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TDA9880 데이터시트(PDF) 7 Page - NXP Semiconductors |
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TDA9880 데이터시트(HTML) 7 Page - NXP Semiconductors |
7 / 40 page 1999 Jul 21 7 Philips Semiconductors Product specification Alignment-free multistandard vision and FM sound IF-PLL demodulator TDA9880 FUNCTIONAL DESCRIPTION Figure 1 shows the simplified block diagram of the integrated circuit. The integrated circuit comprises the following functional blocks: 1. VIF amplifier 2. Tuner-AGC and VIF-AGC 3. VIF-AGC detector 4. Frequency Phase-Locked Loop (FPLL) detector 5. VCO and Travelling Wave Divider (TWD) 6. Digital acquisition help and AFC 7. Video demodulator and amplifier 8. Sound carrier trap 9. Intercarrier mixer 10. FM demodulator and acquisition help 11. Audio amplifier 12. Internal voltage stabilizer. VIF amplifier The VIF amplifier consists of three AC-coupled differential amplifier stages. Each differential stage comprises a feedback network controlled by emitter degeneration. Tuner-AGC and VIF-AGC The AGC capacitor voltage is converted to an internal VIF gain control signal, and is fed to the tuner AGC to generate the tuner AGC output current at pin TAGC (open-collector output). The tuner AGC takeover point can be adjusted with RTOP. This allows the tuner to be matched to the SAW filter in order to achieve the optimum IF input level. VIF-AGC detector The AGC detector generates the required VIF gain control voltage for constant video output by charging or discharging the AGC capacitor. Gain control is performed by sync level detection. The newly developed AGC circuit provides fast reaction time to cope with ‘aeroplane fluttering’. The time constants for decreasing or increasing gain are nearly equal. Frequency Phase-Locked Loop (FPLL) detector The VIF amplifier output signal is fed into a Frequency Detector (FD) and into a Phase Detector (PD) 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 the frequency detector or the phase detector is converted into a DC voltage via the VIF-PLL filter, which controls the VCO frequency. VCO and Travelling Wave Divider (TWD) The Resistor Capacitor (RC) VCO operates as an integrated relaxation oscillator at double the picture carrier frequency. The control voltage required to tune the VCO to actually double the picture carrier frequency is generated by the FPLL detector and fed via the loop filter to the VCO control input terminal. The oscillator signal is divided-by-two with a TWD which generates two differential output signals with a 90 degrees phase difference independent of the frequency. Digital acquisition help and AFC The integrated relaxation oscillator has a very wide frequency range from approximately 30 to 70 MHz (after the TWD). To prevent false locking of the FPLL and with respect to the catching range of the frequency detector of maximum ±2.5 MHz, the Digital Acquisition Help (DAH) provides current into the loop filter until the VCO is in a frequency window of ±2.3 MHz around the wanted VIF frequency. In this case the analog operating FPLL will lock the VCO to the VIF carrier and the acquisition help does not provide any current to the loop filter. The principle of the digital acquisition help is as follows: the VCO is connected to a downcounter, which is preset depending on the required VIF frequency. The counting time, as well as the counter control, is derived from a 4 MHz reference signal. This signal can be supplied from the internal 4 MHz crystal oscillator or from the 4 MHz reference oscillator of an external tuning system. The counting result after a counting cycle corresponds to the actual VCO frequency. The digital AFC is also derived from the counting result after a counting cycle by digital-to-analog converting the last four bits of the counter. 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. |
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