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F800 CIRCUIT DESCRIPTION
16 (25)
Uppgjord (även faktaansvarig om annan) -
Prepared (also subject responsible if other)
Nr
-
No.
KL/ECS/S/LT Magnus Lindahl
ECS/S/LT-96:5044
Dokansv/Godk -
Doc respons/Approved
Kontr
-
Checked
Datum
-
Date
Rev
File
KI/ECS/S/LTC
1996-06-06
A
965044.DOC
Squelch and AF circuits
The AF data signal from the 2:nd IF is connected to the CPU board and the option
board as well as to the squelch and AF filter circuits on the radio board. The AF data
signal has a straight frequency response from 0 - 3000 Hz. The AF data signal is also
passed through an amplifier stage acting as a band pass filter. The filtered signal is
called AUDIO and has a straight frequency response from 300 Hz to 3000 Hz.
To the input of the squelch circuit the AF data signal is connected. The actual squelch
function consists of one active high pass filter, a noise amplifier, a limiter and finally a
comparator. The high pass filter designed to allow frequencies higher then 8 kHz to
pass. After the HP filter there is a noise amplifier followed by a limiter. The limited AF
signal is rectified and then connected to one of the inputs of the comparator. The
other comparator input is connected to a reference value. The comparator output as in
high state when there is noise present at the input ( no carrier ). The squelch threshold
level is set by altering the amplification in the noise amplifier. The threshold level must
be set to 20 dB SINAD.
The SQ hysteresis is in some way dynamic, depending on the input noise level etc.
The hysteresis is set by a fixed resistor in the limiting amplifier stage. Normally the
hysteresis is 0,5 - 4 dB. The lower hysteresis level is present when the received
carrier is high ( = low noise level in the squelch circuit when carrier is present ). When
the received carrier has a lower level, close to the squelch threshold level, the
hysteresis is larger.
HP filter
8 kHz
-
+
SQ out
Limiter
Noise Amplifier
From
Detector
Fig K: IF circuits, block diagram.