DDS can refer to either Direct Digital Synthesis
or Divide Down Sampling
Many NI cards use direct digital synthesis (DDS) to generate periodic signals at precise frequencies. Direct digital synthesis works by storing a repetitive waveform to the onboard memory of the signal generator. For NI signal generators, typically this onboard memory stores 16,384 unique points and is used as a lookup table. Every time there is a rising edge of the sample clock, the signal generator will reference the lookup table and determine which point to generate that achieves the desired frequency of the waveform being produced.
Divide down sampling is a technique for deriving a sample rate from the NI signal generators main internal timing source. Divide down sampling divides the internal sample clock by an integer value N. The internal sample clock rate and the values of N are device specific. Divide down sampling mode is software-selectable with the use of the niFgen Configure Clock Mode.vi
. This mode provides a high-quality clock with the lowest jitter.
As an alternative, niFgen Configure Clock Mode.vi
can be used to configure the NI signal generator to use the High-Resolution Sampling clock mode to utilize a direct digital synthesis method to generate sample clock rates at very fine frequency resolution increments, typically in the order of microhertz.