Maxon Telecom FM Mobile radio SM-2000 - Manual (Page 24)

The quadrature detector is also configurable for narrow or wide channel spacing by software control of
Q219 and diode D127. These components with R219 parallel R211 to maintain audio output at the proper
level for the selected bandwidth.
Q134 inserts wideband squelch adjustment RV102 into the circuit whenever standard filter CF104 is
selected. This tends to broaden the filter response and provide the higher frequency noise components
needed to activate the squelch.
Squelch (Mute) Circuit
The squelch output is sent to microcontroller pin 7 after processing by the squelch circuits. The squelch
circuit consists of a double-tuned BPF (Band Pass Filter) and also a noise detector circuit. Details of these
circuits are discussed next.
Squelch Noise Band Pass Filter
The output signal from pin 9 of IC109 (MC3371) is filtered to remove the 455 kHz IF variations. The
remaining audio is then filtered by a double-tuned BPF consisting of resonator L228-C191, resonator
L229-C193 and coupling capacitor C192. The noise in the IF passband is accepted and voice frequencies
are rejected. Any noise present at the filter output is applied to the noise detector via RV103. RV103 is
used to set the squelch circuit sensitivity and is normally adjusted to produce a noise squelch opening
sensitivity of 10dB to 12dB SINAD.
Noise Detector
The noise detector operating in conjunction with IC109 consists of Q132, Q133, D125, D129, TH101 and
their associated biasing components. Noise from RV103 is amplified by an op amp within IC109. This
noise sprectrum is further amplified by Q133 and rectified by D125. This output is buffered by Q132 and
fed to the SQ IN (IC109 pin 12) through R200. When the mute is activated, (IC109 pin9) goes low to pull
the base of inverter Q131 low. Q131 collector goes high thereby signalling the microprocessor to mute the
receiver audio.
Low Pass Filter
The audio output of (IC109 pin9) is fed to a LPF (Low Pass Filter) consisting of R206 and C201. This RC
attenuates 455 kHz IF energy which normally appears at the output of the FM receiver chip. The audio is
then routed to the input of IC406A LPF for audio processing.
Speaker Audio Amplifier
After signal detection and audio filtering, the low level audio is returned to pin 1 of IC108, the Audio PA.
IC108, a TDA2003 device, provides sufficient power gain to drive the mobile speaker. Digital transistor
Q137 mutes the audio amplifier by grounding R157 when the transistor turns on. When Q137 base is high,
the transistor is on and the mobile is muted. Conversely when Q137 is off, the mute is released and audio is
heard.
MAXON SM-2000 MOBILE
THEORY OF OPERATION
Page-20-
December 98