Specifications

Basics 2-3
DS360 Ultra Low Distortion Function Generator
Introduction to Precision Waveform Synthesis
The DS360 uses Direct Digital Synthesis and analog signal processing to generate an extremely pure
sinewave with extraordinary frequency resolution and stability. Traditional function generators typically
use one of several methods to generate sinewaves, each having one or more major limitations.
Traditional Generators
Frequency synthesized function generators typically use a phase-locked loop (PLL) to
lock to a stable reference, and use wave shaping circuits to produce the desired function.
This solution often has limited frequency resolution. Typically frequency resolution is
limited to about 1:10
6
(some advanced PLL circuits have much higher resolution).
Distortion performance is limited due to the wave shaping circuits, often to as low as
-40dB.
Arbitrary function generators eliminate the need for wave-shaping circuitry. Normally a
PLL is used to create a variable clock that increments an address counter. This counter
addresses memory location in a waveform RAM that produces data for a DAC. This
waveform RAM can be filled with any data, to create “arbitrary” waveforms, as well as
sines, squares or other common waveforms. Since this is a sampled data system, it
requires an anti-imaging filter to create an accurate waveform. Sampling theory states
that a waveform can be accurately reproduced, as long as it is sampled more than twice
as fast as its highest frequency component. Since arbitrary function generators vary their
clock frequency, they must also modify their output anti-imaging filter.
Direct digital synthesis, a relatively new technique, overcomes many of these problems.
DDS works by generating addresses to a waveform RAM to produce data for a DAC.
Unlike PLL based techniques, the clock is a fixed frequency reference. Instead of using a
counter to generate addresses, an adder is used. On each clock cycle, the contents of a
Phase Increment Register is added to the contents of the Phase Accumulator. The output
of the Phase Accumulator is the address to the waveform RAM. By changing the value
of the Phase Increment, the number of cycles required to step through the entire
waveform RAM changes, thus changing the output frequency. Since a fixed frequency
clock is used, only one anti-imaging filter is required. This technique features excellent
frequency resolution, as good as 1:10
14
and reasonable distortion performance, down to
-70dB.
Low distortion oscillators normally use some variety of R-C circuit in a Wein Bridge
configuration to generate a pure, low distortion sinewave. This solution suffers from
poor frequency accuracy, resolution and stability, due to component aging and drift.
Frequency stability and accuracy for these oscillators is normally measured in 100’s to
1000’s of PPM. Frequency resolution is typically between 0.1% to 1%. This technique
features excellent distortion performance, as low as -100 dB or better.