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TDA9812 датащи(PDF) 7 Page - NXP Semiconductors |
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TDA9812 датащи(HTML) 7 Page - NXP Semiconductors |
7 / 32 page 1995 Mar 21 7 Philips Semiconductors Preliminary specification Multistandard VIF-PLL and FM-PLL/AM demodulator TDA9812 FUNCTIONAL DESCRIPTION Vision IF amplifier The vision IF amplifier consists of three AC-coupled differential amplifier stages. Each differential stage comprises a feedback network controlled by emitter degeneration. Tuner and VIF AGC The AGC capacitor voltage is transferred to an internal IF control signal, and is fed to the tuner AGC to generate the tuner AGC output current (open-collector output). The tuner AGC take-over point can be adjusted. This allows the tuner and the SWIF filter to be matched to achieve the optimum IF input level. The AGC detector charges/discharges the AGC capacitor to the required voltage for setting of VIF and tuner gain in order to keep the video signal at a constant level. Therefore for negative video modulation the sync level and for positive video modulation the peak white level of the video signal is detected. In order to reduce the reaction time for positive modulation, where a very large time constant is needed, an additional level detector increases the discharging current of the AGC capacitor (fast mode) in the event of a decreasing VIF amplitude step. The additional level information is given by the black-level detector voltage. Frequency-Phase detector (FPLL) The VIF-amplifier output signal is fed into a frequency detector and into a phase detector via a limiting amplifier. During acquisition the frequency detector produces a DC current proportional to the frequency difference between the input and the VCO signal. After frequency lock-in the phase detector produces a DC current proportional to the phase difference between the VCO and the input signal. The DC current of either frequency detector or phase detector is converted into a DC voltage via the loop filter, which controls the VCO frequency. In the event of positive modulated signals the phase detector is gated by composite sync in order to avoid signal distortion for overmodulated VIF signals. VCO, travelling wave divider and AFC The VCO operates with a resonance circuit (with L and C in parallel) at double the PC frequency. The VCO is controlled by two integrated variable capacitors. The control voltage required to tune the VCO from its free- running frequency to actually double the PC frequency is generated by the Frequency-Phase detector and fed via the loop filter to the first variable capacitor (FPLL). This control voltage is amplified and additionally converted into a current which represents the AFC output signal. The VCO centre frequency can be decreased (required for L/L accent standard) by activating an additional internal capacitor. This is achieved by using the L/L accent switch. In this event the second variable capacitor can be controlled by a variable resistor at the L/L accent switch for setting the VCO centre frequency to the required L/L accent value. At centre frequency the AFC output current is equal to zero. The oscillator signal is divided-by-two with a Travelling Wave Divider (TWD) which generates two differential output signals with a 90 degree phase difference independent of the frequency. Video demodulator and amplifier The video demodulator is realized by a multiplier which is designed for low distortion and large bandwidth. The vision IF input signal is multiplied with the ‘in phase’ signal of the travelling wave divider output. In the demodulator stage the video signal polarity can be switched in accordance with the TV standard. The demodulator output signal is fed via an integrated low-pass filter for attenuation of the carrier harmonics to the video amplifier. The video amplifier is realized by an operational amplifier with internal feedback and high bandwidth. A low-pass filter is integrated to achieve an attenuation of the carrier harmonics for B/G and L standard. The standard dependent level shift in this stage delivers the same sync level for positive and negative modulation. The video output signal is 1 V (p-p) for nominal vision IF modulation. |
Аналогичный номер детали - TDA9812 |
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Аналогичное описание - TDA9812 |
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