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5.3 Power Amplifier
The 12.8MHz output of XO1 is amplified by Q22 to drive the reference input of the
PLL synthesizer IC U6. This IC is a single chip synthesizer which includes a 1.1 GHz
pre-scaler, programmable divider, reference divider and phase/frequency detector. The
frequency data for U6 is supplied through serial data link by the microprocessor.
The phase detector output signals of U6 are used to control two switched current
sources. The output of the positive and negative sources (Q3 and Q6) produces the
tuning voltage which is smoothed by the loop filter components to bias the VCO
varactor diode D3.
5.3
Power Amplifier
The 4 mW output from the main board connects to the power amplifier board through a
short miniature 50Ω coaxial cable.
Q2 on the power amplifier board increases the signal to approximately 200 mW. The
bias current of Q2 is controlled by Q1 and the power leveling circuitry to adjust the
drive to the output module U2.
U2 increases the power to 10--30 watts (depending upon options) before it is fed to the
directional coupler, low pass filter and output connector. The directional coupler
detects the forward and reverse power components and provides proportional dc
voltages which are amplified by U1a and U1b. The forward and reverse voltages from
U1a and U1b are compared to the DC reference voltage from RV1. The difference is
amplified by U1c, Q3 and Q4.
The resulting control voltage supplies Q2 through R10, R12 and completes the power
leveling control loop.
5.4
Temper atur e Pr otection
Thermistor RT1 on the power amplifier board is used to sense the case temperature of
the output module U2. If the case temperature rises above 90 degrees C., the voltage
across RT1 will increase and transistor Q5 will be turned on.
reference voltage to the power regulator which in turn reduces the outpower by 6-10dB.
5.5
600ΩLine Input
The 600Ω balanced line input connects to line isolation transformer T1. T1 has two
150Ω primary windings which are normally connected in series for 600Ω lines. The
dual primary windings can be used to provide DC loop PTT signaling or a 2/4 wire
hybrid connection. All four leads are available at the rear panel system connector.
The secondary of T1 can be terminated with an internal 600Ω load through JP5 or left
un-terminated in high impedance applications.
Page 14
5 CIRCUIT DESCRIPTION
This reduces the dc
RF Technology T220
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