C2.12 T854 Circuit Operation M8SL2-00-002-812
© Copyright Tait Electronics Limited August 2004. All rights reserved.
2.8 T854 Drive Amplifier, Fan Control and PA
Refer to Figure 2.1 and the exciter and PA circuit diagrams (sheets 3 and 4) in Section
6.2.
The output power of the PA is maintained at a constant level via a power control loop
applied to the two-stage, wide band exciter amplifier (Q337, Q336). The forward and
reverse RF power levels are sensed via a dual directional coupler and detector diodes
(D440, D420 in the PA cavity). The detected DC signals are buffered (IC330 pins 3 and 5)
and then summed with the ‘power control’ level and fed to the control integrator (IC350
pin 6).
Note: Forward and reflected power signals are summed so that, under high
VSWR, the power control will turn the output RF level down.
To reduce the spurious output level when the synthesiser is out-of-lock, the Tx-Reg. and
Lock-Detect signals are gated to inhibit the PA control circuit and to switch off the RF
signal at the input to the drive amplifier. This is achieved by a PIN switch attenuator
(D340, D380, D360).
Cyclic keying control is provided by additional circuitry consisting of several time
delay, ramp and gate stages:
• Q330, IC350 power ramping
• Q326, Q325, Q341 Tx-Reg. and Lock-Detect
gate
• Q335, Q340, Q345 delay and PIN switch drive.
This is to allow the RF power circuits (both exciter and PA) to ramp up and down in a
controlled manner so that minimal adjacent channel interference is generated during
the transition.
The output of the wide band amplifier is approximately 1W (+30dBm) for an input of
100mW (+20dBm) from the VCO, when the power control is set to maximum.
A temperature sensor (R481) in the PA cavity presents a voltage to the comparators of
IC340 pin 2 and IC350 pin 2. The fan is switched on and off by the comparator of IC340
and the power fold-back is enabled by the comparator of IC350. Power fold-back
reduces the output power to a preset level that protects the PA semiconductors from
excessive temperatures. Under normal operating conditions, the fan will be activated
before power fold-back can occur and it will prevent the temperature from reaching
damaging levels. The voltage divider ratio of R311 and R312 sets the the temperature at
which the fan is switched on and off. R306 and R340 set the temperature at which power
is reduced. Depending on ambient temperature and ventilation, it may take several
minutes of continuous operation for the fan to be activated.
#R517, #R518 and #R519 form an attenuator to provide good VCO/exciter isolation as
well as the correct exciter drive level.
The attenuator (#R395, #R396 and #R397) aids in producing the correct exciter drive
level to the PA over the three frequency ranges.
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