Service Manual Part 1
8+
L114 and L116 form a 50Ω band pass filter which provides the spurious response rejec-
tion necessary for the second mixer. The local oscillator signal of 51.2MHz for the sec-
ond double balanced mixer is provided by the 12.8MHz reference via a times 4
multiplier 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.
818 111/11'<#
The VCO output from the PIN switch is amplified to 0.5W by a three stage broad band
exciter (Q300, Q301 and Q302) with approximately 21dB of gain. Q300 is operating into
compression to prevent changes in VCO drive affecting output power.
This output power is controlled by varying the voltage on IC405 pin2, 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.
818 111/117!&
The 0.5W exciter output is amplified to 20W by IC405. The T2000-800 power amplifier
has no tuning, and when the output power is set to 15W the variation across the 70MHz
switching range should be less than ±1W.
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.