Specifications

CIRCUIT ANALYSIS
RECEIVER
RF Amplifier
Incoming signals from the antenna jack are routed backwards through the transmitter lowpass filter in PIN diode
switch D12. In receive mode, D12 conducts allowing a low impedance path through the diode to the receiver front
end circuitry. The receiver RF amplifier section is comprised of two bandpass filter sections separated by an
amplifier based around Q1. These two filters allow signals at or near the operating frequency to pass but provide
strong rejection of the mixer’s spurious response frequencies. The first filter section is a two pole design formed
around RF transformers T1, T2 and their associated circuitry. This filter is followed by the RF amplifier transistor
Q1. This device is a dual-gate MOSFET which, with its low noise figure, yields good receiver sensitivity while
showing strong resistance to overload from strong signals. The output of Q1 drives a three-pole filter section formed
around T3, T4 and T5. The output of the RF amplifier stage is routed to the first mixer.
First Mixer and First IF Amplifier
The action of the first mixer transistor Q2 is to convert incoming signals at the operating frequency to the frequency
of the first IF which is 21.4 MHz. The output of the mixer is at a frequency which is equal to the difference between
the frequency of the incoming signal and the local oscillator. In this radio, the local oscillator signal is chosen to be
21.4 MHz below the operating frequency. The device chosen to perform the mixing operation is Q2, a high
performance JFET. The incoming signal is applied to the gate of Q2, and the local oscillator signal is filtered by
transformers T6, T7 and associated circuitry before being supplied to the drain of Q2. The difference frequency
signal at 21.4 MHz exits the mixer at the source of Q2 drives the first IF filter XF1 and XF2. XF1 and XF2 form a
4-pole monolithic crystal filter pair which in part determines the selectivity of the radio. The output of the crystal
filter is routed to the first IF amplifier formed around Q3. RF transformer T8 and T9 provide proper matching of the
crystal filters to insure good pass band response and selectivity.
Second Mixer, Second IF, and FM Detector
The output of Q3 is applied to the input (pin 16) of IC2. IC2 is a single conversion FM receiver on one integrated
circuit chip. The signal at the input is routed straight to a mixer which converts the incoming signal to the second IF
frequency of 455 kHz. The second local oscillator is formed with crystal X1 and circuitry within IC2. The output of
the second mixer is at pin 23 which is connected to a ceramic bandpass filter CF1 and centered at 455 kHz. This
filter, along with XF1 and XF2, determine the adjacent channel selectivity of the radio. The output of CF1 drives a
high gain IF amplifier chain internal to IC2 which in turn drives the quadrature detector. The output of the detector is
amplified and exits IC2 at pin 9.
Audio
Detected audio from IC2 passes through a lowpass filter formed around L7, C53 and C52 which removes IF
frequency components at 455 kHz. The audio signal then passes through buffer amplifier transistor Q6 before being
filtered by a two section, 4-pole high pass filter (IC3) This filter removes DCS and CTCSS low frequency tones
from the recovered audio. Transistors Q7 and Q8 act as switches around volume control VR2 to mute the audio
during squelched receive operation. The audio signal is finally routed to audio power amplifier IC5 and then to the
speaker.
MONOGRAM SERIES LBI-38864B
CIRCUIT ANALYSIS
Page -11-
Nov. 94