Tait T830 Series, Service Manual

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31/01/96 Copyright TEL
2.5 Noise Mute (Squelch)
(Refer to Figure 2.2.)
The noise mute operates on the detected noise outside the audio bandwidth. An opera-
tional amplifier in IC304 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 (Q306 & Q307) with additional filtering. The noise is then rectified
(D301) 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 RV100, the
front panel mute potentiometer. Hysteresis is introduced by the feedback resistor
(R106) to prevent the received message from being chopped when the average noise
voltage is close to the threshold. R111 and R110 determine the mute opening and clos-
ing times. The mute control signal at pin 7 of IC100 is used to disable the speaker and
line audio outputs. The speaker output can be separately enabled for test purposes by
operating the front panel mute disable switch, SW100.
The mute control line is available on pad 101 (Rx gate out) for control of external cir-
cuitry. A high (9V) on pad 101 indicates that the audio is disabled and a low (0V) indi-
cates that a signal above the mute threshold level is being received.
The audio can also be disabled using the "Rx-disable" inputs, pads 100 or 113, having
connected the "Rx-disable" link between pins 1 & 2 of PL100. An adjustable time delay
(RV101) is provided on these lines. In order to disable the audio, either pad must be
pulled to 0V.
The red front panel LED (D102) indicates the status of the mute circuit. When a signal
above the mute threshold is received, the LED is illuminated. An undedicated relay is
provided (RL100) for transmitter keying or other functions and this can be operated
from the mute line by linking PL102.
2.6 Carrier Mute
(Refer to Figure 2.2.)
A high level carrier mute facility is also available. The RSSI (refer to Section 2.10) pro-
vides a DC voltage proportional to the signal strength. This voltage is compared with a
preset level, set up on RV104, and may be linked into the mute timing circuit using
PL104. PL104 selects either the noise mute or the carrier mute. From this point both
mute circuits operate in the same manner, using common circuitry.