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TDA9887 데이터시트(PDF) 10 Page - NXP Semiconductors |
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10 / 58 page 2004 Aug 25 10 Philips Semiconductors Product specification I2C-bus controlled multistandard alignment-free IF-PLL demodulator with FM radio TDA9887 8 FUNCTIONAL DESCRIPTION Figure 1 shows the simplified block diagram of the device which comprises the following functional blocks: • VIF amplifier • Tuner AGC and VIF-AGC • VIF-AGC detector • Frequency Phase-Locked Loop (FPLL) detector • VCO and divider • AFC and digital acquisition help • Video demodulator and amplifier • Sound carrier trap • SIF amplifier • SIF-AGC detector • Single reference QSS mixer • AM demodulator • FM demodulator and acquisition help • Audio amplifier and mute time constant • Radio mode • Internal voltage stabilizer • I2C-bus transceiver and MAD (module address). 8.1 VIF amplifier The VIF amplifier consists of three AC-coupled differential stages. Gain control is performed by emitter degeneration. The total gain control range is typically 66 dB. The differential input impedance is typically 2 k Ω in parallel with 3 pF. 8.2 Tuner AGC and VIF-AGC This block adapts the voltages, generated at the VIF-AGC and SIF-AGC detectors, to the internal signal processing at the VIF and SIF amplifiers and performs the tuner AGC control current generation. The onset of the tuner AGC control current generation can be set either via the I2C-bus (see Table 13) or optionally by a potentiometer at pin TOP (in case that the I2C-bus information cannot be stored). The presence of a potentiometer is automatically detected and the I2C-bus setting is disabled. Furthermore, derived from the AGC detector voltage, a comparator is used to test if the corresponding VIF input voltage is higher than 200 µV. This information can be read out via the I2C-bus (bit VIFLEV = 1). 8.3 VIF-AGC detector Gain control is performed by sync level detection (negative modulation) or peak white detection (positive modulation). For negative modulation, the sync level voltage is stored at an integrated capacitor by means of a fast peak detector. This voltage is compared with a reference voltage (nominal sync level) by a comparator which charges or discharges the integrated AGC capacitor for the generation of the required VIF gain. The time constants for decreasing or increasing the gain are nearly equal and the total AGC reaction time is fast to cope with ‘aeroplane fluttering’. For positive modulation, the white peak level voltage is compared with a reference voltage (nominal white level) by a comparator which charges (fast) or discharges (slow) the external AGC capacitor directly for the generation of the required VIF gain. The need of a very long time constant for VIF gain increase is because the peak white level may appear only once in a field. In order to reduce this time constant, an additional level detector increases the discharging current of the AGC capacitor (fast mode) in the event of a decreasing VIF amplitude step controlled by the detected actual black level voltage. The threshold level for fast mode AGC is typically −6 dB video amplitude. The fast mode state is also transferred to the SIF-AGC detector for speed-up. In case of missing peak white pulses, the VIF gain increase is limited to typically +3 dB by comparing the detected actual black level voltage with a corresponding reference voltage. 8.4 FPLL detector The VIF amplifier output signal is fed into a frequency detector and into a phase detector via a limiting amplifier for removing the video AM. During acquisition the frequency detector produces a current proportional to the frequency difference between the VIF and the VCO signals. After frequency lock-in the phase detector produces a current proportional to the phase difference between the VIF and the VCO signals. The currents from the frequency and phase detectors are charged into the loop filter which controls the VIF VCO and locks it to the frequency and phase of the VIF carrier. For a positive modulated VIF signal, the charging currents are gated by the composite sync in order to avoid signal distortion in case of overmodulation. The gating depth is switchable via the I2C-bus. |
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