Codan NGT 2010 RF Technical & Service Manual page 148

NGT—Technical description (2011/2012)
Output stage and bias regulator
Drawing 04-03369, or
Drawing
The Class B push-pull output stage formed by V211 and V212 is driven from T203 to
produce the nominal 100 W or 125 W PEP.
The bias regulator V209 and V213 provides a voltage to the bases of V211 and V212.
The transistors V209 and V213 form a feedback voltage regulator. The output voltage is
the base–emitter voltage of V209 and is adjustable by the preset potentiometer R239.
The transistor V209 is mounted close to the output transistor and provides temperature
compensation to the bias network.
The transistor V210 modifies the output bias voltage slightly depending on the level of
signal detected by D208. The amount of control voltage is varied by R241 and is
adjusted to minimise high-order intermodulation products.
The balanced to unbalanced impedance-matching output transformer T204 couples the
power amplifier output to the band filters via J2 and the PTT relay K8.
Output filters
Drawing 04-03369, or
Drawing
The transmit frequency range is 1.6 to 30 MHz. Seven LPFs are used to remove the
harmonics generated by the PA. The filters are operated from IC2 and IC3 and are
selected using relays K1 to K7 as required.
The output of the filter circuit passes through the RF bridge formed by T1 and associated
components to the antenna output.
Automatic level control
The ALC is controlled by:
•
•
•
•
•
•
Drawing 04-03369, or
Drawing
All the ALC inputs (except relay detection) are applied to V7 to V10. The ORed output
is connected via R50 and D14 to the positive input pin of the ALC level comparator
IC5/A pin 3. The reference voltage connected to pin 2 is firmware controlled by the
microprocessor IC2.
114
04-03542
04-03542
forward power
reflected power
drain voltage swing of output stage
battery voltage
overtemperature of heatsink
failure of transmit/receive or filter relays
04-03542
NGT Transceiver System Technical Service Manual