User's Manual

SECTION 1: THEORY OF OPERATION
M32450-25-FCCRpt.doc Page 7

Receiver 1 IF
The major contributor of the IF subsystem (U20) 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 U20, 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 Y3
and provides the injection frequency for the mixer. The mixer output passes through a 455 KHz ceramic
filter (FL3). It is then amplified and passed through another ceramic filter (FL4) to a second gain stage.
The Diversity Reception Controller uses BRSSI1 to select the receiver with the best quality signal.
The audio is amplified by two (2) op amps (U19CD) 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 (RV7) 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 op amp (U1D). The output of U1D drives two (2)
amplifiers (U1B and U1C).
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 (U1B) has adjustable gain. The output drives op amp (U1A), 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.
The lower op amp (U1C) amplifies the signal from the low pass filter and applies it to the VCO via the
VCOMOD output. Pot RV2 and RV4 are used to adjust maximum deviation.
Injection Synthesizer
Two dual fractional synthesizer chips (U6 and U8) are the major contributor of the receiver and transmitter
injection oscillators. This device contains the key components of a phase locked loop (PLL), including a
prescaler, programmable divider, and phase detector. The selected frequencies are loaded into U6 and
U8 as a clocked serial bit stream via the PLL_DATA, PLL_CLOCK, and PLL_ENABLE signals.
Frequency stability is determined by a temperature-compensated crystal oscillator module (VCTCXO)
(Y1) at a frequency stability of 1 PPM from –30C to +60C. This device has an input (REFMOD) that
accepts transmit modulation and voltage from a RX FREQ ADJUST pot. The pot allows the receiver to
be fine-tuned to the exact operating frequency.
Two (2) voltage control oscillators (VCO) are formed by integrated low-noise oscillators with buffered
outputs (VCO1 and U9) and associated circuitry. The VCO’s generate receiver and transmit injection
signals. The output of U9 is split by a two-way power divider (U10) leading to outputs RXINJ1 and
RXINJ2. A second output of U9 is returned to the synthesizer FIN input via RXFB. This completes the
loop signal path.