Noise Mute (Squelch); Carrier Mute - Tait T880 II Series Adjustment Manual

B2.6 T885 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 6.2 and
The noise mute operates on the detected noise outside the audio bandwidth. Two oper-
ational amplifiers in IC330 are used as an active band-pass filter centred on 70kHz to fil-
ter out audio components and provide gain. Between the active filter stages is a
variable gain stage which utilises one of the remaining operational amplifiers in IC330.
The noise is then rectified (D330) 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
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 6.2 and
A high level carrier mute facility is also available. The RSSI (refer to
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.
10/07/00
Figure 2.2.)
Figure 2.2.)
Section 3.8). The mute control signal
Section
M880-00
2.12) pro-
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