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B2.6
T855 Circuit Operation
2.5
Noise Mute (Squelch)
(Refer to the audio processor and IF section circuit diagrams (sheets 2 and 3 respec-
tively) in Section 6.3 and Figure 2.2.)
The noise mute operates on the detected noise outside the audio bandwidth. An opera-
tional amplifier in IC310 is used as an active band pass filter centred on 70kHz to filter
out audio components. The noise spectrum is then further amplified in a variable gain,
two-stage amplifier (Q350 and Q360) with additional filtering. The noise is then recti-
fied (D310) and filtered to produce a DC voltage proportional to the noise amplitude.
The lowest average DC voltage corresponds to a high RF signal strength and the highest
DC voltage corresponds to no signal at the RF input.
The rectified noise voltage is compared with a threshold voltage set up on RV230, the
front panel "Gating Sensitivity" potentiometer. Hysteresis is provided by the feedback
resistor (R267) to prevent the received message from being chopped when the average
noise voltage is close to the threshold. R281 and R280 determine the mute opening and
closing times and, in combination with solder links SL210 and SL220, provide three time
delay options (SL210 is linked as standard. Refer to Section 3.8). The mute control signal
at pin 7 of IC270 is used to disable the speaker and line audio outputs. The speaker out-
put can be separately enabled for test purposes by operating the front panel mute disa-
ble switch, SW201.
2.6
Carrier Mute
(Refer to the audio processor and IF section circuit diagrams (sheets 2 and 3 respec-
tively) in Section 6.3 and Figure 2.2.)
A high level carrier mute facility is also available. The RSSI (refer to Section 2.12) pro-
vides a DC voltage proportional to the signal strength. This voltage is compared with a
preset level, set up on RV235, and may be linked into the mute timing circuit using
PL250. PL250 selects either the noise mute or the carrier mute. From this point both the
noise and carrier mute circuits operate in the same manner, using common circuitry.
© Copyright Tait Electronics Limited August 2004. All rights reserved.
M8SL2-00-002-812
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