(Question D) Wireless microphone amplification system (undergraduate)

1. The task is to design and produce a short-range wireless microphone amplification system for venue amplification.
2. Require

(1) The wireless microphone adopts analog FM mode, the carrier frequency range is 88MHz~108MHz, the maximum frequency deviation is 75kHz, the audio signal bandwidth is 50 Hz~15 kHz, and the antenna length is less than 0.5 meters. You can use an ordinary FM radio to listen to the microphone signal, and the audio signal should have no obvious distortion. The wireless microphone is independently powered by 2 1.5V batteries. (15 points)
(2) The wireless microphone carrier frequency can be set arbitrarily between 88MHz and 108MHz, and the channel frequency interval is 200kHz. (15 points)
(3) Make a receiver corresponding to the wireless microphone, with a communication distance greater than 10m. Under 8Ω load, the maximum audio output power is 0.5W. The receiver can be modified from an off-the-shelf radio. (15 points)
(4) Make another wireless microphone that meets the above requirements. By manually setting the carrier frequencies of the two microphones respectively, the two microphones can be used at the same time, and the receiver can be improved to achieve manual amplification of the two microphones separately or mixed sound amplification. (25 points)

(5) The two wireless microphones can automatically detect channel occupancy when turned on. If interference is found with each other or other radio stations, the interference signal can be avoided by automatically selecting the carrier frequency. Response time is less than 1 second. (30 points)
(6) Design report: (20 points)

Question analysis

        The system consists of a sending device and a receiving device. The transmitting end can realize the input of voice signals and the analog modulation and transmission of controllable carrier frequency and can automatically avoid channel interference. The receiving end can receive two different voice signals and achieve separate amplification or mixing. Both the sending device and the receiving device are powered by a single battery power supply, and meet the power supply requirements of each circuit module after passing through the power conversion module.

       The sending device mainly includes a microphone amplifier module, FM sending module and power conversion module. The receiving device mainly includes FM receiving module, adder module, audio reverberator module, power conversion module and audio filter amplifier module. The receiving device receives the FM signal, and the MCU demodulates the signal by controlling the FM receiving module, then mixes the two audio signals through the adder, and finally realizes amplification through the audio power amplifier module, and the power conversion module implements each circuit The power supply requirements of the module. The overall hardware flow chart is shown in Figure 1. Test ports are reserved at the output end of the combining circuit at the transmitting end, the front end of the transmitting antenna, and the output point of the single battery power supply, which are used to observe the combined signal waveform, observe the bandwidth of the modulated signal, and measure the transmitting end. power consumption.

Figure 1 Hardware flow chart

Schematic design instructions

MAX9814

Voice amplification module

Option 1: Use a universal operational amplifier to amplify

A power supply is used to provide bias to the microphone, and then a non-inverting amplifier circuit is used to amplify the microphone signal. The operational amplifier is easy to use, has stable gain, and has good performance.

Option 2: Use MAX9814 to amplify

The MAX9814 is a low-cost, high-quality microphone amplifier with built-in automatic gain control ( AGC )

and low-noise microphone bias. The device integrates a low-noise preamplifier, variable gain amplifier ( VGA ), output amplifier, microphone bias generator and AGC control circuit.

Combining the above two options, option two can directly provide microphone paranoia with extremely low noise and excellent performance, so option two is chosen.

TPS5430

RDA5820

Transmitter main chip selection

Solution 1: Use integrated chip QN8027

The sound-level frequency response range of QN8027 is 50Hz to 18kHz , the transmitting frequency range is 76.0MHz to 108.0MHz , the transmitting power is 100mW , the operating voltage is 3.0V ~ 5.0V , and the measured coverage radius is 100 meters in an open area.

Solution 2: Use integrated chip RDA5820NS

RDA5820NS is an integrated FM transceiver launched by RDA Semiconductor . It contains a fully integrated FM transceiver system and can be directly powered by two dry batteries. The frequency covers 60MHz ~ 108MHz . Only its internal registers need to be configured to complete the configuration of the transmission parameters. Through configuration, RDA5820NS can also switch between transceivers, which is very suitable for completing the power-on channel detection function.

Based on the above two options ,  considering the design requirements that the transmitter needs to automatically avoid channel interference, option two is selected.

OPA1177

Adder module

Option 1: Use OP07 to make an adder

A general-purpose operational amplifier OP07 is used to make an adder. OP07 gain bandwidth product typical value is 0.6MHz,

It has the characteristics of low noise and low offset, but the OP07 conversion rate is low ( 0.3V/uS ) and the large signal bandwidth is small.

Option 2: Use OPA1177 to make an adder

The adder is made using TI 's zero-drift amplifier OPA1177 . OPA1177 offset drift ( typ ) (uv/℃): 0.100 , slew rate 0.3 V/uS .

Based on the above two options, option two has better performance, so option two is selected.

CD2399 mix

Option 1: Use a microcontroller for mixing

The received signal can directly obtain the digital audio signal through the IIS bus. The MCU is used to delay, attenuate, and add the collected audio signal to obtain the digital mixed signal, and then directly output the mixed signal through the DAC .

Option 2: Use integrated chips to achieve mixing

Based on the above two options, option one is more flexible, but the implementation is more complicated and difficult to complete during the competition. Solution two is simple to implement, so option two is chosen.

QN8025

FM receiving module

Solution 1: Use integrated chip QN8025

 The QN8025 is a high-performance, low-power microcontroller stereo FM receiver designed for mobile phones, MP3 players and portable radios. The circuit integration is high and the debugging difficulty is low.

Solution 2: Use integrated chip RDA5820

Using the integrated transceiver function of RDA5820 , it is the same as the transmitter.

Considering the difficulty and cost of circuit debugging, option one is chosen.

AD603

PCB design instructions

A. Create a network table

1. Make sure that the package of the device cannot be selected incorrectly.

2. Pay attention to correctly selecting the position of the origin of the coordinates of the veneer. The setting principles of the origin are: A. The intersection of the extension lines on the left and bottom sides of the veneer. B. The first pad in the lower left corner of the board. The edges of the board and frame are rounded with a radius of 5 mm . For special circumstances, please refer to the structural design requirements.

3. Set the prohibited wiring area and prohibited layout area of ​​the printed board according to the structural diagram and the clamping edges required during production and processing. According to the special requirements of certain components, a prohibited wiring area is set.

4. Basic principles of layout operation A. Follow the layout principle of " large first, then small, first difficult, then easy " , that is, important unit circuits and core components should be laid out first. B. The schematic block diagram should be referred to in the layout, and the main components should be arranged according to the main signal flow pattern of the single board. C. The layout should try to meet the following requirements : the total wiring should be as short as possible, and the key signal lines should be the shortest ; high voltage and high current signals should be completely separated from small current and low voltage weak signals ; analog signals and digital signals should be separated; high frequency signals Separate from low-frequency signals; high-frequency components should be adequately spaced. D. For circuit parts with the same structure, adopt " symmetrical " standard layout as much as possible; E. Optimize the layout according to the standards of uniform distribution, balanced center of gravity, and beautiful layout;

Software description

Sender:

Physical display instructions

Receiving end

The transmitting end