TDA9885TSLASHV5 datasheet(12/56 Pages) NXP

12 / 56 page

TDA9885_TDA9886_3

Product data sheet

Rev. 03 — 16 December 2008

12 of 56

NXP Semiconductors

TDA9885; TDA9886

I2C-bus controlled multistandard alignment-free IF-PLL demodulators

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.

8.12 AM demodulator

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 through the input ampliﬁer of the SIF-AGC detector to the audio

ampliﬁer.

8.13 FM demodulator and acquisition help

The narrowband FM-PLL detector consists of:

• Gain controlled FM ampliﬁer and AGC detector

• Narrowband PLL

The 2nd SIF 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 narrowband 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 2nd SIF

signal (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 narrowband 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 Section 8.6).

8.14 Audio ampliﬁer and mute time constant

The audio ampliﬁer consists of two parts:

• AF preampliﬁer

• AF output ampliﬁer

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