Local Oscillators; Introduction; Vco1 And Pll - Codan X-2 Service Manual

Technical Description 4-10

4.4 Local Oscillators

4.4.1

Introduction

104-02907 Sheet 1
Two digitally controlled synthesized local oscillators drive the first and
second mixers. VCO1 operates between 47 MHz and 63 MHz, moving in
2 kHz steps. VC02 operates between 44.5435 MHz and 44.5455 MHz,
moving in 10 Hz steps.
The synthesizers are each programmed in serial data format from the
microcontroller which accesses channel data stored in memory.
Each synthesizer is locked to the reference oscillator. This consists of a
7.304 MHz crystal oscillator held at a constant temperature of 60°C by a
PTC thermistor oven.
4.4.2

VCO1 and PLL

VCO1
and V4. The frequency of oscillation is determined by the tuned circuit L7,
C23 and the capacitance of the varicaps D1 to D4. Oscillator output power
is set by R14 and R15. Resistor R12 compensates the output level over the
frequency range.
Capacitor divider C29 and C30 couple the oscillator output to the buffer
amplifier V5, where the output drives the two mixers IC1 and IC2 and the
prescaler IC4. In each case pin 5 is used as the input.
The high frequency output from VCO1 is divided by 64/65 prescaler IC4
down to a frequency range of 734 kHz to 984 kHz at pins 2 and 3. A high
or low on pin 4 sets the division ratio.
IC5 is a complex PLL chip that contains two programmable dividers and a
phase comparator. The phase comparator compares two input signals of the
same frequency and outputs a voltage which is dependant on their relative
phase.
One input to the comparator IC5 on pin 4 is from the prescaler IC4. This is
divided to 2 kHz by one of the programmable dividers. Quartz crystal Z3
connected between pins 7 and 8 forms the reference oscillator set to a
frequency of 7304 kHz. The reference oscillator is divided down to 2 kHz
by the second fixed programmable divider.
The two divided signals are applied to the phase comparator and when the
two signals are 'locked' in phase the comparator output is at mid-rail
(2.5 V). As these two frequencies are locked together by the action of the
loop, changing either programmable divider will change the VCO frequency.
is designed around a differential amplifier consisting of FETs V3
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X-2 Technical Service Manual
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Publication No: 15-02047