#FirstLichuangContest#Wireless digital intercom module

** U/V dual-band digital & analog intercom module**       1. ** Introduction to the work * **** 1. Function introduction:** ** * This low-power wireless intercom module can be integrated into various devices , or can be used independently. When talking in digital format, only modules with the same protocol can talk, one-to-one and one-to-many can be realized, and data or text messages can be sent and received. The analog format is compatible with traditional FM walkie-talkies. 2. Principle introduction: This module uses STM32F103R8T6 as the main control, communicates with SX1278 through SPI and I2C, communicates with RDA1846S through I2C, and communicates with WM8974 through SPI and I2C. SX1278 and RDA1846S share the transmitting and receiving circuit and are switched through PE4259. The transmission uses two stages of amplification and is expected to push the power output to 5W. The receiving front stage uses two independent low-noise tubes NE3509 (noise coefficient 0.5), each responsible for one frequency band, and the rear stage uses the MGA-685T6 integrated wideband low-noise amplifier chip (noise coefficient 0.93). The total receiving gain is 40dB, but considering the impact of noise, the signal gain is estimated to be about 30dB. ****        The power supply part uses an automatic step-up and step-down circuit based on XL6007, so that the power amplifier tube can have a stable drain bias voltage of 7.5V, so that the output is relatively stable, and the input voltage of 5-10V can work normally. ** **** STM32 mainly runs the Speex voice coding and decoding algorithm, and passes the compressed voice data to SX1278 for packaging and transmission through the SPI interface. After the peer receives the packet data, it can be decompressed and restored to voice. When only simulating intercom is required, SX1278 should be turned off and the RDA1846 should be controlled separately. Both cannot be used for communication at the same time. STM32 communicates with WM8974 duplex through SPI and additional clock signals. During digital intercom, you can talk and listen at the same time like talking on a mobile phone. **** **** Pin diodes are used for line switching at the antenna end, because high-power multi-channel RF switches are expensive. Both the input line and the output line are equipped with low-pass filter networks to suppress harmonics. **** ****** 3. PCB introduction: ** The overall design adopts a 4-layer board with a board thickness of 1.6mm, a copper thickness of 35um, and three FR-4 compartments with thicknesses of 0.2mm, 1.1mm, and 0.2 respectively. mm. The stacking sequence is signal-ground-power-signal. When the dielectric constant is 4.3, the width of the top 50Ω microstrip line should be 0.3mm, but the power trace can be designed wider, I set it to 0.35mm. ** ** There are a large number of through holes under the power chip, which can not only speed up heat dissipation but also reduce grounding impedance. Place the wire-wound inductors as far away as possible or vertically to reduce unwanted coupling. The matching of the power tube should be debugged on-site with a vector network analyzer and spectrum analyzer. The matching of the low-noise amplifier should be completed by ADS software simulation design. The center frequencies are 144MHz and 430MHz respectively. Pin diodes are used for line switching at the antenna end, because high-power multi-channel RF switches are expensive. The receiving line should be kept as far away from the power amplifier as possible to reduce thermal noise. There is a tin-plated heat dissipation window on the bottom layer of the power amplifier, and a heat sink should be attached. ** 2. ** Highlights of the work 1. Adopt spread spectrum transmission and design PA and LNA circuits, making the communication distance longer; 2. Dual-band circuit design makes the frequency selection range wide, and there is no need to worry about the channel being occupied. Low-frequency antennas with long antennas have better signal penetration and diffraction; 3. There are USB interfaces and emulator interfaces, serial ports, I2C, and SPI interfaces. The whole can be used as a development board, and you can upgrade the firmware or program development by yourself; 4 , Onboard automatic step-up and step-down SEPIC regulated power supply, with a wide range of input voltage selection; 5. External expansion I2C interface equipment, keyboard, display screen, Bluetooth module. 3. ** System Architecture Diagram ** ****                **** 4. Schematic Diagram The values ​​of the radio frequency components in the schematic diagram are for reference only. 5. ** Material list ( BOM list **) 6. ** PCB **Physical picture 7. Description of the software part      The Speex codec is an open-source, patent- and royalty-free software dedicated to speech compression and decompression. Speex is based on CELP (code-excited linear prediction) and designed to compress voice at bitrates ranging from 2 to 44 kbps. The features of Speex include: – Narrowband (8 kHz), wideband (16 kHz), and ultrawideband (32 kHz ) compression in the same bitstream – Intensity stereo encoding – Packet loss concealment – ​​Variable bitrate operation (VBR) – Voice activity detection (VAD) – Discontinuous transmission (DTX) – Fixed-point port – Acoustic echo canceller – Noise suppression Still go here Take a look at http://www.stmcu.org/document/detail/index/id-200260 or http://www.docin.com/p-702959468.html     and http://bbs.eeworld.com. cn/thread-341817-1-1.html 8. Debugging process               After soldering the main components, first test the voltage. After reflow soldering, QFN packaged components are prone to short circuits and weak soldering. The solution is to use a hot air gun to 350 degrees to repair the soldering, lift it up and press it tightly. If there is solder sticking to dense pins, use a soldering iron tip to boil rosin or solder paste and then brush it a few times. The excess solder will be sucked away by the soldering iron tip. Try to avoid hitting the soldering iron as it may damage the heating core. Use high-temperature cotton to remove the solder. If it is oxidized, you can use resurrection cream to repair it for a short period of time. Now the voltages of 3.3V, 5V and 7.5V are all OK, and the quiescent current is 40mA. However, after touching the bias coil with a finger, self-oscillation immediately occurs between the front-stage PA and the rear-stage PA, causing the 7.5V current to reach 1.6A, which is very dangerous. The solution is to re-adjust the matching network between RQA004 and RQA0011 so that the output impedance of RQA0004 and the input impedance of RQA0011 achieve conjugate matching to eliminate signal reflection. Use the network splitter and extension cable, move C25 away, and measure the S parameters of the pre-measurement reflection and the S-parameters of the post-measurement reflection respectively. As shown in the figure, the mismatch is serious. Go to the ADS software and use the Smith Chart tool to complete the front and rear stages. For matching, it should be noted that because the network branch output power is only 10dBm, only small signal impedance can be tested, so the results can only be used as a reference. The bias resistor was resized to ensure that the bias current of the power tube was within the allowable range. The inter-stage matching was re-adjusted based on the test results of the datasheet and network substation. It no longer oscillated. RF power amplifier matching and debugging process: 1) Warm up the machine for half an hour, then perform dual-port calibration on the network points and save. Then connect the open-circuit extension line to port 1, find the extension line compensation option in the network branch, and adjust the compensation parameters so that the impedance values ​​of each frequency point of S11 are concentrated at the right top of the Smith chart, and save; 2) Confirm that the bias voltage of the PA is correct. After reaching the specified value, start adjusting from the front stage. First, ensure that the input reflection of RQA0004 is less than -10dB. Open ADS and import the S2P model of RQA0004 for simulation design, and then apply the results to the board. If there is still oscillation, you can connect a 68-ohm resistor string and a 220pF capacitor in parallel to the input end of the PA, which can eat up most of the low-frequency signals. The source of self-oscillation of low-frequency signals. After slightly adjusting the inductor and capacitor values ​​based on simulation, test 430MHz S11

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