User's Manual

Page 24
SVR-250 Service Manual
Theory of Operation
Receiver:
The receiver is a double-conversion superheterodyne type, designed for narrow band FM reception. The first
local oscillator is derived from the frequency synthesizer. The second LO is crystal controlled.
RF Stage: The incoming RF signal from the antenna jack is directed to the first RF bandpass filter to improve
selectivity and then to the input of the RF amplifier. The output of the RF amplifier is then presented to a second
bandpass filter.
First LO/Mixer: The first LO signal is developed by the synthesizer and is mixed with the incoming signal to
produce the first IF frequency (45 MHz). The IF frequency is filtered by FL1A and FL1B and amplified by the
IF amplifier before being presented to the second LO/Mixer IC.
Second LO/Mixer: The first IF signal is presented to the second IF IC (MC3371) which performs the functions
of second LO, second IF amp and mixer, FM discriminator and squelch. The second LO crystal (44.545 MHz)
is mixed with the 45 MHz first IF signal to produce the second IF frequency of 455 kHz. Two 6 pole ceramic filters
provides selectivity for the 455 kHz signal and are switchable between narrow band and wide band.
Detector/Squelch: The MC3371 demodulates the 455 kHz signal via quadrature coil to produce the audio and
noise components. The output of the MC3371 is the recovered audio and the RSSI voltage (receiver signal strength
indicator) which is compared by the controller board with a threshold voltage level for squelch setting.
VHF Transmitter
The output of VCO buffer U8 is input to the predriver transistor U7. The output of U7 drives the RF driver
Q8. The collector of Q8 is fed by the transmit 9V line from Q10. The final amp Q13 is a class C power amplifier
and drives the output lowpass and harmonic filter, C47-C50 and L20-L22. D3 is the transmit output switch and
L19 is a ¼ wave transmission line to isolate the receiver switching diode D2. RF output power is controlled by
changing the bias on the gate of Q13 via the TX 9V line (pin 3) from the controller PCB.
UHF Transmitter
The output of VCO buffer U6 is input to the predriver transistor U5. The output of U5 drives the RF driver
transistor Q4. The collector of Q4 is fed by the transmit 9V line from Q5. The final amp Q3 is a class C power
amplifier and drives the output lowpass and harmonic filter, C39-C42 and L26-L28. D3 is the transmit output
switch and L14 is a ¼ wave transmission line to isolate the receiver switching diode D2. RF output power is
controlled by changing the bias on the gate of Q3 via the TX 9V line (pin 3) from the controller PCB.
700/800/900 MHz Transmitter
The output of the transmit VCO is buffered by U5 input to the driver transistor U1. The output of U1 drives
the RF hybrid output amp U4 through lowpass filter FL3. The final amp U4 is a class C power amplifier and drives
the output lowpass and harmonic filter FL4. U7 is the Tx/Rx antenna switch. RF output power is controlled by
changing the voltage on pin 1 of U4 via the TX 9V line (pin 3) from the controller PCB.