TDA9885 Philips Semiconductors, TDA9885 Datasheet - Page 12

• Manufacturer Part Number: TDA9885
• Description: I2C-bus controlled single and multistandard alignment-free IF-PLL demodulators
• Manufacturer: Philips Semiconductors

Philips Semiconductors
8.10
SIF gain control is performed by the detection of the
DC component of the AM demodulator output signal. This
DC signal corresponds directly to the SIF voltage at the
output of the SIF amplifier so that a constant SIF signal is
supplied to the AM demodulator and to the single
reference QSS mixer.
By switching the gain of the input amplifier of the SIF-AGC
detector via the I
FM sound is 5.5 dB lower than for AM sound. This is to
adapt the SIF-AGC characteristic to the VIF-AGC
characteristic. The adaption is ideal for a picture-to-sound
FM carrier ratio of 13 dB.
Via a comparator, the integrated AGC capacitor is charged
or discharged for the generation of the required SIF gain.
Due to AM sound, the AGC reaction time is slow
(f
AM sound time constant in the event of a decreasing
IF amplitude step, the load current of the AGC capacitor is
increased (fast mode) when the VIF-AGC detector (at
positive modulation mode) operates in the fast mode too.
An additional circuit (threshold approximately 7 dB)
ensures a very fast gain reduction for a large increasing
IF amplitude step.
8.11
With the present system a high performance Hi-Fi stereo
sound processing can be achieved. For a simplified
application without a SIF SAW filter, the single reference
QSS mixer can be switched to the intercarrier mode via the
I
The single reference QSS mixer generates the 2nd FM
TV sound intercarrier signal. It is realized by a linear
multiplier which multiplies the SIF amplifier output signal
and the VIF-PLL VCO signal (90 degrees output) which is
locked to the picture carrier. In this way the QSS mixer
operates as a quadrature mixer in the intercarrier mode
and provides suppression of the low frequency video
signals.
The QSS mixer output signal is fed internally via a
high-pass and low-pass combination to the
FM demodulator as well as via an operational amplifier to
the intercarrier output pin SIOMAD.
2003 Oct 02
2
c
C-bus.
I
alignment-free IF-PLL demodulators
< 20 Hz for the closed AGC loop). For reducing this
2
C-bus controlled single and multistandard
SIF-AGC detector
Single reference QSS mixer
2
C-bus, the internal SIF level for
12
8.12
The amplitude modulated SIF amplifier output signal is fed
both to a two-stage limiting amplifier that removes the AM
and to a linear multiplier. The result of the multiplication of
the SIF signal with the limiter output signal is
AM demodulation (passive synchronous demodulator).
The demodulator output signal is fed via a low-pass filter
that attenuates the carrier harmonics and via the input
amplifier of the SIF-AGC detector to the audio amplifier.
8.13
The narrow-band FM-PLL detector consists of:
The intercarrier signal from the intercarrier mixer is fed to
the input of an AC-coupled gain controlled amplifier with
two stages. The gain controlled output signal is fed to the
phase detector of the narrow-band FM-PLL
(FM demodulator). For good selectivity and robustness
against disturbance caused by the video signal, a high
linearity of the gain controlled FM amplifier and of the
phase detector as well as a constant signal level are
required. The gain control is done by means of an ‘in
phase’ demodulator for the FM carrier (from the output of
the FM amplifier). The demodulation output is fed into a
comparator for charging or discharging the integrated
AGC capacitor. This leads to a mean value AGC loop to
control the gain of the FM amplifier.
The FM demodulator is realized as a narrow-band PLL
with an external loop filter, which provides the necessary
selectivity (bandwidth approximately 100 kHz). To achieve
good selectivity, a linear phase detector and a constant
input level are required. The gain controlled intercarrier
signal from the FM amplifier is fed to the phase detector.
The phase detector controls via the loop filter the
integrated low radiation relaxation oscillator. The designed
frequency range is from 4 to 7 MHz.
The VCO within the FM-PLL is phase-locked to the
incoming 2nd SIF signal, which is frequency modulated.
As well as this, the VCO control voltage is superimposed
by the AF voltage. Therefore, the VCO tracks with the FM
of the 2nd SIF signal. So, the AF voltage is present at the
loop filter and is typically 5 mV (RMS) for 27 kHz
FM deviation. This AF signal is fed via a buffer to the audio
amplifier.
The correct locking of the PLL is supported by the digital
acquisition help circuit (see Section 8.6).
Gain controlled FM amplifier and AGC detector
Narrow-band PLL.
AM demodulator
FM demodulator and acquisition help
TDA9885; TDA9886
Product specification