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Maxon SP200/210 Radio SP200/210
DETAILED FUNCTIONAL DESCRIPTION Issue 1.0
Page 5-10 03/01
Noise detector circuit
The noise detector circuit, in conjunction with IC5, consists of transistors Q26, Q27, thermistor TH1
and diode D11. Any noise signal present is applied to Q27 from pin 11 of IC5. The signal is amplified
by Q27, rectified by D11 and then buffered by Q26. The buffered signal is applied to pin 12 of IC5
(Squelch input).
The squelch trigger output (pin 14, IC5) is applied to the microcontroller BUSY input on pin 54 of
IC403. The actual audio mute is from the microcontroller.
When noise is present, the voltage at pin 12 of IC5 (Squelch input) exceeds 0.7V. This causes the
squelch trigger output to go open circuit, which turns on Q24 and the busy line goes low, which mutes
the receiver audio circuit.
When no noise is present, the voltage at pin 12 of IC5 is less than 0.7 V and pin 14 of IC5 is
grounded, which switches Q24 off, so the busy line goes high, unmuting the receiver audio circuit.
Resistor R74 is used to provide hysteresis of 3 to 6 dB.
AF Output Low Pass Filter
A low pass filter formed by C115, C116 and R91 removes any extraneous 455 kHz energy from the
AF output of the FM receiver chip.
Speaker Audio Amplifier
After signal detection and audio filtering the signal is passed to the digital board, to the audio ASIC
(via IC408 for SAT). ). After the audio ASIC (see previous description) the signal is then routed, via
the volume control (VR5), to pin 8 of IC402. IC402 is a TDA7233 audio amplifier, which is enabled by
the control line from the microcontroller via Q415 and Q414.