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TDA9881 Datasheet(PDF) 10 Page - NXP Semiconductors |
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TDA9881 Datasheet(HTML) 10 Page - NXP Semiconductors |
10 / 42 page 9397 750 13417 © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Product data sheet Rev. 01 — 16 November 2004 10 of 42 Philips Semiconductors TDA9881 Alignment-free vision and FM sound IF PLL demodulator 8.11 Single reference QSS mixer With the present system high performance Hi-Fi stereo sound processing can be achieved. For a simplified application without an SIF SAW filter, the single reference QSS mixer can be switched to the intercarrier mode via pins SIF1 and SIF2; see Table 7. 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 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 QSS output pin QSSO. 8.12 FM demodulator and acquisition help circuit The narrow-band FM PLL detector consists of: • Gain controlled FM amplifier and AGC detector • Narrow-band PLL. The intercarrier signal from the single reference QSS 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. The VCO control voltage is superimposed by the AF voltage. Therefore, the VCO tracks with the FM of the 2nd SIF signal. Thus, 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. |
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