User Manual
SECTION 1: THEORY OF OPERATION
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Incoming signals pass through a double-tuned filter (FT4A) that selectively provides a high degree of out-
of-band signal rejection. A low noise amplifier (U6A) amplifies the selected signals and a triple-tuned filter
(FT3A) provides additional selectivity. An RF amplifier chip (Q6A) further amplifies the signal. The output
from Q6A passes through another triple-tuned filter (FT2A) and it goes to the mixer (U29A). U29A is a
double-balanced mixer which heterodynes the receive injection (RXINJ1) signal from the synthesizer with
the RF amplifier output. The result is a 45 MHz IF signal. Spurious signals are filtered out by FL5A, a 4-
pole bandpass filter, and the IF signal goes to the Receiver 1 IF section for further processing.
There are five (5) filter sets (FT2A, FT3A, and FT4A) available, and each one covers a 20-30 MHz portion
of the UHF band. Should replacement of the filters be required, exact replacement parts must be used.
Receiver 1 IF
This section consists of Receiver 1 IF subsystem. The major contributor of the IF subsystem is a
complete 45 MHz superheterodyne receiver chip that contains an RF amplifier, mixer, local oscillator, 455
KHz IF, quadrature detector, and received signal strength indication (RSSI) detector (U8A).
Incoming 45 MHz signals appearing at ISTIF pass through an amplifier and an LC matching circuit to a
mixer. A crystal oscillator is controlled by crystal Y1A and provides the injection frequency for the mixer.
This signal drives the oscillator in the Receiver 2 IF subsystem. The mixer output passes through a 455
KHz ceramic filter (FL6A). It is then amplified and passed through another ceramic filter (FL7A) to a
second gain stage. The IF output drives a quadrature detector. The recovered audio appears at pin 9 of
U8A. The RSSI detector converts the AGC voltage generated inside the chip into a DC level
corresponding logarithmically to the signal strength. The Diversity Reception Controller uses RSSI to
select the receiver with the best quality signal.
High frequency deemphasis is provided by a filter consisting of a resistor (R416) and a capacitor (C378).
The audio is amplified by op amps (U9AC and U9AA) and delivered to the baseband routing circuitry via
the BRXMOD1 output. Pot R19A is necessary to adjust the DC levels of U9AC and U9AA output.
Transmit Processing
The analog circuitry in this section modulates the Transmitter. There is also a fine-tuning frequency
adjustment for the Receiver.
The data-bearing audio signal from the modem appears at TXMOD. The audio is amplified by op amp
(U10A) and the DC output voltage of U10A is simultaneously biased to about half of the supply voltage by
the VBIAS input. VBIAS biases the other op amps in the same manner so they operate in a linear region.
This is necessary because these op amps operate from a single power supply. The output of U10A
passes through a low pass filter (R31) and the signal drives two (2) amplifiers.
The upper amplifier (U39B) has adjustable gain. The output drives op amp (U39A), which inverts the
phase of the signal allowing the modulated signal to pass through to the 10 MHz reference oscillator in
the synthesizer. Some makes of 10 MHz oscillators do not require the modulation signal to be inverted
and a jumper block (JMP1) is provided to accommodate the oscillators. The lower op amp (U10B)
amplifies the signal from the low pass filter and applies it to the VCO via the VCOMOD output.