The path of the AM FM car radio on the SHA1238 chip. Scheme, description

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The AM FM path of the car radio on the SHA1238 chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Since in modern equipment the AM path has become additional, and the FM path is the main one, the main attention is paid to its design. The structure of this path is as follows: resonant UHF (possible AGC or discrete gain control), frequency converter, IF piezo filter, broadband IF, frequency detector, stereo decoder. The number of adjustable circuits is from two to four, depending on the requirements for the selectivity of the receiver. UHF and frequency converter are usually made on the same chip (for example, TA7358AP or KA22495), less often - on discrete elements (in high-end models). The IF and the stereo decoder are also separate microcircuits, although there are also combined ones that combine these two nodes.

As an example, let's consider the path of the IF FM and the stereo decoder of the car radio "Road Star" released in 1993 (Fig. 3). From the output of the frequency converter, the IF signal with a frequency of 10,7 MHz is fed to the aperiodic first stage of the IF. Its task is to match the converter with the ZF1 piezoceramic filter and compensate for losses in it. The signal is then sent to the broadband IF. The phase-shifting circuit L1C3, tuned to the IF, is part of the frequency detector. After the detector, the complex stereo signal is fed to the stereo decoder. Setting the mode of its operation is carried out by the resistor R7. Capacitors C11, C12 together with signal switch elements (not shown in the diagram) form pre-distortion compensation circuits.

The structure of the input stages of the FM path - a resonant UHF and a frequency converter with a separate local oscillator - is also traditional. In older models, the VHF unit is made on discrete bipolar transistors and is a single design with a ferrovariometer. Currently, tuning of circuits with varicaps is widely used, and only in radio receiving paths with frequency synthesizers (in a PLL loop). In domestic car receivers, multi-turn resistors are often used for tuning. Tuning with capacitors is now used only in cheap models made with a combined AM-FM path on microcircuits. Since, with such a construction, there is only one tunable circuit at the URF output in the VHF path, the selectivity over the image channel is low.

Rice. 3 (click to enlarge)

The radio receiving path of almost all cheap Asian-made car radios with analog tuning is made according to the same or similar scheme. The AM, FM and stereo decoder paths are made on a single CXA1238 chip from Sony, which is included according to a typical circuit. The restructuring of the receiver is carried out by a quad block of capacitors of variable capacitance. Range switching is internal at pin 15, the only control is switch SA1. The signals of the CB range are selected by the input circuit L1C2L5CP2.1 and fed to the input of the AM path (pin 19). The L7C6CP2.2 local oscillator circuit is completely connected to the microcircuit. The broadband input circuit of the VHF range is formed by the L2C3C1 circuit, then the signal after the resonant UHF (load - L3C5CP1.1 circuit) is fed to the frequency converter. The broadband IF is common for both paths, the selectivity is determined by the piezo filters ZF1 and ZF2. The ZF3 resonator is part of the PLL FM detector. The stereo decoder, in addition to the main function, performs the functions of a linear amplifier in the AM path. Trimmer resistor RP1 sets the mode of operation of the stereo decoder (subcarrier frequency - 38 kHz, synchronized by the pilot tone). Capacitors C21, C22 together with resistors R10, R11 form pre-distortion compensation circuits.

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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