tda9884 NXP Semiconductors, tda9884 Datasheet - Page 11

• Manufacturer Part Number: tda9884
• Description: Tda9884 I2c-bus Controlled Multistandard Alignment-free If-pll For Mobile Reception
• Manufacturer: NXP Semiconductors

Philips Semiconductors
TDA9884_2
Product data sheet
7.10 SIF-AGC detector
7.11 Single reference QSS mixer
7.12 AM demodulator
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
internal SIF level for 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
event of a decreasing IF amplitude step, the charging and discharging currents of the
AGC capacitor are increased by a factor of 12 (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.
For negative modulation and QSS mode the AGC also is set to fast mode. For negative
modulation and mobile mode the currents are increased additionally by a factor of 36.
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.
In the true split sound mode the VIF-PLL VCO is locked by a synthesizer. By this the 2nd
FM TV sound intercarrier signal is generated independently from the vision carrier so that
in the case of a low RF level, the sound demodulation is possible where the VIF-PLL
would unlock. In the true split sound mode the VIF demodulation is not available.
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.
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.
c
< 20 Hz for the closed AGC loop). For reducing this AM sound time constant in the
Rev. 02 — 12 May 2006
I
2
C-bus controlled multistandard alignment-free IF-PLL
2
C-bus.
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
TDA9884
2
C-bus, the
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