Car radio fm circuit board
The particularity of automobile radio application environment has higher requirements on circuit performance, and the design of radio frequency circuit is the key to achieve high performance.This paper introduces the design method of radio frequency circuit of TDA7513, puts forward the design method and measures to improve the EMC and noise characteristics of radio frequency circuit, and points out the key points of radio frequency circuit performance test.Radio frequency circuit is the key point and difficulty in radio circuit design. If radio frequency circuit design is not good, the noise limit sensitivity, signal-to-noise ratio and other technical indicators of radio will be greatly reduced, even only a few radio stations can be received manually, and the function of automatic search radio will fail.From the broadcast field strength of radio antenna, the dynamic range of signal is very large, especially the environment of car radio changes quickly and greatly.
Radio frequency circuits are often difficult to integrate into the IC, and usually consist of separate elements to form a preset low noise amplifier (LNA) and a resonant bandpass filter.The function of automobile radio frequency circuit is to amplify weak broadcast signal in time domain, and provide stable carrier signal intensity for rear mixer through automatic gain control circuit (AGC).In the frequency domain, it should track the selected radio signal, filter out interference signals such as mirror frequency (> 60dB suppression) and local frequency, and improve the quality of radio frequency signal.
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.