M7/M7L/M7LT Modular Satellite Modem MaintenanceM7/M7L/M7LT - Rev. 0.05 3-973.9.1. Modulator Bit TimingWhen the modem is configured with any serial interface option, the modulator always outputs theSend Timing signal on the baseband interface connector. In a typical network, this timing signal isused to generate the bit timing for the transmission toward the satellite. This timing signal may beselected from 1 of 4 sources:1. Internal – locked to a 2.0 ppm Internal Reference. If bit rate timing requirements require abetter reference, the internal oscillator may be phase locked to an External Referenceapplied at the rear panel (J1). Refer to Section 3.7.4 for details on how to activate theExternal Reference.2. RCV Clock – locked to the demodulator Receive Timing (i.e. Loop Timing)3. External – locked to an external timing input at the bit rate on pins 9 and 21 of the overheadchannel interface connector J10. This option allows a station-derived standard clock rate tobe used to clock data out of the FIFO4. Terminal Timing – locked to the Terminal Timing input (pins 24/11)NOTE: Bit timing options for optional interfaces are discussed in the applicable interface appendix.A simplified block diagram of the Modulator clock sources for a Synchronous Interface is shown inFigure 3-50.Transmit Clock SourcesDemodulatorModulatorReceive FIFOBufferDATACLOCKDATAININOUTOUTDATAOptional Reed-Solomon Decoder& IBS MultiplexerCLOCKDemodulator RCVExternalReference Input(Rear Panel)CLOCKInternalReferenceOscillatorExternalReference PLLTerminalTimingTerminalTimingFromInterfaceDemod outputclock is phaseBit Rate NCOFromReceiveClockSend DataFromInterfaceSendTiming ToInterfaceModulator bit clock source isselected from "Internal", "TerminalTiming", "External" or "RCVClock". The Send Timing isalways an output from the modem.ReceiveData ToInterfaceReceiveTiming ToInterfaceExternalFIFO ClockFromInterfaceExternal Send Timing Input(Rear Panel)Figure 3-50 – Modulator Clock Source Options
