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CXA1929M

TV RF MODULATOR/PLL

Sony Semiconductor 

CXA1929M

Functional Description of RF Modulator System

A block diagram of the RF modulator is shown in Fig. 1.

The composite video signal passes through an amplitude limiting circuit and a digitally controlled modulation

depth adjustment circuit. By means of three allocated I2C control bits it is possible to adjust the modulation

depth from a nominal 74 % to 86% in 2.0 % steps. Modulation depth can also be controlled externally by

addition of an external resistor on pin3. The video signal is then passed through to the mixer. In the mixer

stage, both the video composite signal and FM sound carrier amplitude modulate the RF carrier from the UHF

OSC, and the issuing signal is fed to the output via an RF amplifier.

The audio signals enter the chip via pin4. The FM Voltage Controlled Oscillator is band-switched to the

approximate centre frequencies of the four sound carrier frequencies 4.5 MHz (NTSC) or 5.5, 6.0, 6.5 MHz

(PAL). The required correct audio carrier frequency is fine tuned by the adjustment of fsound via I2C, where

upon the FM phase locked loop will lock the VCO onto the exact centre frequency. Minimum distortion of the

FM signal occurs when the centre frequency of the oscillator coincides with a mid-rail control voltage (to 2.0 V).

Hence fine adjustment capacitors are also integrated on the chip, which tag into account any capacitance

variations from one batch to another. This is achieved by fine adjustment of Delta fsound via I2C control. The

design of the FM modulator eliminates the need for an external varactor.

The amplitude of the FM signal is varied via I2C by a digitally controlled logarithmic gain stage, which controls

the picture to sound power ratio (P/S). This signal is then fed into the RF amp. Hence at the RF output the

P/S ratio can be programmed in 1 dB steps between the range 10 dB and 17 dB (see Electrical

Characteristics). Audio sensitivity can be programmed in 8 % steps between the range –32 % and +24 %.

The UHF is phase locked to the reference frequency by means of the UHF phase locked loop. The VCO

output passes through a prescaler (≈8) followed by an I2C controlled programmable divider (≈M), and then

phase compared with that of a reference signal at 15.625 kHz. The reference signal is derived from a 4 MHz

crystal oscillator feeding a –256 divider. The UHF oscillator covers the frequency range 471 MHz to 855 MHz

in frequency steps of 125 kHz. This is adequate for the UHF NTSC and PAL TV channels (471.25MHz (CH21)

to 855.25 MHz (CH69)). By adjustment of the external components around the oscillator and some adjustment

to output matching the RF modulator can also achieve range of frequencies in the VHF range.

The Nicam input (pin 22) accepts the 6.522 MHz Nicam signal subcarrier QPSK at 728 K bits/s data. This is

then buffered and feed into a separate mixer (controlled via VCO). This then feeds to the RF amp with the

composite video and audio. The I2C control will switch the Nicam signal output off or on.

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