2-2
THEORY OF OPERATION
2.0
Low Band Receiver
2.1
Receiver Front-End
The low band receiver is bandsplit into three ranges depending on radio model, covering frequencies
from 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to 50.0 MHz. The circuitry of the three models is
identical except for component value differences. The receiver consists of five major blocks: front-end
bandpass filters and pre-amplifier, first mixer, high-IF and blanker switches, low-IF and receiver back-
end, and “Extender” (noise blanker). Two fixed-tuned bandpass filters perform antenna signal pre-
selection. A cross over quad diode mixer converts the signal to the high - IF of 10.7 MHz. High-side
first injection is used.
Figure 2-1
Low Band Receiver Block Diagram
Demodulator
Delay
Filter
Mixer
Fixed
Tuned Filter
RF Amp
Tuned Filter
Pin Diode
Antenna
Switch
RF Jack
Antenna
First LO
RXINJ
Recovered Audio
RSSI
IF
Second LO
Crystal
Filter
455kHz Filter
455kHz Filter
455kHz Filter
(12.5kHz)
455kHz Filter
(12.5kHz)
Swit
c
h
Swit
c
h
Swit
c
h
Swit
c
h
Limiter
IF Amp
IF Amp
Filter Bank Selection
from Synthesizer IC
Blanker
Noise
Blanking Pulses
Gate
Buffer
Enable
1st
Fixed
(25/20kHz)
BWSELECT
(U1201 Pin 48)
4-pole
(25/20kHz)
