DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

260 SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATIONAnalog OutputsDACSignalConditioningChannel 1SampleClockExternalTriggerCLKEnableCounterResetControl LogicEnd AddressRegisterAddressData OutRAMData InDAC+Σ+SignalConditioningSignalConditioningDigital Markersa x (Channel 1)+ b x (Channel 2)Channel 2Digital BuffersComputerInterfaceFigure 6.9 An arbitrary waveform generator has at its core a RAM that contains the full time-domain digital representation of the waveformdesired. To generate the analog signal, the discretized point-by-point version of the waveform is played in a sequential manner throughthe generator’s DAC. Waveforms can be replayed continuously by looping back from the last point of the waveform’s sequence to the addressof the RAM location for the first point of the sequence. Triggered operation is also possible by cycling through the memory contents onlyupon receiving a trigger signal.waveform. Of course, reproducing a signal requires the stored waveform to have been sampledat a rate of at least twice its highest-frequency component, and an appropriate interpolatinglow-pass filter should be used. In turn, this means that the complexity and timeduration of the reproduced waveform are limited by the arb’s memory size, or depth. Thetime duration of the output waveform is given byT waveform number of waveform pointsfclockSpeech, for example, requires a sampling minimum speed of approximately 8 kilosamples/s.An arb with a depth of 32 kilosamples would then suffice for only 4.096 s of recording.When keeping the memory size fixed, longer waveform durations can be achievedonly by limiting the bandwidth to allow lower sampling rates. Obviously, limiting thebandwidth results in limiting the complexity of the waveform by reducing the number ofspectral components available to describe the details of waveform.end address start addressfclock