Car radio coaxial circuit design guide

In order to improve the EMC performance of the radio and reduce the noise of the radio itself, it is necessary to take the following measures:

 Select the correct power supply mode. When the radio is working, the power supply to other units (such as CD and DVD, etc.) should be turned off, and only the radio unit should be powered. It is best to supply power to the radio frequency circuits of FM and coaxial cable assemblies separately. In the FM mode, only the radio frequency circuit of FM is powered, and the power supply to the radio frequency circuit of AM is turned 1off. In the AM mode, power is only supplied to the rg405 radio frequency circuit of AM and the radio frequency circuit of FM is turned off powered by. After taking such measures, the signal-to-noise ratio of the whole machine can be improved by at least 10dB.

 The wiring of the PCB board is also very important. The power lines rg402 and ground lines of the FM radio frequency circuit, AM radio frequency circuit, OSC oscillator circuit, intermediate frequency circuit, and power amplifier circuit should be isolated from coax cable, and finally connected to the power terminal of the whole machine. Under space permitting conditions, FM and AM radio frequency circuits can be shielded with shielding boxes. The use of rg405 shielding box can improve the signal-to-noise ratio of the whole machine by about 4dB. Both the crystal and the local oscillation circuit should be surrounded by their own ground wires, and the output oscillation signal wire should be as short as possible during the component layout, which is beneficial to improve the carrier to noise ratio index.

Coaxial cable assemblies should be selected, and even EMC commands are adopted in the MCU program to reduce the MCU's interference to the radio. Use the button interrupt mode to replace the common button scan mode, let the MCU turn off in the radio state (that is, the MCU stops this oscillation) and so on. Taking these measures can increase the receiving sensitivity of the radio by about rg402.

To prevent this lightning and lightning strike, it is necessary to add a transient voltage breakdown (TVS) coax cable, such as SA5.0CA, to the antenna end, which is one of the ESD protection measures. TVS can absorb up to several kilowatts of surge power. Under reverse application conditions, when it is subjected to a high-energy large pulse, its operating impedance immediately drops to a very low conduction value, thereby allowing large currents to pass through while reducing the voltage Clamp at a predetermined level. Its response time is only rg405 10-12 seconds, so it can effectively protect the precision components in the electronic circuit, and the lightning protection effect is obvious.

The working environment temperature of car radio varies greatly, so the selection of radio frequency circuit components should also consider the rg402 temperature characteristics

A good coax cable design must be verified by correct tests. There are many factors that affect the radio test. First of all, it is necessary to test the correct connection of the cable. For example, impedance matching. The RF signal fed into the radio through a BNC connector or SMA connector is better than using an alligator clip to clamp the antenna end to feed the radio. The rg402 RF signal is fed through an alligator clip. The RF signal into the antenna of the radio will lose about 6dB; secondly, the frequency spectrum of the audio signal source of the AM/FM signal generator should be pure about rg405 90dB signal-to-noise ratio). This can be checked from the user manual of the AM/FM signal generator. You can test its performance with a spectrum analyzer yourself. The audio signal source of the general audio signal analyzer is relatively pure, and it can be used as an external audio signal into the AM/FM signal generator, so that it is relatively simple to test the signal-to-noise ratio and distortion.