Preface

High-frequency questions in the 2019 e-sports competition 双路同传的语音收发系统can be solved with this board.But the test paper questions I did at that time were so simple. Don’t you feel comfortable taking the test?

This thing is actually based on the modified chip I made before qn8027和qn8035的DSP信号收发板. The difference is that the main control is changed from stc to art_pi, and the FM transceiver is changed to RDA5820.

Overview

Use RDA5820 FM radio transceiver chip to realize the reception and transmission of wireless signals. The frequency band for reception and transmission can be broadened by mixing with other local oscillators.

use

• Listen to radio programs as an FM wideband radio
• As a FM wide-band radio station , transmitting FM signals
• As a radio recorder , the program controls the program you want to record. The I2S interface can transmit the received radio signal, decode it through ARTPI and save it to the SD card. You no longer have to worry about missing the program you want to listen to.
• As a walkie-talkie , communicate with others on the FM band
• Participate in e-sports

Signal flow:

• Reception: The signal is amplified by the antenna and mixed with the active local oscillator in ADE-1, then sent to RDA5820 for processing, and the audio signal is sent to the headset or transmitted to ARTPI through I2S;
• Transmission: The microphone signal from the headphone jack is amplified/LR input to the RDA5820, and the resulting FM signal is mixed with the active local oscillator. After LC frequency selection, it is amplified again by the HMC580 and transmitted to the antenna;

The role of the mixer: it can expand the receiving and transmitting frequency bands, not limited to the 87-108MHz specified by FM

Control interface:

• Control signal input: I2C (PH12 SCL/PH11 SDA)
• Display output: OLED (shared I2C bus)
• Audio signal input: 3.5mm audio interface or analog signal (L/R)
• Audio signal output: 3.5mm audio interface/I2S (PA6 SDI/PG11 CK/PA15 WS) (port multiplexing)

Updated on 2021-3-27:

The basic part has been completed. It can communicate with artpi through i2c and realize the transmitting and receiving functions of FM band normally. The following is the test environment: the spectrum analyzer is connected to the transmit output to test the transmit signal, channel one (yellow) of the oscilloscope is connected to the transmit output, channel two (blue) is connected to the modulation signal input, and the signal generator is connected to the modulation signal input. Set the transmission frequency to 89.8MHz, and the peak signal output from the chip is 4.3dBm: Increase the output signal frequency to 100MHz, input 15KHz sine audio signal, and transmit normally:

The reception was not tested because there is no signal source that can modulate the FM band, but the station can be found using the automatic station search function, which is feasible.

### The spread spectrum part is not completed!

Preparing for the postgraduate entrance examination (Fishing for too long) No time for debugging, but it is available based on previous experience with another board. Note that the inductors L13, L14, and L15 are the inductors of the frequency selection circuit. When using chip inductors, the Q value may be too low and the wave cannot be generated, so they need to be replaced. Wirewound inductors with higher Q values.

The register description for controlling rda5820 is placed below, you can take it by yourself if needed: