Analog Circuits Cookbook

Direct Digital Synthesis 73

Direct Digital Synthesis

(Part

2)

Spurious outputs can be due to a variety of causes, principally 'amplitude

truncation' (only a finite number of voltage levels available out of the DAC

to represent a sinewave) exacerbated by DAC non-linearity and glitches,

and phase truncation (only a finite number of phase steps available to

represent the sinewave). In many ways, spurious outputs due to phase

truncation are the most troublesome, as will appear later.

Frequency

-n units of

Phase

accumulator

N-bit

frequency

register

N-Dtt

phase

register

P-bits

of

phase

Sine

lookup

table

A-bits

of

amplitude

DDS

output

Figure

3.9 Generic ...