User Manual

SECTION 1: THEORY OF OPERATION
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Receiver 1 IF
This section consists of 1 IF subsystem. The major contributor of the IF subsystem (U33) a complete 45
MHz superheterodyne receiver chip incorporating a mixer/oscillator, two limiting intermediate frequency
amplifiers, quadrature detector, logarithmic received signal strength indicator (RSSI), voltage regulator
and audio and RSSI op amps (U33).
Incoming 45 MHz signals appearing at RX1_45MHz pass through a low-voltage high performance
monolithic FM IF system (U33). Within U33, the signals pass through a simple LC filter and are boosted
by the RF amplifier. The output of the RF amplifier drives a mixer. A crystal oscillator is controlled by
crystal Y4 and provides the injection frequency for the mixer. The mixer output passes through a 455
KHz ceramic filter (FL6). It is then amplified and passed through another ceramic filter (FL5) to a second
gain stage. The IF output drives a quadrature detector. The phase shift elements for the detector are
C123 and FLT5. 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 BRSSI1 to
select the receiver with the best quality signal.
The audio is amplified by an op amp and delivered to the baseband routing circuitry via the RXMOD1
output. High frequency de-emphasis is provided by a filter consisting of a resistor and a capacitor. In
order to match the audio signal levels with the other circuitry, a gain control is included. A pot is
necessary to adjust gain.
Transmit Modulation
The analog circuitry in this section modulates the Transmitter. The data-bearing audio signal from the
modem appears at TXMOD. The audio is amplified by op amp (U9D). The output of U9D drives two
amplifiers (U9B and U9C).
The upper amplifier (U9C) has adjustable gain. The output drives op amp (U9A), which inverts the phase
of the signal. Upon the start of a transmission, the modulating signal passes through to the 12 MHz
reference oscillator in the synthesizer. Some makes of 12 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 (U9B) amplifies the signal from the low pass filter and applies it to the VCO via the VCOMOD
output.
Pot RV1 and RV2 are used to adjust maximum deviation.
Baseband Routing
This circuitry routes the audio signal from a Receiver to the modem circuit. Provisions are also made to
route an analog modulation source attached to the radio to the transmitter.
During the receive operation, data-bearing audio signals from the two (2) receivers pass through an IC
analog switch (SW2). The microcontroller makes the TXKEYOUT* line high and the RX1/RX2* line high
or low to pass data from Receiver 1 or Receiver 2. The Receiver audio signal appearing on pin 5 of SW2
is routed directly to the modem circuit via DIVAUDIO. Simultaneously, the data-bearing audio signal is
routed through an eighth order Bessel filter (U6). The filter removes high frequency signals from the data.
In the transmit mode, when external modulation is selected with the DIP Switch, voice audio arriving on
pin 5 of the RS232 data connector appears at the input of SW2. The controller makes TXKEYOUT* low,
and RX1/RX2* high in order to pass the audio signal through SW2.