The special nature of the automotive radio application environment has higher requirements, while the design of the RF circuit is the key to achieving high performance. This paper introduces the design method of the TDA7513 radio frequency circuit. It is proposed to improve the design method and measures to improve the EMC characteristics and noise characteristics of RF circuits according to the actual design experience, and point out the attention points of the RF circuit performance test. RF circuits are the key and difficulties of radio circuit design. If the RF circuit is not designed, the noise limit sensitivity and signal-to-noise ratio of the radio and other technical indicators will be greatly reduced, and even a few broadcast stations can be received. Search for radio function invalid. From the broadcast signal field of the radio antenna, the dynamic range of the signal is very large, especially the environment changes in the car radio.
Radio RFCs are typically difficult to integrate into ICs, typically consisting of a front low noise amplifier (LNA) and resonance band pass filter by the separation element. The role of the car radio rass circuit is to enlarge the weak broadcast signal from the time domain. The automatic gain control circuit (AGC) provides a stable carrier signal intensity for the primary mixer; see it from the frequency domain, it is to track The selected radio signal is filtered off the interference signal such as a mirror frequency ("60dB suppression) and the vibration frequency, improve the quality of the radio frequency signal.
RF circuit design
Figure 1 is a block diagram of the car radio RF circuit we design, which consists of antenna filter and radio frequency low noise amplifier and a resonant band pass filter. The design goal of the car radio is that noise limit is 0 dBu (30dB S / N), audio signal to noise ratio 64dB, automatic search sensitivity is less than 10 dBU, has strong anti-adjacent channel interference and other interference signal capability, implementing MCU automatic Adjust the function.
Fig. 2 is a schematic diagram of a portion of the FM radio part of the automotive radio TDA7513. C31, C32, D2 (1Sv172), C44 constitute a frequency modulation circuit, 1Sv172 is a VHF to UHF band antenna signal attenuator, which is a current control type element, and the impedance is reduced as the current increases its impedance. It is controlled by the FMAGC current generated by the primary FM broadband AGC and the narrowband AGC synthesis, and the control point is the antenna signal level 57dBu. L5, C36, V2 (KV1410), C43, R19, C45 constitute an antenna band pass filter with a bandwidth of about 12 MHz. The antenna filter can be artificially free to adjust the radio frequency coil L5, or can be adjusted by the MCU to adjust the automatic adjustment function.
Q2 (3sk126), C38, R15, R20, C46, R21, C47, C41, R17 form a low noise amplifier, gain of 30 dB. This design uses the N-channel field effect tube 3Sk126 to ring the input impedance, high gain, high gain, and low noise, and is a voltage controlled device, simple design. Q2 is controlled by the FMAGC voltage generated by the primary FM broadband AGC and the narrowband AGC synthesis, and the control point is an antenna signal level 78dbu. T3, C34, V1 (KV1410), C28, C35 constitute an RF resonance band pass filter, with a bandwidth of about 8 MHz, T3 is an FM RF transformer. The band pass filter can also be artificially unharitable to adjust T3, or automatically adjust the varactor diode V1 by MCU. The receiving limit of the receiver is determined by the receiver's own noise performance, so it is required to use a low noise element in the radio frequency circuit of the radio.
Measures to improve radio frequency circuit performance
To improve the EMC performance of the radio and reduce the noise of the radio itself, it is necessary to take the following measures:
1. Select the correct power mode. Power supply to other units (such as CD and DVD, etc.) should be turned off when the radio works, and only the radio unit is powered. The RF circuit of the FM and AM is powered off, and only the RF circuit of the FM is powered, and the radio frequency circuit of the AM is powered. Only only the radio frequency circuit of the AM is powered, and the RF circuit of the FM is powered. After taking such a measure, you can make the whole machine signal to noise ratio to at least 10 dB.
2. The wiring of the PCB board is also very important. The FM RF circuit, the AM RF circuit, the OSC oscillator circuit, the intermediate frequency circuit, the power supply line and the ground wire of the power amplifier circuit are isolated from each other, and the power supply is supplied to the whole machine. In space permission conditions, the FM and AM RF circuits can be shielded with a shield box. Use a shield box to improve the signal to noise ratio to about 4dB. Both the crystal and the local oscillator circuit are surrounded by respective ground wires, and the component layout should make the output oscillating signal line as short as possible, so that the noise ratio indicator is improved.
3. The low EMI MCU should be selected, even in the MCU's program, EMC instructions should be selected to reduce the interference of the MCU on the radio. The key interrupt mode is used instead of a common key scan mode, so that the MCU is turned off in the radio (ie, the MCU is oscillated). Taking these measures can increase the reception sensitivity of the radio to about 15 dBu.
4. To prevent lightning and lightning strikes, it is necessary to add transient voltage breakdown (TVS) diodes in the antenna, such as SA5.0CA, one of the ESD electrostatic protection measures. The TVS can absorb the surge in surges of thousands of watts, under reverse application conditions, when to withstand a high-energy large pulse, its operating impedance immediately reduces the extremely low conduction value, thereby allowing large current through while using voltage Clamp at a scheduled level. Its response time is only 10-12 seconds, so it can effectively protect the precision element in the electronic circuit, and the lighting strike effect is obvious.
5. The temperature of the car radio work changes greatly, so the choice of RF circuit components should also consider temperature characteristics.
RF circuit performance test
A good RF circuit design must be verified by the correct test. There are many factors affecting the radio test. First, the correct connection of the test cable is necessary, such as impedance matching, the radio frequency signal is fed into the radio into the radio with the crocodile clip, and the radio frequency signal is fed by a crocodile. The radio antenna end RF signal will lose 6dB; secondly, the audio signal source spectrum of the AM / FM signal generator should be left with a net 90dB signal-to-noise ratio. This can be viewed from the user manual of the AM / FM signal generator. You can test your performance with spectrum analyzers. The audio signal source of the general audio signal analyzer is relatively pure, and it can input an AM / FM signal generator as an external audio signal, so that the test signal-to-noise ratio and distortion are relatively simple.
Source: Wiku Electronic Market Network
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