User`s manual

Memory Concurrent with the DAC, the frequency generator also clocks the segment
memory to output the next code to set the DAC bits to the next point on the
waveform. By clocking both the memory and DAC at a certain clock rate
(i.e., sample rate), the AFG outputs a waveform at a frequency determined
by the length and number of waveform segments and the sample rate. (See
“AFG Memory Description” on page 452 for more information on how the
memory operates.)
Waveform segments and segment sequences can also be stored into memory
using external sources, like the VXIbus, for user generated waveforms.
Reference
Oscillator
The reference oscillator provides the clock signal for the frequency
generator. Thus, frequency stability depends on the stability of the
reference oscillator. The oscillator also determines the frequency range of
the frequency generators. The AFG allows for user supplied reference
oscillator sources for custom frequency values.
Frequency
Generators
The frequency generator generates the clock pulses to enable both the
output DAC and memory to output a segment. The frequency generator
thus determines the rate (i.e., the sample rate) at which the points of a
waveform segment are output.
The AFG uses two different generators. One generator (Frequency1
generator) uses a Direct-Digital-Synthesis (NCO) technique to generate the
sample frequencies. The other generator (Frequency2 generator) is a
Divide-by-n generator.
448 Block Diagram Description Chapter 10