RELM SMX920 Technical Manual
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Contents
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SMX920 Base Station & Repeater Technical ManualRELM/ BK RADIO61.2 Module Functional Description1.2.1 RF Module and Main Controller AssemblyThe SMX920 consists of a full duplex RF module with its own shielded metal housingand a Main Controller board integrated on a single PCB. Using advanced yetsimplified circuit designs, the size and complexity is reduced. This affords a numberof advantages including;♦ Cost reduction♦ Reduced number of components improves reliability and MTBF♦ Consistent and improved manufacture♦ Elimination of connectors and cabling♦ Reduction of human error♦ Faster maintenance or swap outThe Exciter module generates the low level, on frequency, RF transmitter signalwhich is later amplified to nominal output power level by the Power Amplifiermodule. The exciter consists of a Voltage Controlled Oscillator (VCO) and associatedmain RF board, which, in conjunction with the reference oscillator and the PLLcircuitry, forms a two-point modulation programmable frequency synthesiser.Frequency programming data is received from the Micro Controller via an 3 wireserial data bus.The exciter circuitry features a modulation bandwidth from DC for accurate CTCSSand DCS generation and a wide RF switching bandwidth which covers the entire sub-band. The average maximum RF output power is > +24.7 dBm / 300 mW. Normallyno adjustments are required, however should the carrier freq need re-alignment forfuture maintenance, the TCXO reference oscillator frequency can be adjustedmanually or electronically adjusted via software settings.The fractional N synthesiser provides ultra low spurii while still maintaining fast locktimes. The frequency step size is auto determined at 5.0 kHz or 6.25 kHz.The receiver section accepts the low level RF input signal and amplifies, filters andconditions the signal prior to detecting the wanted audio component. The Receiverfeatures the same advanced synthesiser and wide bandwidth as the exciter. The frontend bandpass filter uses very high tolerance components to minimise productionspread variations has a wide bandwidth and eliminates the need for future alignments.The bandwidth is equal to the band allocation (refer to Section 8.5 for details of theband allocations). The VCO has low phase noise and covers the full sub-band.The receiver has high sensitivity while maintaining excellent intermodulationimmunity and adjacent channel rejection. A dual first IF filter provides excellentrejection to common known spurious responses. High blocking of over 100 dB typicalensures that strong interfering signals do not desensitise the receiver when receivingweak signals.The Micro Controller section is physically located towards the centre on the mainboard and controls all signal connections (apart from the RF connections). It controlsthe operation of the RF sections and acts as the interface between the user controls,indicators and the RF sub sections. Together with the VF ASIC chip, processedtransmit and received audio is passed to and from the Exciter and Receiver sections aswell as providing all other audio signalling functions of the transceiver. |
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