The introduction of the second VHF band into the car radio with a synthesizer. Scheme, description

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The introduction of the second VHF band into the car radio with a synthesizer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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I got a decent SANYO car radio for repairs with one big drawback (as its owner thought): VHF reception was only in the bourgeois range (88-108), and in our village only domestic (64-74). Moreover, the owner wanted to switch VHF bands with one button. The first thought - to get into the VHF unit and make two coils switched by a relay - did not materialize. The block used a frequency synthesizer chip - LM7000. I told you - it was a decent car radio: 1 IF - 10,7 MHz, 2 IF - 455 kHz. The front panel was non-removable, which made things a lot easier.

The second thought is to put a mixer on quartz and 174PS1 and switch this business with a relay, well, to hell with it - an integral optical switch. The idea did not come true - low sensitivity, interference from the SB station, climbing through the mirror channel of a powerful carrier of a local television transmitter. The third thought is to deal with the HOST processor program. It also did not give a result - the processor is not known, the program is sewn up in its insides. Playing with jumpers, diodes and buttons surrounding the processor did not give anything.

The fourth thought - "bro, do not powder your brains, buy another radio tape recorder" also did not materialize. Okay, let's intercept the data going into the synthesizer and upgrade it with our processor. All TTL levels, there is a description of the synthesizer, there is +5 volt power supply, we will take the clock pulses for our processor from the crystal oscillator of the HOST processor. The step on the bourgeois VHF is 50 kHz, on the domestic one - 10 kHz; on the MW and LW bands, another synthesizer prescaler is used, as indicated by one of the bits in the control word. On the face of the radio there was a rarely used LO / DX button (near / far reception), which was cut off from the HOST processor and connected to the conversion processor. As an indication of VHF on, I used a red LED in a surface mount case crammed under the VHF switch button. It was decided not to give a damn about the frequency indication on the native LCD. Since the number of steps is different (for the bourgeois - 400, for the domestic - 1000), I limited myself to the range of 70-74 MHz (this is where we broadcast).

What the program does:

get the control word, if there is no synchronization from the HOST processor (i.e. the control word is not output), then go to button analysis;
if it is AM band (SV, DV) - output;
if the flag for enabling the Russian VHF band is set, then calculate your control word, otherwise do not change;
output: output the control word to the synthesizer;
if the mode control button is pressed, throw the flag and change the state of the LED.

As a processor, I used an Atmel AVR AT90S1200.

As a result, the revision came down to:

installing a flashed AVR (I glued it in the free space of the board);
installing SMD LED on the front panel;
cutting several tracks (from the button and controlling the synthesizer) and forwarding several wires;
I had to wind one turn on the VCO coil and play by pulling / pushing the turns, otherwise the PLL would break.

In the same way, I rebuilt a musical center of Chinese or Indian origin on the same synthesizer chip, the VCO did not have to be twisted.

Author: Sergey Gimaev, RW9UAO; Publication: N. Bolshakov, rf.atnn.ru

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