SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATION - Design and Development of Medical Electronic Instrumentation - page 256

maximum frequency of spectral components for the arbitrary signal. Even an assembly-
language program copying the contents of sequential addresses on RAM to an I/O location
would result in DAC writing rates of a few megapoints per second at the most.
Instead of having a DAC interfaced to memory through a microprocessor, arbs have ded-
icated RAM that is interfaced directly to the DAC. In this way, update rates are limited only
by the access time of the RAM and by the speed of the DAC. As such, commercial arbs can
currently be purchased with maximum writing rates in the vicinity of 1 gigapoint/s, yielding
bandwidths of up to 500 MHz.

Direct Digital Synthesis

A generator capable of directly synthesizing an analog signal from digital data has at its
core a memory that contains the full time-domain digital representation of the waveform
desired. To generate the analog signal, the discretized point-by-point version of the wave-
form is played in a sequential manner through the generator's DAC. A simple form of such
generator is a direct digital synthesizer (DDS). As shown in Figure 6.7, an address gener-
ator circuit controls the way in which samples stored in ROM are delivered to the DAC's
input. On each clock pulse delivered to the address generator, a new address is issued to
the ROM such that data for the next point in the sequence are presented to the DAC.
The ROM in a DDS generator most often contains data for generation of a complete
single cycle of a sinusoidal waveform. The address generator is a simple counter, and the
addresses it generates constitute the various phase angles φ for which samples of the sine

wave sin are available in ROM. The series of values coming out of this ROM lookup

table as a function of incrementing phase angles is translated into an analog sine wave by
a D/A converter (DAC).
Obviously, if the clock presented to the phase accumulator counter remains constant,
the rate at which phases are generated remains constant, and the end result is a sine wave

φ

Phase Accumulator

V out

t

n bits

Counter

Low-Pass
Filter

RO M DAC

Output

Sine Look-up Table

Phase
Increment
Register

Address
Generator
Ci rcu i t

Figure 6.7 In a direct digital synthesizer (DDS), an address generator circuit or phase accumulator controls the way in which samples
stored in a ROM lookup table are delivered to the DAC's input. Control over the output frequency is achieved by selecting an appropriate
phase accumulator increment.