Operations Manual

Page 6 RF Technology PA50
5 CIRCUIT DESCRIPTION
R213, R230 and R208, R215 provide the bias voltage to the transistors so that quiescent
current is maintained across the operating temperature range.
5.5 Reflectometer
The reflectometer provides an indication of load mismatch. A200 is a current
transformer with a single turn primary being passed through the centre of a toroid. The
multiturn secondary has an output voltage that is proportional to the current flowing in
the primary. The voltages appearing at the anodes of D205 and D206 are of equal
amplitude and opposite phase. A sample of the line voltage is applied to the junction of
the divider resistors R218 and R220. C209 is adjusted for equal amplitude with the
transformer. Since the transformer has 180 degree phase shifted outputs, the application
of the line voltage will cause cancellation at D205 and addition at D206. This phase
relationship will change when a mismatch occurs resulting in an increase in the voltage
at D205. D205 and D206 rectify these voltages so that a DC level proportional to the
forward and reflected voltages are available for the metering circuits. L200 provides a
DC reference point for the diodes.
5.6 Metering Circuits
These circuits are included so that the exciter can interrogate the PA and read all the
vital operating levels. U203 is an A to D converter with the following inputs:
Reverse voltage
Forward voltage
Temperature
DC Input volts
O/P drain current
Driver drain current
Q100 bias voltage
Q101 bias voltage
Model identification jumpers
Forward and reverse voltages are supplied from the reflectometer outputs and factory
set to the correct levels by R227 and R228. U204 ( temperature sensor ) is mounted on
the same pad as the source lead of Q101 to provide a temperature reading of the O/P
transistor cases. DC Power Input voltage is provided by a resistive divider (R3 and R4).
The O/P drain current is provided by measuring the small voltage drop across R113 and
R128. Driver drain current is provided by measuring the small voltage drop across R112
and R138. The bias voltages are read directly from the bias supplies. The model
identification is determined by the jumper settings of J203, 204 and 205.
PA50A 25 – 32 MHz J203 on, J204 and J205 off
PA50B 30 – 40 MHz J204 on, J203 and J205 off
PA50C 38 – 50 MHz J205 on, J203 and J204 off
U202 and associated components provide a 3 wire serial interface to allow the exciter to
interrogate the A to D converter. U200C is a comparator that switches the fan on when
the temperature rises above 40 degrees C. U201D is a comparator that switches the