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TUA6012 Datasheet(PDF) 11 Page - Siemens Semiconductor Group |
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TUA6012 Datasheet(HTML) 11 Page - Siemens Semiconductor Group |
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11 / 31 page  TUA 6012 TUA 6014 Semiconductor Group 11 1998-09-01 2 Circuit Description 2.1 Mixer-Oscillator Block The mixer oscillator section includes two balanced mixers (double balanced mixer), two balanced oscillators for VHF low and/or VHF high band and UHF, a reference voltage source and a band switch. Filters between tuner input and IC separate the TV frequency signals into two bands. The band switching in the tuner front-end is done by using two, three or four port outputs. In the selected band the signal passes a tuner input stage with MOSFET amplifier, a double-tuned bandpass filter and is then fed to the balanced mixer input of the IC which has in case of VHF low/VHF high a high-impedance input and in case of UHF a low- impedance input. The VHF low/VHF high input can be used unsymmetrically by capacitively grounding one of the input pins. The input signal is mixed there with the signal from the activated on chip oscillator to the IF frequency and is available at the balanced high-impedance output pair (IFout/IFout). 2.2 PLL Block The mixer-oscillator signal VCO/VCO is internally DC-coupled as a differential signal at the programmable divider inputs. The signal subsequently passes through a programmable divider with ratio N = 256 through 32767 and is then compared in a digital frequency/phase detector to a reference frequency f ref = 4 MHz / reference divider ratio ( f ref = 31.25 kHz, 50 kHz or 62.5 kHz). This frequency is derived from a unbalanced, low- impedance 4 MHz crystal oscillator (pin Q) divided by reference divider ratio (programmable reference divider ratio = 128, 80 or 64). The phase detector has two outputs, UP and DOWN that drive two current sources I+ and I- of a charge pump. If the negative edge of the divided VCO signal appears prior to the negative edge of the reference signal, the I+ current source pulses for the duration of the phase difference. In the reverse case the I- current source pulses. If the two signals are in phase, the charge pump output (CHGPMP) goes into the high-impedance state (PLL is locked). An active low-pass filter integrates the current pulses to generate the tuning voltage for the VCO (internal amplifier, external pullup resistor at TUNE and external RC circuitry). The charge pump output is also switched into the high-impedance state when the control bit T0 = 1. Here it should be noted, however, that the tuning voltage can alter over a long period in the high-impedance state as a result of self- discharge in the peripheral circuity. TUNE may be switched off by the control bit OS to allow external adjustments. If the VCO is not working the PLL locks to a tuning voltage of 33 V. By means of control bit 5I the pump current can be switched between two values by software. This programmability permits alteration of the control response of the PLL in the locked-in state. In this way different VCO gains can be compensated, for example. |
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