Sailor RM2042 Technical Manual page 73

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RM2042
CHANNEL-70 CARRIER DETECT
The receiver module is constructed with two identicale carrier detect circuits - one for the build-in channel-
70 receiver and one for the external connected VHF radiotelephone.
The carrier detect circuit for the build-in channel-70 receiver is included to avoid emission of a DSC call,
while an other unit is transmitting. In addition the CH70 CARRIER DETECT signal is used to generate
the MODULATION DETECT signal, which is described later on.
The carrier detect circuit is constructed as an ordinary noise triggered squelch, which in principle is
measuring the noise level above the upper frequency component in the information signal.
From the output of the FM detector, the received signal is led to a band pass filter with a center frequency
of about 45 kHz, formed by the two operational amplifiers U8.1 and U8.2. The output signal from this filter
is rectified by the double diode D4 and the resulting DC voltage is compared to a reference voltage by
means of U4.3.
The level of the rectified voltage and thereby the trigger level of the carrier detect circuit can be adjusted
by the trimming resistor R36.
If only a noise signal is received, the rectified input signal to the inverting terminal at the voltage
comparator will be large enough to keep the output low. But if a carrier is received, the output noise from
the FM detector will be reduced and the comparator output will change to logical high.
VHF CARRIER DETECT
The VHF carrier detect circuit is used to detect whether a carrier signal is received by the external
connected VHF radiotelephone. This information is required to control the termination of a radiotelephone
contact on a working channel between a ship station and a cost station. According to the recommendation
689 given by the international organisation CCIR, a radiotelephone contact, which is iniatated by a DSC
call, shall be considered to be complete, if the ship station equipment detects the absence of the cost
station's carrier for a periode greather than 5 seconds.
As mentioned above the VHF carrier detect circuit is identical to the corresponding carrier detect circuit
for the build-in channel-70 receiver. The band pass filter is build-up around the two operational amplifiers
U7.1 and U7.2, while U4.4 is used as the voltage comparator.
The trigger level of the VHF carrier detect circuit is adjusted by the trimming resistor R93.
MODULATION DETECT
To initialize the sampling of the received bit stream, a MODULATION DETECT signal is generated by
means of the two NOR gates U9.2 and U9.3. The truth tabel for the MODULATION DETECT signal is
given on the following page.
Please note the bar above the signal name: "AF SIGNAL DETECT" in the truth tabel, which is indicating
that this signal is inverted.
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