EFJohnson 9800 SERIES User Manual

3-22

February 2001

Part No. 001-9800-001

Over Current Shutdown

Current to final amplifier Q601 on the PA board 

is monitored by sensing the voltage drop across R680. 
Pins 5 and 6 of U500B are effectively connected 
across this resistor. As current increases, the voltage 
on U500B, pin 6 decreases which causes the output 
voltage on pin 7 to increase. The gain of each U500B 
input is set at ten by R509/R504 and R507/R502. 

Emitter biasing for Q501 is provided by R506 

and R511. Normally, the output voltage of U500B is 
not high enough to turn on Q501. However, if current 
becomes excessive, for example because of an antenna 
mismatch, Q501 begins turning on. This decreases the 
base voltage of Q502 which turns off Q500 slightly 
and cuts back power output.

3.10.1 INTRODUCTION

The synthesizer block diagram is part of the RF/

PA board diagram shown in Figure 3-7. The synthe-
sizer output signal is produced by a VCO (Voltage 
Controlled Oscillator) located on 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 
reference 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 800 and 
900 MHz channels. Therefore, with 900 MHz models, 
the reference divider in U804 divides the 14.850 MHz 
reference oscillator signal by 297, and with 800 MHz 
models, it divides the 17.500 MHz reference oscillator 

signal by 350. The TCXO frequency stability is 1.5 
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 3.10.6 for more information on U804
operation.

3.10.2 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 L907. 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 (Q902)

The VCO is formed by bipolar transistor Q902, 

ceramic resonator L907, and several capacitors and 
varactor diodes. It oscillates at the transmit frequency 
in the transmit mode and 52.950 (800 MHz) or 45 
MHz (900 MHz) below the receive frequency in the 
receive mode (the first injection frequency).

Biasing of Q902 is provided by R906 and R908, 

and stabilization is provided by R912. Inductor L906 
functions as an RF choke, and C924 is an AC bypass 
capacitor. An AC voltage divider formed by C913, 
C917, and C921 starts and maintains oscillation and 
matches Q902 to the tank circuit.

The tank circuit consists of laser tuned inductor 

L907, varactor diodes CR902-CR904, and several 
capacitors. Inductor L907 is laser trimmed to set the