TDA9812 datasheet(8/32 Pages) PHILIPS

8 / 32 page

1995 Mar 21

Philips Semiconductors

Preliminary speciﬁcation

Multistandard VIF-PLL and

FM-PLL/AM demodulator

TDA9812

Video buffer

For an easy adaption of the sound traps an operational

amplifier with internal feedback is used in the event of B/G

and L standard. This amplifier is featured with a high

bandwidth and 7 dB gain. The input impedance is adapted

for operating in combination with ceramic sound traps. The

output stage delivers a nominal 2 V (p-p) positive video

signal. Noise clipping is provided.

SIF ampliﬁer and AGC for AM sound

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 signals

(average level of AM carrier) and controls the SIF amplifier

to provide a constant SIF signal to the AM demodulator.

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.

Intercarrier mixer

The intercarrier mixer is realized by a multiplier. The VIF

amplifier output signal is fed to the intercarrier 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.

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 detector

The FM detector consists of a limiter, an FM-PLL and an

AF amplifier. The limiter provides the amplification and

limitation of the FM sound intercarrier signal before

demodulation. 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

and to save pins for DC decoupling.

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

The AF amplifier consists of two parts:

• The AF preamplifier 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, 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.

• 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 and 1

⁄2VP-reference

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.

For all audio output signals the constant reference voltage

cannot be used because large output signals are required.

Therefore these signals refer to half the supply voltage to

achieve a symmetrical headroom, especially for the

rail-to-rail output stage. For ripple and noise attenuation

the 1

⁄2VP voltage has to be filtered via a low-pass filter by

using an external capacitor together with an integrated

resistor (fg = 5 Hz). For a fast setting to 1⁄2VP an internal

start-up circuit is added.

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