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Chapter 7 Tutorial
Direct Digital Synthesis
Direct Digital Synthesis
Digital signal processing methods are used in many everyday applications.
Whether it is a digital audio compact disc player, an electronic
synthesized piano, or a voice-synthesized telephone message system,
complex waveforms can be easily created or reproduced using digital
signal-generation methods.
The 33250A uses a signal-generation technique called Direct Digital
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Synthesis (DDS) for all waveform functions except pulse. As shown below,
a stream of digital data representing the desired waveform is sequentially
read from waveform memory and is applied to the input of a digital-to-
analog converter (DAC). The DAC is clocked at the function generator's
sampling frequency of 200 MHz and outputs a series of voltage steps
approximating the desired waveform. A low-pass "anti-aliasing" filter
then smooths the voltage steps to create the final waveform.
Waveform
Memory
Anti-Aliasing
200 MHz
Filter
Address
Data
Waveform
DAC
Direct Digital Synthesis Circuitry
The 33250A uses two anti-aliasing filters. A ninth-order elliptical filter is
used for continuous sine waves because of its nearly flat passband and
sharp cutoff above 80 MHz. Since elliptical filters exhibit severe ringing
for waveforms other than continuous sine waves, a seventh-order linear-
phase filter is used for all other waveforms functions.
For standard waveforms, and arbitrary waveforms that are defined with
fewer than 16,384 (16K) points, the function generator uses waveform
memory that is 16K words deep. For arbitrary waveforms that are
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defined with more than 16K points, the function generator uses
waveform memory that is 65,536 (64K) words deep.
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