Frequency synthesizer and microcomputer for Yamaha YX-9500 car radio. Scheme, description

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Frequency synthesizer and microcomputer for Yamaha YX-9500 car radio. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Frequency synthesizers of modern receivers are made according to the PLL circuit (in English terminology PLL - Phase Locked Loop). The principles of building such systems are known: the local oscillator signal after frequency division is compared in frequency and phase with the reference signal, the frequency of which is equal to the frequency grid step in the selected range. The resulting error signal changes the local oscillator frequency so that it becomes equal to the reference frequency multiplied by the division factor. The speed of the first generation synthesizers was insufficient, therefore, in the VHF range, they were used in conjunction with an external frequency divider. The feature set was extremely limited.

Synthesizers of the second generation are already made entirely on a single chip. They include a control microprocessor and settings memory cells. Typically, 5-6 memory cells are used in each of the AM bands and from 10 to 30 or more in the VHF band. Cells in the VHF range are usually divided into groups for ease of use. To indicate the tuning frequency in the first generation synthesizers, LED indicators were used, then they switched to the use of liquid crystal screens (LCD display) with backlighting and cathodoluminescent indicators (in expensive models). Changing the frequency grid (European or American standard) was previously performed by external jumpers or switches on the radio board, in new models this operation is carried out from the keyboard purely by software.

In addition to controlling the actual tuning frequency of the receiver, the microprocessor of the frequency synthesizer also performs a number of service functions. The algorithm of work and the name of the functions for different manufacturers are quite different. The usual set of functions is as follows: switching bands (band), manual tuning (manual tuning) with the possibility of memorizing (memory), automatic tuning and storing all available stations (auto tuning, auto memory store - AMS) or stations with a maximum signal level (best stations memory, BSM), automatic tuning to the next frequency station (seek), scanning memory cells forward (scan up) or backward (scan down) with listening for 5-10 seconds. In addition, the last tuning on each of the bands is automatically remembered (in receivers with analog tuning, this feature was taken for granted).

The functions of the microprocessor also include keyboard scanning, indication of the range, tuning frequency, memory cell numbers, receiver or tape recorder operating modes, the set of which can be quite different from model to model, even among the products of the same company. With the spread of digital controllers (volume, balance, timbre) in the audio path, their control was entrusted to the microcomputer of the frequency synthesizer. Tape drives with logical control and a number of external devices are also serviced by this microprocessor, which gives grounds to classify such control systems as third generation.

Radio data transmission systems (RDS) that have appeared in recent years use the same display and microprocessor to display information. Traffic reports for drivers, weather forecast, financial news and other information that can be stored in memory is transmitted. Data decoding is still performed by a separate device, but it can be assumed that its functions will also soon be transferred to the main microprocessor. Unfortunately, in Russia this system is still at the first stage of development.

The automatic tuning algorithm for modern radio receiving paths is approximately the same and differs only in details. Tuning, for example, is first performed in the local reception mode (Local) with reduced sensitivity of the receiving path, and only then in the long-range reception mode (DX). Some modern receivers can search for stations broadcasting certain programs (sports, news, music of certain genres). Unfortunately, domestic radio stations do not yet transmit identification signals, and the musical vinaigrette on the air does not contribute to the use of this function. The processor tunes the receiver in range until it receives a stop signal from it. It is generated by the coincidence of two conditions - the capture of the frequency and the achievement of a given level of the IF signal. In the VHF band, this is usually done using the silent tuning system signal available on most microcircuits. Further, depending on the selected algorithm, other conditions are analyzed. For example, in the VHF band, in addition to the signal level, you can control the presence and level of the pilot tone. Then, with a weak signal, the stereo decoder is forced into mono mode. If the station satisfies the set conditions, its frequency is stored in the processor's memory.

As an example, consider the frequency synthesizer and control microcomputer UPD1719G-014 of the Yamaha YX-9500 radio recorder, manufactured in 1996 (Fig. 5). This microcircuit is now somewhat outdated, but using its example it is easy to disassemble the construction of a simple frequency synthesizer and its interaction with the radio receiving path.

Rice. 5 (click to enlarge)

The clock frequency of the microprocessor is 4,5 MHz, stabilized by a quartz resonator. Most of the inputs and outputs of the microcircuit are occupied by servicing the liquid crystal display and keyboard, 16 buttons of which are combined into an incomplete 6x4 matrix. When switching to the cassette playback mode, the supply and control voltages are removed from the radio receiving path, the keyboard scanning stops and only the direction of tape movement is indicated.

Depending on the tuning range selected from the keyboard, a set of signals at pins 12 and 13, through switches on bipolar transistors (not shown in the diagram), supplies power to the corresponding stages of the receiver. The local oscillator signal of the AM path is fed to pin 5, the FM path - to pin 6. The width-modulated signal for controlling the frequency of the local oscillators from pin 3 is fed to an integrator made on transistors VT4, VT5. The tuning voltage for varicaps is taken from capacitor C1. This microcomputer does not automatically switch the sensitivity of the receiving path and stereo mode during the setup process; Local / DX and mono-stereo (only for VHF) modes are switched manually. The corresponding signals are generated at pins 10 and 18. In the process of searching for stations or switching fixed settings, the microcomputer outputs a mute signal at pin 14, which controls the electronic keys at the UMZCH input (not shown in the diagram). At pin 63, stop signals are active for the FM path (from the silent tuning system) and the AM path. Additionally, an intermediate frequency is received from the AM path (pin 16). Pin 64 receives a signal from the stereo decoder's pilot tone detector to indicate stereo reception.

Several sources are used to power the microprocessor. Firstly, this is a 3,6 volt voltage regulator on a VD20 zener diode, from which the microprocessor itself is powered in operating mode. To power the memory cells, a stabilized voltage source of 5 volts was used, made on the basis of a 78L05 micropower voltage regulator. Power is constantly supplied to it from the car battery through the VD18 diode. When removing the main battery, you can connect a galvanic battery with a voltage of 9-15 volts through the VD19R13 circuit. Finally, in the event of a complete shutdown of power sources (removable radio), an ionistor C8 with a capacity of 0,22 F is provided. The energy stored by it is enough to power the memory cells for 4-5 days.

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

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