Tait T850 II Series, Service Manual

Page: 252 / 370

D2.6 T858/859 Circuit Operation
M850-00
31/09/98 Copyright TEL
2.3.1 Power Control
The DC voltages from the directional coupler representing forward and reflected power
are buffered by the two voltage followers IC3 pins 1, 2 & 3 and pins 5, 6 & 7. Their out-
puts are summed at an integrator (IC1 pins 8, 9 & 10), which drives the series pass con-
trol element (Q16).
Forward and reflected power are summed so that, under high output VSWR, the power
control turns the PA down. This is because the control loop adjusts for the same DC
voltage from the directional coupler that would have been present if there were no
reflected power.
2.3.2 T859 Driver Power Level
A ceiling is placed on the output power available from driver stages Q1, Q3 and Q4 to
ensure final stages Q6 and Q7 are not overdriven. This is achieved by RV69 and IC4
controlling the reference voltage of power control error amplifier IC1c, which in turn
limits the maximum supply voltage that can be applied to driver Q4 by power control
transistor Q16.
Note: T859 PAs with serial numbers prior to 217262 do not have this feature.
Caution:
The driver power level clamp (RV69) is factory set to give a maximum
power output of 110W in the T859. The unit may be damaged if this
level is increased.
2.3.3 Thermal Protection
At excessively high temperatures, the output power will automatically reduce to a pre-
set level, thus preventing the PA from overheating.
A thermistor controlled voltage divider (R68, R72) applies a voltage to a comparator
with hysteresis (IC1 pins 12, 13 & 14).
Note: In all T858 PAs and T859 PAs with serial numbers prior to 217262, the
threshold of the comparator is independently set by RV69 which sets the
shutdown temperature. On later model T859 PAs this threshold is fixed.
The output current from the comparator is summed into the power control network via
RV74 so that the power level to which the PA must turn down may be set.