, the parameters are difficult to control, and the anti-interference ability is poor; Solution 2, the circuit is relatively simple, the anti-interference ability is strong, and the carrier frequency can be easily adjusted. After comprehensive consideration, use Solution 2.

2. Solution description

The system block diagram is shown in Figure 1. The electret microphone converts the vibration of the sound wave into the corresponding audio voltage signal, which is amplified by OPA365 and sent to the integrated FM transmitter based on Si4713. The carrier parameters are controlled by Arduino. When the power is turned on, if there is an interference signal in the current frequency band, a new carrier frequency is automatically generated randomly and the LED is used to prompt. The modulated signal is transmitted through the antenna. The receiver circuit built based on STM32 and the integrated tuning chip Si4703 can recover the audio signal from the modulated signal, amplify it and input it into the power amplifier circuit, and finally play it through the speaker.

Figure 1 System Block Diagram

II. Theoretical Analysis and Calculation

1. Communication Distance Analysis

Electromagnetic waves have power loss when propagating in the air. Therefore, to obtain a sufficiently large communication distance, the output power of the transmitter should be set to a sufficiently large value. According to the WIM transmission model, it can be approximately obtained:

[font=微软雅黑, Wherein Lp is the transmission loss, which is defined as the logarithm of the ratio of transmitter power to receiver power, f is the electromagnetic wave frequency, and d is the transmission distance, with units of dB, MHz, and km respectively. From the law of conservation of energy, we know that:

Where Prec, Pout, and Pl are the receiver power, transmitter power, and transmission loss power, respectively. If the communication distance is to be more than 10 meters, the transmitter power should be set to more than 3dBm by substituting it into the formula. 2. Load power analysis Under 8 load, the maximum audio output power is required to be 0.5W. From P=U^2/R, we can know that when the maximum audio input is at the maximum, the effective value of the voltage across the load is 2V, and then the peak-to-peak value of the output voltage of the power amplifier is 2.83V. 3. Automatic interference avoidance analysis "]When the transmitter is turned on, the frequency value is preset to obtain the channel occupancy and signal interference in the current environment. If the interference is serious, a pseudo-random number algorithm is used to make the transmitter carrier frequency jump to another value, and this is repeated until the interference of the carrier at the current frequency in the current environment reaches the allowable value. The system reflects the avoidance situation through the LED. If the LED is on, the system is still looking for a carrier frequency with less interference. If the LED is off, it means that the interference of the carrier of the system transmitter has reached the allowable range, that is, automatic avoidance is completed.

III. Circuit and Program Design

1. Signal Acquisition Circuit Design

The signal acquisition circuit is shown in Figure 2. The minimum supply voltage of OPA365 is 2.2V, which meets the power supply requirements of two dry batteries in the system. The unit gain bandwidth is 50MHz, which can achieve distortion-free amplification of audio signals. The audio signal bandwidth is 40Hz~15KHz, and the cutoff frequency of the high-pass filter is

So it is advisable

Figure 2 Signal acquisition circuit

2. Transmitter circuit design

The transmitter circuit is shown in Figure 3. Si4713 integrates a VCO frequency synthesizer and a built-in AFC automatic frequency control module, which can generate a carrier with a bandwidth of 76~108MHz. The input audio signal and the carrier generated by the chip are processed by the internal multiplier to achieve FM frequency modulation, generate a modulated signal, and output it through the TXO pin antenna.

Figure 3 Transmitter circuit

3. Receiver circuit design

The receiver circuit is shown in Figure 4. Si4703 can demodulate the frequency modulated signal to generate an audio signal. The weak signal entering the antenna and the tuning signal are processed by the multiplier inside the chip and then passed through the programmable gain amplifier. After that, they are demodulated through mathematical operations to extract the audio signal and finally achieve dual-channel output.

Figure 4 Receiver circuit

4. Software programming

The flowcharts of transmitter and receiver programming are shown in Figures 5 and 6 respectively. After the wireless microphone is turned on, the channel occupancy is detected first. After ensuring that the interference is small, the user's required frequency is set through the human-machine interface, and the real-time parameters are displayed. After starting the device, the receiver detects the user's signal reception method, that is, manual setting or automatic reception, displays the reception status through the human-machine interface, and selects the amplification method through the human-machine interface for corresponding processing.

Figure 5 Transmitter program design flow chart

Figure 6 Receiver program design flow chart

IV. Test plan and test results

1. Test environment

Oscilloscope: Tektronix MDO2002B digital oscilloscope;

Signal generator: RIGOL DG4162 160M arbitrary waveform generator;

Power supply: ZhongCe DF1743003C voltage regulator.

2. Test plan

2.1 Signal transmission test plan

Connect the audio signal source, set ten frequency points within the required range, listen to the audio signal through the homemade receiver, change the distance between the transmitter and the receiver until the sound is obviously distorted, and record the distance between the two at this time.

2.2 Output power test plan

Connect the audio signal source, change the amplitude of the input signal, connect the load resistor to the output of the receiver, use the two channels of the oscilloscope to measure the signals at both ends of the load respectively, and display them on the image. There is a phase difference between the two. The difference between the peak and the trough of the two signals is the voltage at both ends of the load. Calculate the output power and record it.

2.3 Automatic avoidance function test plan

Set the carrier frequency of the two transmitters to be the same, take five frequency points within the required range, turn on one transmitter first, wait for it to stabilize, then turn on the second transmitter, observe whether it can automatically switch to other frequencies, record the actual frequency of the second transmitter, and determine whether the switching time is less than 1s.

2.4 Bandwidth Test Scheme

Use a signal generator to generate a sinusoidal signal with a frequency of 40Hz to 15kHz and connect it to the reserved interface, then output it to the test interface of the transmitter. Use an oscilloscope to observe and record the signal at the test interface of the receiver, and compare the two signals to determine the distortion.

3. Test Results and Data

3.1 Signal Transmission Test

Table 1 Signal transmission test table

3.2 Output power test Table 2 Output power test table"][table=98%]

Frequency/MHz	88.0	90.0	92.0	94.0	96.0	[size= 3]98.0	100.0	102.0	[align=left ] 104.0	106.0	108.0 [/t d][/tr]
distance/m	15	14	15	12	11	12	14	15	[align=right ] 14	13	11
Peak-to-peak value of 8 ohm load/V	Output power/W
5.96	0.56

3.3 Automatic avoidance function test

Table 3 Automatic avoidance function test table

Transmitter 1	Set carrier frequency / MHz	88.2	92.2	96.2	100.2	104.2
Transmitter 2	Set carrier frequency/MHz	88.2	[align=center ]92.2	96.2	100.2	[align =right] 104.2
	Actual carrier frequency/MHz	88.6	92.4	96.4	100.4	104.4

4. Test result analysis

4.1 Signal transmission test analysis: From the data results, we know that within the required frequency range, the communication distance is greater than 10 meters, which meets the requirements of the topic. The factors that limit the communication distance mainly include limited transmission power, channel interference, and antenna gain.

4.2 Output power test analysis: From the data results, we know that the maximum audio output power is greater than 0.5W, which meets the requirements of the topic.

4.3 Automatic avoidance function test analysis: From the data results, we know that when the transmitter detects signal interference, it can automatically avoid interference within 1s and switch to other carrier frequencies to work, which meets the requirements of the topic. 4.4 Bandwidth test analysis: It is observed that for audio signal input from 40Hz to 15kHz, there is no obvious distortion in the output, that is, the audio signal bandwidth of the system meets the requirements of the topic. V. References [1]. Luo Jie, Xie Zimei. Electronic Circuit-Design·Experiment·Test (Fifth Edition), 2015, Electronic Industry Press."] [2]. Kang Huaguang. Fundamentals of Electronic Technology (Analog Part) (Sixth Edition). 2013, Higher Education Press.

[3]. [US] Bruce Carter. The Definitive Guide to Operational Amplifiers (Fourth Edition) 2014, Posts and Telecommunications Press.

[4]. Fan Changxin. Principles of Communications (Sixth Edition) 2001, National Defense Industry Press.

Final work picture:

btty038

It ruined my first prize