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TDA8349A Folha de dados(PDF) 5 Page - NXP Semiconductors |
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TDA8349A Folha de dados(HTML) 5 Page - NXP Semiconductors |
5 / 17 page February 1991 5 Philips Semiconductors Product specification Multistandard IF amplifier and demodulator TDA8349A bandgap also reduces the supply ripple on the video output to values less than −30 dB. The video amplifier has a typical bandwidth of 10 MHz which allows application for all new video standards with bandwidths of up to 10 to 12 MHz. The video output signal has an amplitude of 2 V (p-p). White spot protection comprises a white spot clamp system combined with a delayed-action inverter which is also highly resistant to high sound carriers. A switchable DC shift for positively modulated IF signals ensures correct signal handling. This switching is obtained via pin 20, which is the same pin used for switching the demodulation polarity in the demodulator. The circuit also has a noise clamp which prevents the video output becoming less than ±400 mV below the top sync level at noise peaks. The output buffer of the video amplifier consists of a class A/B circuit which can handle large source as well as large sink currents. This makes the circuit more flexible in several applications with one or more ceramic filters connected to this output buffer. AGC control circuit This converts the AGC detector voltage (pin 5) into a current signal which controls the gain of the IF amplifier. It also provides a tuner AGC control output from pin 4, current limiting is incorporated to prevent internal damage. The AGC starting point is adjusted by a voltage between 3 and 5 V for pnp tuners and between 7 and 9 V for npn tuners via pin 3. AGC circuit A new AGC system has been designed for the AGC. It will be a top sync-detector for negatively modulated signals and a top white level AGC for positively modulated signals. For optimal flexibility reasons the load and unload currents of the AGC are chosen such that both, a relatively fast set, as well as a set with a low tilt are possible for positive (L) and negative (B/G) modulated signals. For this reason a tilt ratio between positive (L) and negative (B/G) of approximately 3:1 has been chosen. This means that in a fast set the choice of a typical tilt for negatively modulated signals of 2% will obtain a typical tilt for positively modulated signals (L) of 6%. For a digital set which requires a small tilt the choice of tilt can be a factor of 5 or 10 smaller by increasing the AGC capacitor. The chosen AGC currents: Note 1. As long as no signal has been identified by the identification detector the unload current will be 50 µA. MODE UNLOAD CURRENT LOAD CURRENT TILT AT 2.2 µF B/G 50 µA 1.5 mA typ. 0.5% (line tilt) L 500 nA (note 1) 1.5 mA typ. 1.5% (field tilt) MAC(positive) 200 nA 1.5 mA typ. 1.2% (frame tilt) MAC(negative) 500 nA 1.5 mA typ. 1.5% (field tilt) Switching between the first three modes can be achieved by the system switch. This is a 3-level switch which when grounded selects B/G; open or 5 V selects L, and with pin 20 connected to VCC selects positively modulated MAC. The IC operates in a fourth mode if the identification capacitor at pin 19 is connected to VCC, it can be used for negatively modulated MAC. During channel switching a situation can occur that requires the AGC to increase the gain more than for example 50 dB. If this increase of gain has to be done for a positively modulated (L) signal, it will be achieved by the 500 nA load current and is therefore extremely slow. Because the identification information can be used to indicate that the signal is too small, in this event the identification circuit will mute, it is possible to increase the positive unload current to the same value as that used for negatively modulated signals. This switching is fully automatic and cannot be switched off. AFC circuit The AFC circuit consists of a demodulator stage which is fed with signals 90 ° out of phase. A very accurate internally realized 90 ° phase shift circuit makes it possible to use the demodulator IF regenerator tuned circuit for tuning the AFC circuit. To prevent video ripple on the AFC output voltage a sample-and-hold circuit is used for negatively modulated signals. The output signal of the demodulator is sampled during sync level of the video signal and will be stored with the aid of an external capacitor. |
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