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TDA9812T

Multistandard VIF-PLL and FM-PLL/AM demodulator

Philips Electronics 

Philips Semiconductors

Multistandard VIF-PLL and

FM-PLL/AM demodulator

Preliminary specification

TDA9812

FUNCTIONAL DESCRIPTION

Vision IF amplifier

The vision IF amplifier consists of three AC-coupled

differential amplifier stages. Each differential stage

comprises a feedback network controlled by emitter

degeneration.

Tuner and VIF AGC

The AGC capacitor voltage is transferred to an internal IF

control signal, and is fed to the tuner AGC to generate the

tuner AGC output current (open-collector output). The

tuner AGC take-over point can be adjusted. This allows the

tuner and the SWIF filter to be matched to achieve the

optimum IF input level.

The AGC detector charges/discharges the AGC capacitor

to the required voltage for setting of VIF and tuner gain in

order to keep the video signal at a constant level.

Therefore for negative video modulation the sync level and

for positive video modulation the peak white level of the

video signal is detected. In order to reduce the reaction

time for positive modulation, where a very large time

constant is needed, an additional level detector increases

the discharging current of the AGC capacitor (fast mode)

in the event of a decreasing VIF amplitude step. The

additional level information is given by the black-level

detector voltage.

Frequency-Phase detector (FPLL)

The VIF-amplifier output signal is fed into a frequency

detector and into a phase detector via a limiting amplifier.

During acquisition the frequency detector produces a DC

current proportional to the frequency difference between

the input and the VCO signal. After frequency lock-in the

phase detector produces a DC current proportional to the

phase difference between the VCO and the input signal.

The DC current of either frequency detector or phase

detector is converted into a DC voltage via the loop filter,

which controls the VCO frequency. In the event of positive

modulated signals the phase detector is gated by

composite sync in order to avoid signal distortion for

overmodulated VIF signals.

VCO, travelling wave divider and AFC

The VCO operates with a resonance circuit (with L and C

in parallel) at double the PC frequency. The VCO is

controlled by two integrated variable capacitors. The

control voltage required to tune the VCO from its free-

running frequency to actually double the PC frequency is

generated by the Frequency-Phase detector and fed via

the loop filter to the first variable capacitor (FPLL). This

control voltage is amplified and additionally converted into

a current which represents the AFC output signal. The

VCO centre frequency can be decreased (required for

L/L accent standard) by activating an additional internal

capacitor. This is achieved by using the L/L accent switch.

In this event the second variable capacitor can be

controlled by a variable resistor at the L/L accent switch for

setting the VCO centre frequency to the required

L/L accent value. At centre frequency the AFC output

current is equal to zero.

The oscillator signal is divided-by-two with a Travelling

Wave Divider (TWD) which generates two differential

output signals with a 90 degree phase difference

independent of the frequency.

Video demodulator and amplifier

The video demodulator is realized by a multiplier which is

designed for low distortion and large bandwidth. The vision

IF input signal is multiplied with the ‘in phase’ signal of the

travelling wave divider output. In the demodulator stage

the video signal polarity can be switched in accordance

with the TV standard.

The demodulator output signal is fed via an integrated

low-pass filter for attenuation of the carrier harmonics to

the video amplifier. The video amplifier is realized by an

operational amplifier with internal feedback and high

bandwidth. A low-pass filter is integrated to achieve an

attenuation of the carrier harmonics for B/G and

L standard. The standard dependent level shift in this

stage delivers the same sync level for positive and

negative modulation. The video output signal is 1 V (p-p)

for nominal vision IF modulation.

1995 Mar 21

7

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