User's Manual

F32700N25.doc Page 8
Receiver IF
The major contributor of the IF subsystem (U13) a complete 45 MHz super-heterodyne
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.
Incoming 45 MHz signals appearing at RX1_45MHz pass through the low-voltage high
performance monolithic FM IF system. Within U13, 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 Y2 and provides the injection
frequency for the mixer. The mixer output passes through a 455 KHz ceramic filter
(FL8). It is then amplified and passed through another ceramic filter (FL7) to a second
gain stage. One of the outputs of U13 is BRSSI1 that is the measure of receive signal
strength.
The audio is amplified by two (2) op amps (U9C and U9B) and delivered to the power
and analog ground 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 (RV3) is necessary to
adjust gain and pot (RV4) is used to adjust the level.
Transmit Processing
The audio DC level is adjusted through OPAMP (U19), then the signal is delivered to
the transmit modulator. The reference frequency correction data is applied to DAC
(U21). The output B of the DAC then scaled down to fine tune the frequency adjustment
to about 10 ppm over the entire range of the DAC. This DC signal is applied to the OP
amps as a reference voltage.
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 (U23D).
The output of U23D drives two (2) amplifiers (U23B and U23C).
The transmitter uses dual-point modulation meaning the modulation is applied both to
the VCO as well as the reference oscillator (VCTXO).
The upper amplifier (U23B) has adjustable gain. The output drives op amp (U23A),
which inverts the phase of the signal. Upon the start of a transmission, the modulating
signal passes through to the VCTXO reference oscillator in the synthesizer. Some
makes of VCTXO oscillators do not require the modulation signal to be inverted and a
jumper block (JMP1) is provided to accommodate the oscillators.