The particularity of the automotive radio application environment has higher requirements for circuit performance, and the design of the RF circuit is the key to achieving high performance. This paper introduces the RF circuit design method of TDA753. Based on the actual design experience, the author puts forward the design method and measures to improve the EMC characteristics and noise characteristics of RF circuits, and points out the key points of RF circuit performance test.
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RF circuit is the key point and difficulty of radio circuit design. If the RF circuit design is not good, the noise sensitivity and signal-to-noise ratio of the radio and other technical indicators will be greatly reduced, and even a few radio stations can be received manually. The search radio function is disabled. From the broadcast signal field strength of the radio antenna end, the dynamic range of the signal is very large, especially the environment in which the car radio is located changes rapidly and large.
Radio RF circuits are often difficult to integrate into an IC, typically consisting of a discrete low-noise amplifier (LNA) and a resonant bandpass filter. The role of the radio frequency circuit of the car radio is to amplify the weak broadcast signal from the time domain, and provide a stable carrier signal strength for the post-stage mixer through the automatic gain control circuit (AGC); from the frequency domain, it is to be tracked. The selected radio signal filters out interference signals such as image frequency (>60dB rejection) and local oscillator frequency to improve RF signal quality.
RF circuit design Â
Figure 1 is a block diagram of the RF circuit of the car radio we designed. It consists of an antenna filter and an RF low noise amplifier and a resonant bandpass filter. The car radio is designed with a noise sensitivity of 0dBu (30dB S/N), an audio signal-to-noise ratio of 64dB, and an automatic search sensitivity of less than 10dBu. It has strong anti-adjacent channel interference and other interference signals, and is fully automatic. Adjustment function.
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Figure 2 is an electrical schematic diagram of the RF front end of the FM radio part of the car radio TDA7513. C31, C32, D2 (1SV172), C44 constitute the FM band antenna signal conditioning circuit, 1SV172 is the VHF ~ UHF band antenna signal attenuator, it is a current-controlled component, its impedance decreases with increasing current. It is controlled by the FMAGC current generated by the synthesis of the downstream FM wideband AGC and the narrowband AGC, and the starting point is the antenna signal level of 57dBu. L5, C36, V2 (KV1410), C43, R19, and C45 form an antenna bandpass filter with a bandwidth of about 12 MHz. The antenna filter can manually adjust the RF coil L5 with a non-inductive adjustment, or adjust the varactor V2 through the MCU to realize an automatic adjustment function.
Q2 (3SK126), C38, R15, R20, C46, ​​R21, C47, C41, and R17 form a low-noise RF amplifier with a gain of 30dB. In this design, the N-channel FET 3SK126 is used as the RF amplifier, which has the advantages of high input impedance, high gain and low noise, and is a voltage-controlled device with simple design. Q2 is controlled by the FMAGC voltage generated by the combination of the post-stage FM wideband AGC and the narrowband AGC, and the starting point is the antenna signal level of 78dBu. T3, C34, V1 (KV1410), C28, C35 form an RF resonant bandpass filter with a bandwidth of about 8 MHz, and T3 is an FM RF transformer. The band-pass filter can also manually adjust the T3 with a non-inductive adjustment, or the varactor V1 can be automatically adjusted by the MCU.
The receiver's receiving limit is determined by the receiver's own noise performance, so it is required to use low-noise components in the radio frequency circuit of the radio.
Measures to improve the performance of RF circuits
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:
1. Select the correct power mode. When the radio is working, turn off the power to other units (such as CDs and DVDs) and only power the radio unit. It is better to separately supply the RF circuits of FM and AM. In the FM mode, only the RF circuit of the FM is powered, and the RF circuit of the AM is powered off. In the AM mode, only the RF circuit of the AM is powered, and the RF circuit of the FM is turned off. powered by. By taking such measures, the overall signal-to-noise ratio can be improved by at least 10 dB.
2. The wiring of the PCB board is also very important. The FM radio circuit, the AM RF circuit, the OSC oscillator circuit, the IF circuit, the power line and the ground line of the power amplifier circuit should be isolated from each other and finally connected to the power supply end of the whole machine. The FM and AM RF circuits can be shielded with a shielded box under space permitting conditions. Using the shielding box can improve the signal-to-noise ratio of the whole machine by about 4dB. Both the crystal and the local oscillator circuit are surrounded by their respective ground lines. The layout of the components should be such that the output of the oscillating signal line is as short as possible, which is beneficial to improve the carrier-to-noise ratio.
3. The MCU with low EMI should be selected, and even the EMC command should be adopted in the MCU program to reduce the interference of the MCU to the radio. The button interrupt mode is used instead of the common button scanning mode, so that the MCU is turned off in the radio state (that is, the MCU stops the oscillation). Taking these measures can increase the receiving sensitivity of the radio by about 15dBu.
4. To prevent this lightning and lightning strike, it is necessary to add a transient voltage breakdown (TVS) diode at the antenna end, such as SA5.0CA, which is one of the ESD static protection measures. TVS can absorb up to several kilowatts of surge power. In reverse application conditions, when subjected to a large pulse of high energy, its operating impedance immediately drops to a very low conduction value, allowing large currents to pass while simultaneously applying voltage. Clamp at a predetermined level. The response time is only 10-12 seconds, so it can effectively protect the precision components in the electronic circuit, and the lightning protection effect is obvious.
5. The temperature of the working environment of the car radio varies greatly, so the choice of RF circuit components must also consider the temperature characteristics.
RF circuit performance test
Good RF circuit design must be verified by proper testing. There are many factors that affect the radio test. It is necessary to test the correct connection of the cable first. For example, impedance matching, RF signal is fed into the radio through the BNC or SMA socket. It is better to feed the radio with the alligator clip. The RF signal passes through the crocodile clip. The RF signal at the radio antenna will lose about 6dB. Secondly, the spectrum of the audio source of the AM/FM signal generator should be about 90dB pure signal-to-noise ratio. This can be viewed 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 can be input 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 the distortion.
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