EFJohnson 001-9800-001 User Manual

3-11

February 2001

Part No. 001-9800-001

The functions controlled by U2 are as follows: 

•

Display controller U1 programming

•

Backlight control

•

Transmit/Busy indicator CR4 control

•

Front panel option switch detection

•

Front panel Select switch detection

•

Microphone hanger off-hook detection.

The use of a separate microcontroller on the 

display board minimizes the number of interconnec-
tions required with the audio/logic board because 
separate lines are not required for each of the 
preceding functions.

When the front panel Select switch is pressed, the 

two PBNO lines are shorted together. Then when it is 
rotated clockwise, low pulses appear on the CW line, 
and when it is rotated counterclockwise, low pulses 
appear on the CCW line. The contrast and viewing 
angle of the display are set by potentiometer R46.

3.7.1 INTRODUCTION

The synthesizer block diagram is part of the RF/

PA board diagram shown in Figure 3-4. The synthe-
sizer output signal is produced by a VCO (Voltage 
Controlled Oscillator) located in a separate module 
attached to the RF board. The frequency of the VCO is 
controlled by a DC voltage from the phase detector in 
synthesizer integrated circuit U804. 

The phase detector senses the phase and 

frequency difference between a highly stable signal 
from the reference oscillator (fR) and a frequency 
produced by dividing down the VCO signal (fV). 
When the signal from the VCO is the same as the refer-
ence frequency, the VCO is on the correct frequency. If 
the VCO-derived signal is not the same, the VCO 
control voltage increases or decreases to change the 
VCO frequency until they are the same. The VCO is 
then “locked” on frequency.

The reference input (fR) to the phase detector is 

produced by dividing down the signal from reference 
oscillator U806. The fR input is 50 kHz for all UHF 
channels. Therefore, the reference divider in U804 

divides the 14.850 MHz reference oscillator signal by 
297. The TCXO frequency stability is 2.0 PPM, so this 
is also the stability of the synthesizer (and the second 
injection signal which is derived from the TCXO 
frequency).

The VCO-derived input to the phase detector (fV) 

is the VCO frequency divided down by programmable 
dividers in synthesizer U804. The prescaler and main 
divider are programmed for each channel to produce 
an input frequency to the phase detector (fV) that is 
the same as the 50 kHz reference frequency (fR) when 
the VCO is oscillating on the correct frequency. Refer 
to Section 3.7.5 for more information on U804 
operation.

3.7.2 VOLTAGE-CONTROLLED OSCILLATOR

Introduction

The VCO module is a separate assembly that is 

soldered directly to the RF board and covered by a 
metal shield. It uses a ceramic substrate that can easily 
be damaged by excessive heat; therefore, it is recom-
mended that modules which have been removed using 
a standard soldering iron not be reused. In addition, the 
VCO center frequency is set by laser tuning ceramic 
resonator L101. Therefore, it is not possible to adjust 
this frequency if it changes as the result of changing a 
part. For these reasons, the VCO is considered not field 
serviceable.

Oscillator (Q102)

The VCO is formed by bipolar transistor Q102, 

ceramic resonator L101, and several capacitors and 
varactor diodes. It oscillates at the transmit frequency 
in the transmit mode and 45 MHz below the receive 
frequency in the receive mode (the first injection 
frequency).

Biasing of Q102 is provided by R108 and R112, 

and stabilization is provided by R114. Inductor L102 
functions as an RF choke, and an AC voltage divider 
formed by C112, C115, and C119 starts and maintains 
oscillation and matches Q102 to the tank circuit.

The tank circuit consists of laser tuned inductor 

L101, varactor diodes CR101, CR103-CR106, and 
several capacitors. Inductor L101 is laser trimmed to