VHF radio unit Yamaha YX-9500. Scheme, description

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VHF radio unit Yamaha YX-9500. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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As an example, consider the VHF unit of the Yamaha YX-9500 car radio, manufactured in 1996 (Fig. 4). It has several interesting technical solutions that are also characteristic of equipment from other manufacturers.

The signal from the antenna through the coupling capacitor C1 is fed to the input circuit L1C2C3VD1. The unit is tuned in frequency by changing the control voltage on the varicaps VD1-VD3. The resonant URC is made on a double-gate field-effect transistor VT1. The peculiarity of the cascade is that the input signal is applied to the second gate, and the first gate is used to control the gain. Transistor VT2 is a key that changes the bias on the first gate VT1 (and, consequently, the gain) on command from the control microprocessor. To obtain optimal matching and stable operation over the entire frequency range, the load is switched on - the L3VD2 circuit - through the L2 coupling coil.

At the mixer input, the L4C8 rejector circuit is turned on, tuned to an intermediate frequency. It reduces the likelihood of overloading the mixer with signals close to the intermediate frequency. The amplified input signal and the local oscillator signal are fed to the base of the mixer transistor VT3. The IF signal with a frequency of 10,7 MHz is selected in the collector circuit and fed to the IF through the coupling coil L6.

The local oscillator is assembled on a VT4 transistor according to the traditional capacitive three-point circuit. The L7VD3 local oscillator circuit, in order to obtain the highest possible quality factor, is weakly connected both to the local oscillator transistor and to the buffer stage on the VT5 transistor. The design of the IF path and the stereo decoder is similar to that already considered - a matching stage on a transistor, two piezo filters, a booster on the LA1140 chip and a stereo decoder on the LA3375 chip.

Loop coils are wound with copper enameled wire with a diameter of 0,8 mm, a coil diameter of 5 mm and have the following data: L1 - 6,5 turns, L2 - 2,5 turns, L3 - 6,5 turns, L7 - 5,5 turns. Filter coils: L4 - standard inductor with an inductance of 0,68 μH, L5, L6 - standard IF filter 10,7 MHz (capacitor C * is included in the filter design). The sensitivity of the path is 2,5 μV, the selectivity for the adjacent channel is 45 dB.

The considered construction of the radio receiving path is typical mainly for the equipment of European manufacturers. In modern mass-produced models of Japanese-made car radios, combined radio receiving paths of the second generation, completely made on a single chip, are increasingly being used. For example, Sanyo manufactures the LA1883M chip, housed in a 64-pin package, that works with a microprocessor. Similar paths are used in their radio tape recorders by Sony, Kenwood, Pioneer.

Rice. 4 (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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