SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATION - Design and Development of Medical Electronic Instrumentation - page 260

Analog Outputs

Signal
Conditioning

DAC

Channel 1

+

Σ

Signal
Conditioning

a x (Channel 1)
+ b x (Channel 2)

+

Sample
Clock

CLK

Signal
Conditioning

Counter

DAC

Channel 2

Enable Reset

External
Trigger

Control Logic

RAM

Data In

End Address
Register

Digital Buffers

Computer

Interface

Figure 6.9 An arbitrary waveform generator has at its core a RAM that contains the full time-domain digital representation of the wave-
form desired. To generate the analog signal, the discretized point-by-point version of the waveform is played in a sequential manner through
the generator's DAC. Waveforms can be replayed continuously by looping back from the last point of the waveform's sequence to the address
of the RAM location for the first point of the sequence. Triggered operation is also possible by cycling through the memory contents only
upon receiving a trigger signal.

waveform. Of course, reproducing a signal requires the stored waveform to have been sam-
pled at a rate of at least twice its highest-frequency component, and an appropriate inter-
polating low-pass

filter should be used. In turn, this means that the complexity and time
duration of the reproduced waveform are limited by the arb's memory size, or depth . The
time duration of the output waveform is given by

fi

number of waveform points
f clock

start address
f clock

end address

T waveform

Speech, for example, requires a sampling minimum speed of approximately 8 kilosam-
ples/s. An arb with a depth of 32 kilosamples would then su ce for only 4.096 s of record-

ing. When keeping the memory size fixed, longer waveform durations can be achieved

only by limiting the bandwidth to allow lower sampling rates. Obviously, limiting the
bandwidth results in limiting the complexity of the waveform by reducing the number of
spectral components available to describe the details of waveform.

fi