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Appendix A
Common Questions
NI-DSA Software User Manual
HW Sampling Rate
ratio between these two parameters is a power of 2.
What is the maximum sine output frequency I can obtain using
NI-DSA?
The maximum output frequency you can generate with NI-DSA is 23 kHz.
This limit is governed by the maximum output sampling rate of 51.2 kS/s.
Because the digital-to-analog converters on the NI 455X DSA devices
feature sharp roll-off, anti-imaging filters, you can generate a spectrally
pure sine tone up to 23 kHz with minimal quantization noise, so when using
the NI 455X devices, you do not need to worry about the output signal
becoming choppy or noisy when approaching the Nyquist frequency.
Why does NI-DSA sometimes produce a slightly different output
frequency than the one I request?
The sine signal generated by NI-DSA always consists of 4,096 data points.
This buffer must contain an integer number of cycles of the sine wave to
avoid adding unwanted noise to the signal. Also, the physical sample rate
on the device is tied to the analog input rate. Under some conditions, these
two constraints prevent NI-DSA from generating the precise analog
frequency you request. If you are unsure about the frequency that is being
generated at any time, you can query the software for the actual analog
frequency by calling
Get Sine Source Settings
DSA device generates the requested frequency exactly, and if it does not,
the discrepancy is rarely exceeds a few hertz.
There are also a few cases in which NI-DSA is unable to coerce the output
frequency to a nearby value and returns an error. For example, the baseband
span is 12,000 Hz, corresponding to a hardware sampling rate of
30,720 S/s. You request an output sine frequency at 16,000 Hz, which
requires an analog output sampling rate greater than 32,000 S/s because of
the Nyquist frequency 30,720 S/s (the input rate) is not fast enough.
Doubling this output sampling rate to a value greater than 64,000 S/s is also
not feasible because the hardware supports a maximum output rate of
51,200 S/s. In this example, the best solution is to increase the input
baseband span to 16,000 Hz or greater, speeding up the input sampling rate
and allowing you to obtain higher output sampling rates.
and
Get HW Update Rate
A-6
, you will see that the
. In many cases, the
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