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TDA9882 Scheda tecnica(PDF) 11 Page - NXP Semiconductors |
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TDA9882 Scheda tecnica(HTML) 11 Page - NXP Semiconductors |
11 / 45 page 9397 750 13507 © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Product data sheet Rev. 01 — 16 November 2004 11 of 45 Philips Semiconductors TDA9882 Multistandard vision and QSS FM sound IF PLL demodulator 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. 8.14 Audio amplifier and mute time constant The audio amplifier consists of two parts: • AF preamplifier • AF output amplifier. 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 using a DC operating point control circuit (with external capacitor CAF), the AF preamplifier is 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 de-emphasis time constant with 50 µs or 75 µs depends on the FM carrier selection via pins FM0 and FM1; see Table 5. 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 the FM sound, AM sound and mute state. Switching to the mute state is controlled automatically, depending on the digital acquisition help circuit should the VCO of the FM PLL not be in the required frequency window. This is done by a time constant: fast for switching to the mute state and slow (typically 40 ms) for switching to the non-mute state. Auto mute can be disabled via pins SIF1 and SIF2 with resistor; see Table 6. |
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