User's Manual
SECTION 1: THEORY OF OPERATION
IPM8-FCCRpt.doc Page 7
Receiver 1 Front-End
This section contains components that include several RF Bandpass filters, a low-noise amplifier, and a
MMIC mixer.
Incoming signals pass through one (1) pre-selector band pass filter (FL3) that selectively provides a high
degree of out-of-band signal rejection. An RF amplifier (U35) amplifies the selected signals and is
followed by a SAW filter (FL4). The output from FL4 passes through a mixer (U4). U4 is a MMIC mixer
which mixes the receive injection (RXINJ1) signal from the synthesizer and the RF signal from the
antenna to produce a 45 MHz IF signal. This 45 MHz signal passes through a matched pair of monolithic
filters (FLT3 and FLT4) to the Receiver 1 IF section to provide the bulk of the Receiver’s selectivity.
Receiver 2 Front-End operates identical to Receiver 1 Front-End.
Receiver 1 IF
The major contributor of the IF subsystem (U34) 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.
Incoming 45 MHz signals appearing at RX1_45MHz pass through the low-voltage high performance
monolithic FM IF system. Within U34, 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 Y5
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.
The IF output drives a quadrature detector. The phase shift elements for the detector are a capacitor
(C140) and a ceramic filter (FLT6). 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 a quad rail-to-rail op amp (U19D) 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 pot (R99) is necessary to adjust gain.
Receiver 2 IF operates identical to Receiver 1 IF.
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 a quad rail-to-rail op amp (U9D). The output of
U9D drives two (2) amplifiers (U9C and U9B).
The transmitter uses dual-point modulation meaning the modulation is applied both to the VCO as well as
the reference oscillator (VCTCXO).
The upper op amp (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 VCTCXO
reference oscillator in the synthesizer. Some makes of VCTCXO oscillators do not require the modulation
signal to be inverted and a jumper block (JMP1) is provided to accommodate the oscillators.