Midland Radio SP-300 - Manual (Page 15)

MAXON
SP-300 HAND HELD
Page -12-
May 2001
filters provide the adjacent channel selectivity of 25 kHz or
12.5 kHz bandwidth.
Squelch (mute) Circuit
The squelch circuit switches off the power amplifier when
no audio signal is present. The squelch circuit consists of a
16 kHz band pass filter and a noise detector circuit.
16 kHz Band Pass Filter
The audio signal from Pin 9 of IC5 (MC3371) is filtered by a
16 kHz band pass filter consisting of L16 and L17. 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.
RV2 is used to adjust the squelch circuit sensitivity and is
normally adjusted to produce a noise squelch opening sensi-
tivity of 10 dB to 12 dB SINAD.
Noise Detector
The noise detector in conjunction with IC5 consists of Q26,
Q27, D8, D11, TH1, and their associated biasing compo-
nents. Noise fed from the output of RV2 is amplified by
Q27, then rectified by D11. This output is then buffered by
Q26 and fed to Diode D8, which controls Q24 providing
ground to the mute control Pin 14 of IC5.
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 low level audio
is returned to the RF circuit via VR3. This is then routed to
Pin 3 of IC6, (LM386N-3), to provide speaker audio. IC6 is
enabled by a logic high applied to Q34 which in turn enables
Q33, applying power to Pin 7 of IC6.