Standard Function Mode

Standard Function output mode is used to generate standard function waveforms such as sine, square, triangle, and so on.

NI 5401/5402/5406/5411/5431/5441/5442

For the NI 5401/5402/5406/5411/5431/5441/5442, the Standard Function mode is implemented through direct digital synthesis (DDS), which is a technique for deriving, under digital control, an analog frequency source from a single Reference clock frequency. This technique produces high-frequency accuracy and resolution, temperature stability, wideband tuning, and rapid, phase-continuous frequency switching.

In DDS mode, a fixed–size memory (called lookup memory) stores one cycle of a periodic waveform. A phase accumulator indexes the lookup memory. For each cycle of the device Sample clock, the sample of the waveform in lookup memory that is addressed by the phase accumulator is returned. The accumulator is then incremented by the value in the frequency control word (FCW). By adjusting the Frequency property or the NIFGEN_ATTR_FUNC_FREQUENCY attribute, NI-FGEN calculates the corresponding FCW, and you can vary the output frequency of the waveform in lookup memory. The phase accumulator increments in smaller steps for smaller FCWs. Accordingly, you need more samples to generate one waveform cycle, so the frequency is lower. A higher FCW results in a higher frequency. In DDS mode, the Sample clock does not vary with the frequency of the generated waveform. At higher frequencies, some waveform samples in lookup memory are skipped; at lower frequencies, some samples output multiple times in succession.

NI 5412/5421/5422

For the NI 5412/5421/5422, the Standard Function output mode is implemented primarily in software. Instead of using separate DDS hardware to generate standard function waveforms, NI-FGEN creates and downloads standard function waveforms to arbitrary waveform memory and generates them just like arbitrary waveforms. NI-FGEN automatically selects the best clock mode, sample rate, and buffer size to produce the most accurate waveform possible. To determine how much arbitrary memory is used, query the Buffer Size property or the NIFGEN_ATTR_FUNC_BUFFER_SIZE attribute. To adjust the maximum amount of memory used by NI-FGEN for generating standard function waveforms, set the Max Buffer Size property NIFGEN_ATTR_FUNC_MAX_BUFFER_SIZE attribute. Increasing the maximum buffer size may result in a more accurate waveform.

Note  Frequency hopping and sweeping is not supported on the NI 5412/5421/5422.