311
Chapter 7 Tutorial
Direct Digital Synthesis
4
7
Direct Digital Synthesis
The Keysight 33210A 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
s
teps 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 33210A uses two anti-aliasing filters. An elliptical filter is used for
continuous sine waves because of its nearly flat passband and sharp
cutoff above 10 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, the function generator uses waveform memory
that is equivalent to 16,384 (16K) words deep. For optional arbitrary
waveforms, the function generator uses waveform memory that is
equivalent to 8,192 (8K) words deep.
50 MHz
Anti-Aliasing
Waveform
Data
ddress
Waveform
DAC
Filter
Memory
33210A users guide.book Page 311 Wednesday, July 16, 2008 11:16 AM
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