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SP200/210 Maxon SP200/210 Radio
Issue 1.0 DETAILED FUNCTIONAL DESCRIPTION
03/01 Page 5-9
First Mixer
D9, T1 and T2 form a double balanced mixer which provides the 45.1MHz intermediate frequency
output. The filtered frequency from the front end module is coupled to T1.
The Local oscillator input from the VCO is coupled to T2.
The output of the mixer is taken from the tap on transformer T1 and fed to the single pole diplexer,
comprising L15 / C93 and R65 (High-pass terminating filter) and L14 / C92 (Low-pass coupling filter)
The 45.1MHz crystal filter provides a bandwidth of +/-7.2 kHz. This filter provides a high degree of
protection from spurious and intermodulation products. Additionally, a 90 MHz trap (C93 / L15) is
placed at the filter input.
The output of the filter is coupled by C43 to the base of the post filter IF amplifier Q25.
Second mixer, Second IF, FM detector
The output of the IF amplifier is fed into the narrowband FM IF Integrated Circuit, IC5 (MC3372). This
is a single conversion FM receiver which contains the second mixer, second IF amplifier, and FM
detector.
The second local oscillator frequency is determined by the crystal X1 connected to pin 1 of IC5. In this
case the crystal has a frequency of 44.645MHz. The first IF signal is applied to the mixer and the
resultant frequency of 455KHz, is the difference between the IF signal and second local oscillator.
The 455KHz IF signal is output from pin 3 and is applied to a 455KHz band-pass filter, CF1 (20/25 kHz
channel spacing) or CF2 (12.5 kHz channel spacing). The selection of the filters is accomplished by
diodes D13 (input) and D14 (output) whose bias is controlled by the N/S SW line on the
microcontroller (IC403, pin 25).
The microcontroller takes the relevant channel spacing data from the EEPROM. A High on the N/S
SW line switches the 455kHz signal to CF1, a low switches the signal to CF2.
The output of the relevant IF filter is passed to pin 5, which is the input to the limiting amplifier.
The limiting amplifier is biased externally by R19 & R95 and connected by C52 to the quadrature
detector (X2). The output connects to pin 8. The quadrature circuitry provides a 90° phase shift at the
IF centre frequency, which enables audio to be recovered. Any detected signal is produced at pin 9 of
IC5 and applied to the Receiver Audio Circuit and the Mute (Squelch) Circuit.
Squelch (Mute) Circuit
The mute circuit switches off the audio amplifier when no audio signal is present. The squelch circuit
consists of IC5 and RV2 (RV4) and their associated components. The noise signal from pin 9 of IC5
is amplified by an internal amplifier in IC5 and fed to the 16kHz bandpass filter.
16kHz Band Pass Filter
The audio signal from pin 9 of IC5 is filtered by a 16 kHz band pass filter consisting of L16, L17 / C111
to C113. The noise in the IF passband is accepted and voice frequencies and their products are
rejected.
Any noise present at the output of the filter is applied to the noise detector circuit via RV2 (RV4). RV2
(RV4) is used to adjust the squelch circuit sensitivity and is normally adjusted to produce a noise
squelch opening sensitivity of 10 to 12 dB SINAD.
For 12.5 kHz channel spacing, RV4 is switched in, by the microcontroller and Q28, to attenuate signal
at RV2 for 25 kHz channels. A High on the N/S SW line switches in RV4.
The output from the band pass filter is applied to the filter amplifier input on pin 10 of IC5.