Service Manual Part 1
8.
L114 and L116 form a 50Ω delay line which provides the spurious response rejection
necessary for the second mixer. L114 and L116 also provide the delay necessary for the
noise blanker to detect and clamp noise pulses. The local oscillator signal of 38.4MHz
for the second double balanced mixer is provided by the 12.8MHz reference via a tripler
circuit, and the 10.7MHz output feeds the plug-in IF PCB. Each sub-block within the
front end has been designed with 50Ω terminations for ease of testing and fault finding.
8-8 111114"11'<#
The VCO output from the PIN switch is amplified to 900mW by a two stage broad band
exciter (Q301 and Q302) with approximately 22dB of gain. Q301 is operating into com-
pression to prevent changes in VCO drive affecting output power.
This output power is controlled by varying the collector voltage on Q302, via Q303,
Q304 and the feedback loop from the PA. The power settings (maximum, high and low)
are under microprocessor control and are set for each channel by programming.
R308, R309, R310 and R313 form a 2dB attenuator, reducing the output to approximately
550mW and also providing good isolation between the exciter and PA.
8-8 111114"117!&
The 550mW exciter output is amplified to 30W by Q305 and Q306. Q305 has a small
amount of bias applied (via R319, D302, R322 and R315) to provide stability at low
power levels.
The T2000-100 power amplifier has no tuning, and when the output power is set to 25W
the variation across the 50MHz switching range should be less than ±2W.
The high level RF signal passes via the Tx/Rx PIN switch, the low pass filter and direc-
tional coupler to the antenna connector. The power level is sensed by D301 on the direc-
tional coupler and this voltage is fed back to the power control operational amplifier via
RV324. RV324 on the RF PCB sets the high power and RV507 on the logic PCB will
adjust the low power.