Car radio fm circuit board
The particularity of the application environment of the FM circuit board of a car radio places higher requirements on the circuit performance. The design of the RF circuit is the key to achieving high performance.
The FM circuit board of the car radio improves the EMC and noise characteristics of the RF circuit, and points out the focus of the RF circuit performance test. RF circuit is the focus and difficulty of RF circuit design. If the RF circuit design is not good, it will greatly reduce the radio's noise limit sensitivity, signal-to-noise ratio and other technical indicators, and even only manually receive a few broadcast stations. The dynamic range is very large, especially the environment of car radio changes rapidly and hugely.
RF circuits are often difficult to integrate into ICs, and usually consist of separate components to form a preset low-noise amplifier (LNA) and resonant band-pass filter. The function of the automotive RF circuit is to amplify the weak broadcast signal in the time domain and provide a stable carrier signal strength to the post-mixer through an automatic gain control circuit (AGC). In the frequency domain, it should track selected radio signals, filter out interfering signals such as image frequencies (> 60dB rejection) and local frequencies, and improve the quality of RF signals.
1. Select the correct power supply mode.Turn off power to other units (CD, DVD, etc.) while the radio is working, only to the radio unit.It is better to supply power to the radio frequency circuit of FM and AM separately. In FM mode, power is only supplied to the radio frequency circuit of FM. In AM mode, power is only supplied to the radio frequency circuit of AM.After taking such measures, the signal-to-noise ratio of the whole machine can be improved by 10dB to a minimum.
2. The wiring of PCB board is also very important. The power cord and ground wire of FM radio frequency circuit, AM radio frequency circuit, OSC oscillation circuit, intermediate frequency circuit and amplifier circuit should be isolated from each other and connected to the power end of the whole machine.The FM and AM rf circuits can be shielded with a shield box under an empty license.Using the shield box can improve the signal-to-noise ratio by about 4dB.Both the crystal and the local oscillating circuit should be surrounded by the ground line. When the components are arranged, the output oscillating signal line should be as short as possible, which is helpful to improve the signal-to-noise ratio.
3. Low EMI MCU should be selected, and even EMC instruction should be adopted in MCU program to reduce the interference of MCU on radio.The key interrupt mode is adopted to replace the common key scanning mode, so that MCU is turned off in the radio state (that is, MCU stops the oscillation), etc.These measures can increase the receiver sensitivity of the radio by about 15dBu.
4. In order to prevent this lightning and lightning strike, it is necessary to add transient voltage breakdown (TVS) diode at the antenna end, such as SA5.0CA, which is one of ESD electrostatic protection measures.TVS can absorb up to several kilowatts of surge power, and under reverse application conditions, when receiving a high-energy large pulse, its operating impedance immediately drops to a very low conductance value, thus allowing large current to pass, and simultaneously clamping the voltage to a predetermined level.Its response time is only 10-12 seconds, so it can effectively protect the precision components in the electronic circuit and has obvious lightning protection effect.
5. The operating environment temperature of car radio varies greatly, so the selection of radio frequency circuit elements should also consider the temperature characteristics.
Radio frequency circuit performance testing
Good rf circuit design must be verified by correct test.There are many factors influencing radio test. It is necessary to test the correct connection of cables first. For example, impedance matching, rf signals fed into the radio through BNC or SMA socket are more effective than using crocodile clip to feed into the radio.Secondly, the frequency spectrum of AM/FM signal generator is about 90dB signal-to-noise ratio (this can be seen from the user manual of AM/FM signal generator, or its performance can be tested by the spectrum analyzer.Generally, the audio signal source of the audio signal analyzer is relatively pure, which can be used as the external audio signal input AM/FM signal generator, so it is easier to test the SNR and distortion.