TECHNICAL MANUAL FOR COMPACT VHF DSC RM2042

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RM2042CHANNEL-70 CARRIER DETECTThe 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 generatethe MODULATION DETECT signal, which is described later on.The carrier detect circuit is constructed as an ordinary noise triggered squelch, which in principle ismeasuring 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 frequencyof about 45 kHz, formed by the two operational amplifiers U8.1 and U8.2. The output signal from this filteris rectified by the double diode D4 and the resulting DC voltage is compared to a reference voltage bymeans of U4.3.The level of the rectified voltage and thereby the trigger level of the carrier detect circuit can be adjustedby the trimming resistor R36.If only a noise signal is received, the rectified input signal to the inverting terminal at the voltagecomparator will be large enough to keep the output low. But if a carrier is received, the output noise fromthe FM detector will be reduced and the comparator output will change to logical high.VHF CARRIER DETECTThe VHF carrier detect circuit is used to detect whether a carrier signal is received by the externalconnected VHF radiotelephone. This information is required to control the termination of a radiotelephonecontact on a working channel between a ship station and a cost station. According to the recommendation689 given by the international organisation CCIR, a radiotelephone contact, which is iniatated by a DSCcall, shall be considered to be complete, if the ship station equipment detects the absence of the coststation’s carrier for a periode greather than 5 seconds.As mentioned above the VHF carrier detect circuit is identical to the corresponding carrier detect circuitfor the build-in channel-70 receiver. The band pass filter is build-up around the two operational amplifiersU7.1 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 DETECTTo initialize the sampling of the received bit stream, a MODULATION DETECT signal is generated bymeans of the two NOR gates U9.2 and U9.3. The truth tabel for the MODULATION DETECT signal isgiven on the following page.Please note the bar above the signal name: “AF SIGNAL DETECT” in the truth tabel, which is indicatingthat this signal is inverted.9341PAGE 5-17