Receiver (Module 3) Part No. 626943 - Sailor RM2042 Technical Manual

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RM2042

5.3 RECEIVER (MODULE 3) PART NO. 626943

The receiver module contains a fully channel-70 receiver, FSK decoder/encoder, switches and controle
logic for automaticly change over between the build-in channel-70 receiver and the connected VHF
radiotelephone.
The block diagram of the build-in channel-70 receiver is shown below. This receiver is constructed in
accordance with the double super heterodyne principle, which is characterized by having two interme-
diate frequencies. The input signal to the receiver, which is an FM signal, is received by means of a
separate antenna connected directly to the RM2042.
First Mixer Second Mixer
Receiver
1. IF. 2. IF. Quadrature
Front-end
15,3 MHz 450 kHz Detector
156.525 MHz
f = 141,225 MHz f = 14,85 MHz
LO1 LO2
Channel-70 receiver
27677
RECEIVER FRONT-END
The receiver front-end is tuned to channel 70 (i.e. 156.525 MHz) and consist of the low noise dual gate
MOSFET transistor Q1 surrounded by two double tuned bandpass filters with high quality factor Q.
From the antenna the signal is led to the input bandpass filter, that consist of two resonance circuits, which
is critically coupled to each other by the series connection of the two capacitors C2 and C3.
The two resonance circuits is tuned by the two adjustable coils L1 and L2 and has a total bandwidth of
about 10 MHz. The two diodes D1 and D2 is located in the second section of the input bandpass filter to
protect the RF-amplifier from damage by high voltages.
The configuration of the intermediate filter at the output of the RF-amplifier is identical to the input
bandpass filter. The intermediate filter is tuned by the two adjustable coils L3 and L4 and has a bandwidth
of about 4 MHz.
The front-end selectivity gives the necessary attenuation of unwanted out of band signals, which as an
exampel could be a signal at the image frequency (i.e. f = 125.925 MHz).
IM
FIRST MIXER
The first mixer is an balanced active J-FET mixer with good large signal properties and low noise factor.
The signal from the receiver front-end is led through the unbalanced to balanced transformer TR1 to the
gates of the two J-FET's Q2 and Q3. These J-FET's are switched by injecting the 1st. LO signal to the
sources and a multiplum of the RF- and LO-signal is then generated. This new signal is led to the output
transformer TR2, where the wanted signal at the first intermediate frequency at 15.3 MHz is selected by
the tuned circuit consisting of TR2 and C24.
FIRST LOCAL OSCILLATOR
The first local oscillator is generating the injection signal at 141.225 MHz to the first mixer. The oscillator
is crystal controlled and is oscillating directly at the wanted LO frequency.
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