Service Manual Part 1
8
The VCO output is fed to the first double balanced mixer via a low pass filter to reduce
harmonic content. The VCO signal is 49.lMHz below the RF signal and the first mixer
output (at 49.lMHz) feeds the first post-mixer buffer, Q103. The output of Q103 feeds
both a delay line and the noise blanker circuit.
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.
808 111+11:-11:.110114111'<#
The VCO output from the PIN switch is amplified to 2W by a three stage broad band
exciter (Q300, Q301 and Q302) with approximately 28dB of gain. The first stage (Q301 in
the T2000-500/600, and Q300 in the T2000-700) operates into compression to prevent
changes in VCO drive affecting output power.
This output power is controlled by varying the collector voltage on Q300, 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.
808 111+11:-11:.11:01141117!&
The 2W exciter output is amplified to 30W by Q305 and Q306. The T2000-500/600/700
power amplifier has no tuning, and when the output power is set to 25W the variation
across the switching range should be less than ±2W.
The high level RF signal passes via the directional coupler, the Tx/Rx PIN switch and
the low pass filter to the antenna connector. The power level is sensed by D301 on the
directional coupler and this voltage is fed back to the power control operational ampli-
fier via RV324. RV324 on the RF PCB sets the high power and RV507 on the logic PCB
will adjust the low power.