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TDA9817T

Single/multistandard VIF/SIF-PLL and FM-PLL/AM demodulators

Philips Electronics 

Philips Semiconductors

Single/multistandard VIF/SIF-PLL and

FM-PLL/AM demodulators

Product specification

TDA9817; TDA9818

Sound IF amplifier and SIF AGC

The sound IF amplifier consists of two AC-coupled

differential amplifier stages. Each differential stage

comprises a controlled feedback network provided by

emitter degeneration.

The SIF AGC detector is related to the SIF input signal

(average level of AM or FM carrier) and controls the SIF

amplifier to provide a constant SIF signal to the

AM demodulator and single reference QSS mixer. At

L standard (AM sound) the SIF AGC reaction time is set to

‘slow’ for nominal video conditions. But with a decreasing

VIF amplitude step the SIF AGC is set to ‘fast’ mode

controlled by the VIF AGC detector. In FM mode this

reaction time is always ‘fast’.

Single reference QSS mixer

The single reference QSS mixer is realized by a multiplier.

The SIF amplifier output signal is fed to the single

reference QSS mixer and converted to intercarrier

frequency by the regenerated picture carrier (VCO). The

mixer output signal is fed via a high-pass for attenuation of

the video signal components to the output pin QSS. With

this system a high performance hi-fi stereo sound

processing can be achieved.

For a simplified application without a sound IF SAW filter

the single reference QSS mixer can be switched to the

intercarrier mode by connecting pin SIF2 to ground. In this

mode the sound IF passes the vision IF SAW filter and the

composite IF signal is fed to the single reference QSS

mixer. This IF signal is multiplied with the 90 degree TWD

output signal for converting the sound IF to intercarrier

frequency. This composite intercarrier signal is fed to the

output pin QSS, too. By using this quadrature detection,

the low frequency video signals are removed.

AM demodulator

The AM demodulator is realized by a multiplier. The

modulated SIF amplifier output signal is multiplied in

phase with the limited (AM is removed) SIF amplifier

output signal. The demodulator output signal is fed via an

integrated low-pass filter for attenuation of the carrier

harmonics to the AF amplifier.

FM-PLL demodulator

The FM-PLL demodulator consists of a limiter and an

FM-PLL. The limiter provides the amplification and

limitation of the FM sound intercarrier signal. The result is

high sensitivity and AM suppression. The amplifier

consists of 7 stages which are internally AC-coupled in

order to minimize the DC offset.

Furthermore the AF output signal can be muted by

connecting a resistor between the limiter input pin FMin

and ground.

The FM-PLL consists of an integrated relaxation oscillator,

an integrated loop filter and a phase detector. The

oscillator is locked to the FM intercarrier signal, output

from the limiter. As a result of locking, the oscillator

frequency tracks with the modulation of the input signal

and the oscillator control voltage is superimposed by the

AF voltage. The FM-PLL operates as an FM demodulator.

Audio Frequency signal processing

The AF amplifier consists of two parts:

1. The AF pre-amplifier for FM sound is an operational

amplifier with internal feedback, high gain and high

common mode rejection. The AF voltage from the

PLL demodulator, by principle a small output signal, is

amplified by approximately 33 dB. The low-pass

characteristic of the amplifier reduces the harmonics of

the intercarrier signal at the sound output terminal

pin Vde-em at which the de-emphasis network for FM

sound is applied. An additional DC control circuit is

implemented to keep the DC level constant,

independent of process spread.

2. The AF output amplifier (10 dB) provides the required

output level by a rail-to-rail output stage. This amplifier

makes use of an input selector for switching to AM,

FM de-emphasis or mute state, controlled by the

standard switching voltage and the mute switching

voltage.

Internal voltage stabilizer

The bandgap circuit internally generates a voltage of

approximately 1.25 V, independent of supply voltage and

temperature. A voltage regulator circuit, connected to this

voltage, produces a constant voltage of 3.6 V which is

used as an internal reference voltage.

2004 Jun 29

8

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