Vco2 And Pll - Codan X-2 Service Manual

X-2 Technical Service Manual
Two separate outputs are available from the phase detector. PDB (pin 2 of
IC5) is a coarse control that outputs a mark to space ratio proportional to
the difference frequency between the divided signals. PDA (pin 1 of IC5) is
the fine control and gives an analogue output that takes over from PDB
when the two signals are close to phase lock.
Transistors V6, V7
The input at the base of V6 is biased to 2.5 V by H1. The output DC can
swing between ground and the positive rail and is used to control the
varicaps and consequently the frequency of the VCO.
When a new channel is selected, the microcontroller changes the value of the
programmable counter fed by the prescaler IC4. Because the two signals
driving the phase detector are now no longer at the same frequency, the
phase detector outputs pulses to the loop filter where the output ramps in the
direction necessary to establish lock.
4.4.3

VCO2 and PLL

104-02907 Sheet 1
VCO2
consisting of L15, C65 and C66 sets the appropriate frequency of operation.
The network consisting of C63, Z4, D8 and L17 is equivalent to a voltage
controlled, very high Q series tuned circuit. This network effectively
grounds the gate of V9 at the series resonant frequency and determines the
frequency of oscillation.
The drive to mixers IC8 and IC9 and the prescaler IC10 is via the capacitor
divider C66 and C67, using pin 5 of each IC.
The phase locked loop incorporating VCO2 works in a very similar manner
to PLL1. The main differences are:
•
•
Allowing the phase detector frequency to vary enables this loop to move in
10 Hz steps while having a high phase detector frequency.
The 7304 kHz reference oscillator output from pin 8 of IC5 is applied to pin
2 of PLL IC11.
The DC output at pin 1 of loop filter IC12/A
D8 [E6]
Publication No: 15-02047
with C38, R23, R26, and R27 form the loop filter.
[C3]
is a crystal controlled Colpitts oscillator. The tuned circuit
[E7]
only one phase detector output is used to drive the loop amplifier
IC11 pin 17
[G7]
the phase comparator varies from 1.1 kHz to 2.0 kHz.
and controls the frequency of the VCO.
Technical Description 4-11
[H7] , is applied to the varicap