Frequency code loader for UHF modulator based on TA1243CF chip. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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To transmit television signals over a coaxial cable from a satellite receiver to a TV located in another house at a distance of 11 m, I decided to use a UHF modulator from a faulty VCR. It turned out that the found modulator was assembled on a TA1243CF chip containing a built-in synthesizer of the TV signal carrier frequency, which must be set digitally via the I interface2C.
For this reason, a microcontroller was required, which, at the beginning of the modulator's operation, will send it via the interface I2C five control bytes. The first three of them in my case have fixed values:
- 0xCA - address of the TA1243CF chip on the I bus2C;
- 0xF8 - the first byte of the command (C1); 0x7F - the second byte of the command (C2).
The reference data of the TA1243CF chip (URL: digchip. com/datasheets/download_datasheet.php?id=948 544&part-number=TA1243CF) indicates the purpose of all bits of the C1 and C2 bytes, and if necessary, by correcting their values, you can change the power of the generated signal, ratio of image and sound carrier levels and some other parameters.
The remaining two bytes (PD1 and PD2) contain the code of the synthesized image carrier frequency according to the table. In it N1 (younger) - N10 (higher) - binary digits of the number N=F-0,25, where F is the frequency, MHz; TSG - bit, the unit value of which includes the generation of a television test signal in the form of vertical stripes. For the transmission of two programs, I used the frequencies of standard television channels: channel 36 - 591,25 MHz (PD1=0x24, PD2=0xF4) and channel 50 - 807,25 MHz (PD1=0x32, PD2=0x74).
For convenience, I placed these bytes in the EEPROM of the microcontroller. At address 0 - byte PD1, at address 1 - PD2 channel 36 (591,25 MHz). At addresses 2 and 3 - respectively PD1 and PD2 of channel 50 (807,25 MHz).
By calculating the number N and placing it in the EEPROM at the specified addresses, you can set in 1 MHz steps any desired frequency in the range from 471,25 MHz to 855,25 MHz, shifted by 250 kHz relative to the integer value.
The scheme of the loader of control bytes into the modulator chip TA1243CF is shown in fig. 1. One of the two channels is selected by supplying power to the device while pressing the SB1 button. It should be kept pressed for about 2 seconds after turning on the power. Each such operation alternately selects one of the two channels. If you turn on the power without pressing the button, the modulator will work on the previously selected channel.
Rice. 1. Scheme of the loader of control bytes into the modulator chip TA1243CF
The completion of the transfer of five bytes LED HL1 signals a series of flashes. Then the microcontroller DD1 "falls asleep" and remains in this state until the power is turned off.
The device is powered by a constant voltage of 12 V. But it has a step-up voltage converter on the DA2 chip (MC34063), the output voltage of which +33 V is necessary for the modulator varicaps. The +5 V voltage received from the integrated stabilizer DA1 (78L05) powers the microcontroller and the modulator unit.
All elements, including the modulator unit removed from the VCR, I placed on a circuit board of suitable dimensions (Fig. 2).
Rice. 2. Elements on the circuit board
The microcontroller program can be downloaded from ftp://ftp.radio.ru/pub/2016/10/uhf.zip.
Author: A. Uskov
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