UHF/VHF The tuner signal is capacitively coupled at the UHF/VHF-input and
The reference input REF should be decoupled to ground using a capacitor
of low series inductance. The signal passes through an asynchronous
divider with a fixed ratio of P = 8, an adjustable divider with ratio N = 256
through 32767, and is then compared in a digital frequency/phase
detector to a reference frequency fREF = 7.8125 kHz.
This frequency is derived from a balanced, low-impedance 4-MHz crystal
oscillator (pin Q1, Q2) by dividing its output signal by Q = 512.
The phase detector has two outputs UP and DOWN that drive the 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 (PD) 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 output transistor at UD 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 circuitry. UD may be switched off
by the control bit OS to allow external adjustments.
By means of a 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 in the different TV-bands can be compensated, for example.
P0, P1, P2 The software-switched outputs P0, P1, P2 can be used for direct band
selection (20 mA current output).
P4 and P7 are general-purpose open-collector outputs. The test bit T1 = 1
switches the test signal Cy (divided input signal) to P7.
Four different chip addresses can be set by appropriate connection of pin