tda9884 NXP Semiconductors, tda9884 Datasheet - Page 12

• 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
7.13 FM demodulator and acquisition help
7.14 Audio amplifier and mute time constant
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 MHz 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
The audio amplifier consists of two parts:
The AF preamplifier used for FM sound is an operational amplifier with internal feedback,
high gain and high common mode rejection. The AF voltage from the PLL demodulator is
5 mV (RMS) for a frequency deviation of 27 kHz and is amplified by 30 dB. By the use of a
DC operating point control circuit (with external capacitor C
decoupled from the PLL DC voltage. The low-pass characteristic of the amplifier reduces
the harmonics of the sound intercarrier signal at the AF output terminal.
For FM sound a switchable de-emphasis network (with external capacitor) is implemented
between the preamplifier and the output amplifier.
The AF output amplifier provides the required AF output level by a rail-to-rail output stage.
A preceding stage makes use of an input selector for switching between FM sound,
AM sound and mute state. The gain can be switched between 10 dB (normal) and 4 dB
(reduced).
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Gain controlled FM amplifier and AGC detector
Narrow-band PLL
AF preamplifier
AF output amplifier
Section
7.6).
Rev. 02 — 12 May 2006
I
2
C-bus controlled multistandard alignment-free IF-PLL
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
AF
), the AF preamplifier is
TDA9884
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