tda9817t NXP Semiconductors, tda9817t Datasheet - Page 8

• Manufacturer Part Number: tda9817t
• Description: Single/multistandard Vif/sif-pll And Fm-pll/am Demodulators
• Manufacturer: NXP Semiconductors

Philips Semiconductors
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.
2004 Jun 29
Single/multistandard VIF/SIF-PLL and
FM-PLL/AM demodulators
8
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 FM
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
2. The AF output amplifier (10 dB) provides the required
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.
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 V
sound is applied. An additional DC control circuit is
implemented to keep the DC level constant,
independent of process spread.
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.
de-em
at which the de-emphasis network for FM
TDA9817; TDA9818
Product specification
in