TECHNICAL MANUAL FOR COMPACT VHF DSC RM2042

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5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RM2042SIGNAL SWITCHES AND CONTROL LOGICThe receiver module contains two analog switches, which are included in the CMOS-IC U6.The first switch is only used for loop back test purpose and is controlled by the microprocessor.The second switch is used for switching between the build-in channel-70 receiver and the connected VHFradiotelephone. This switch is controlled by the logical signal Q, which is generated by combinating theoutput of the channel-70 carrier detect circuit and the two logic signals A and B.The function of the signal switches and the corresponding control logic is described by the block diagrambelow, which also include the logical circuits used to genereate the CH70 CARRIER DETECT, VHFCARRIER DETECT and MODULATION DETECT signals.RM2042 Receiver (Module 3)U7.3de-emphasisfilterRxTxExternalConnectedVHFRadiotelephoneBuild-inChannel-70ReceiverU6U3.3de-emphasisfilterU6U7.4Tx AFfilterRx AFfiltersU2FSKencoderFSKdecoderXD (Tx data)RSI (Request to send)RD (Rx data)CD1 (AF Signal detect)Channel-70carrierdetectCDU5.1U5.2QVHFcarrierdetectU9.2VHF-Carrier detectU9.3Modulation detectChannel-70 Carrier detectU9.1 U9.4BALoop back test27542The truth table for the switch controle signal Q is shown below.NOTE! The signal CD is generated by means of the Channel-70 Carrier Detect circuit and indicatewhether a carrier on channel 70 is received (CD = ‘1’ if a carrier is detected and CD = ‘0’ if no carrier isdetected).A B Q REMARKS0 0 1 CH70 used for reception of DSC.0 1 0 VHF used for reception of DSC.1 0 CD VHF used as default for reception ofDSC, but CH70 used instead if carrier1 1 CD detected.PAGE 5-169341
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
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TECHNICAL MANUALFORCOMPACT VHF DSC
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RM2042CONTENTS1 INTRODUCTION1.1 GEN
Page 9 and 10:
RM20421 INTRODUCTIONThe RM2042 VHF
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1 INTRODUCTION RM20421.2 TECHNICAL
Page 13 and 14:
1 INTRODUCTION RM2042LASTThis butto
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1 INTRODUCTION RM2042MICROPROCESSOR
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RM2042CONTENTS2 INSTALLATION2.1 MOU
Page 22 and 23: 2 INSTALLATION RM2042H2067 MOUNTING
Page 24 and 25: 2 INSTALLATION RM2042This mounting
Page 27 and 28: 2 INSTALLATION RM20422.5 ELECTRICAL
Page 29: RM2042CONTENTS3 SERVICE3.1 MAINTENA
Page 32 and 33: 3 SERVICE RM20423.3 PROPOSAL FOR NE
Page 34 and 35: 3 SERVICE RM2042CHECK OF RECEIVER S
Page 36 and 37: 3 SERVICE RM2042CHECK OF TX AF DIST
Page 38 and 39: 3 SERVICE RM20423.6.3 ADJUSTMENT OF
Page 40 and 41: 3 SERVICE RM20423.9 NECESSARY ADJUS
Page 42 and 43: 3 SERVICE RM20423.10 PIN CONFIGURAT
Page 44 and 45: 3 SERVICE RM2042INTEGREATED CIRCUIT
Page 46 and 47: 3 SERVICE RM204271055/NEC & 82C55/O
Page 48 and 49: 3 SERVICE RM204274HC32 (SO-14 PACKA
Page 51: RM2042CONTENTS4 MECHANICAL DESCRIPT
Page 55: RM2042CONTENTS5 CIRCUIT DESCRIPTION
Page 58 and 59: 5 CIRCUIT DESCRIPTION AND SCHEMATIC
Page 85: RM2042CONTENTS6 MICROTELEPHONE INST
Page 88 and 89: 6 MICROTELEPHONE INSTALLATION RM204
Page 95 and 96: RM20427 PARTS LISTVHF DSC RM2042 SA
Page 97 and 98: 7 PARTSLIST RM2042POSITION DESCRIPT
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TECHNICAL MANUAL FOR COMPACT VHF DSC RM2042

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