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Chapter 7 Tutorial
Direct Digital Synthesis
4
7
Direct Digital Synthesis
The Agilent 33220A uses a signal-generation technique called Direct
Digital
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 50 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.
Direct Digital Synthesis Circuitry
The 33220A uses two anti-aliasing filters. An elliptical filter is used for
continuous sine waves because of its nearly flat passband and sharp
cutoff above 20 MHz. Since elliptical filters exhibit severe ringing for
waveforms other than continuous sine waves, a linear-phase filter is
used for all other waveform 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
defined with more than 16K points, the function generator uses
waveform memory that is 65,536 (64K) words deep.
50 MHz
Anti-Aliasing
Waveform
Data
ddress
Waveform
DAC
Filter
Memory
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