Tait T850 II Series, Service Manual

Page: 152 / 370

C2.12 T856/857 Circuit Operation
M850-00
31/09/98 Copyright TEL
2.8 T856 Drive Amplifier & PA
(Refer to Figure 2.1 and the exciter and PA circuit diagrams (sheets 3 & 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 (Q350, Q355). 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 & 5)
and then summed with the ’power control’ level and fed to the control integrator (IC350
pin 6). The output control voltage is buffered by Q310 and Q315, and applied to the col-
lector of the wide band exciter amplifier.
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:
• Q325, IC350 power ramping
• Q355, Q325 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) is provided so that the RF output power can be reduced to
a preset level when a set temperature is exceeded. This is a protection circuit (IC350 pin
1, Q320) to prevent overheating, as the unit is
not
rated for continuous operation (refer
to Section 1.2.3 for duty cycle specifications).
#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.
The RF output from the exciter is fed to the driver stage (Q410) and then to the final
(Q420). DC is fed to the final via a low pass filter with special low frequency decou-
pling. CV475 tunes the output matching across the entire band.