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TDA9881HN Datasheet(PDF) 11 Page - NXP Semiconductors |
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TDA9881HN Datasheet(HTML) 11 Page - NXP Semiconductors |
11 / 42 page 9397 750 13417 © Koninklijke Philips Electronics N.V. 2004. All rights reserved. Product data sheet Rev. 01 — 16 November 2004 11 of 42 Philips Semiconductors TDA9881 Alignment-free vision and FM sound IF PLL demodulator 8.13 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 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 pin AMUTE0; see Table 6. 8.14 Internal voltage stabilizer The band gap circuit internally generates a voltage of approximately 2.4 V, independent of the supply voltage and the temperature. A voltage regulator circuit, connected to this voltage, produces a constant voltage of 3.55 V which is used as an internal reference voltage. 8.15 Logic The logic circuit detects the logic level at the logic ports VIF0, QSSO, FM0, FM1 and AMUTE0 and controls the internal functions; see Table 4 to Table 6. In the event that all logic ports are open-circuit (high-ohmic or CMOS HIGH level) TV standard NTSC with a vision carrier frequency of 45.75 MHz, an FM sound carrier frequency of 4.5 MHz, de-emphasis with 75 µs time constant and auto mute on is selected. Table 4: VIF frequency selection QSSO VIF0 VIF frequency (MHz) No resistor at pin pin open-circuit 45.75 No resistor at pin pin connected to ground 38.9 2.2 k Ω resistor to ground at pin pin open-circuit 58.75 2.2 k Ω resistor to ground at pin pin connected to ground 38.0 |
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