Rev. 1.0
Principles of Operation
4—11
The RF low-pass filter/reflectometer are located in the right-hand compartment on
the top of the chassis.
A ninth-order, elliptic, low-pass filter attenuates harmonics generated in the power
amplifier. The capacitors for the filter are circuit board pads.
The reflectometer uses printed circuit board traces for micro-strip transmission
lines. Transmission line segments (with an impedance of about 82 ohms) on either
side of a 50–ohm conductor provide sample voltages representative of the square
root of forward and reverse power.
DC voltages, representative of forward and reflected power, are routed through a
bulkhead filter board to the motherboard, then to the metering board, where they
are processed for power control and metering and for SWR metering and protection.
This option allows the transmitter to be used as a translator. The receiver board
receives terrestrially fed RF signal and converts it to composite audio which is then
fed into the exciter board. Microprocessor controlled phase lock loop technology
ensures the received frequency will not drift, and multiple IF stages ensure high
adjacent channel rejection.
The square shaped metal can located on the left side of the receiver board is the
tuner module. The incoming RF signal enters through the BNC connector (top left
corner) and is tuned through the tuner module. Input attenuation is possible with
jumper J1 on the top left corner of the receiver board. Very strong signals can be
attenuated 20 dB automatically by placing the jumper on the left two pins (“LO”
position). An additional 20 dB attenuation is also available with the jumpers in the
top left corner of the board. The frequencies are tuned by setting switches SW1 and
SW2 (upper right corner). The switches frequency range is 87.9 Mhz at setting “00”
to 107.9 Mhz at setting “64”. Other custom ranges are available. These two switches
are read upon power up by the microprocessor (U4). The microprocessor then tunes
Illustration 4-6 Receiver Board
Receiver
Module
RF In
