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How to choose DFT Type with SA option in PNA-X？

Question asked by mindhen on Nov 1, 2019
Latest reply on Nov 4, 2019 by daras

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In SA mode , four DFT types are given for user . How to choose which type of DFT to use in the actual application？

Different DFT types , different FFT resolutions; It will affect the detector results? and thus affect the amplitude and frequency accuracy? How to evaluate the results？

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daras Nov 4, 2019 1:57 PM
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Sorry for the late reply, but due to the fires in Northern California, the SA expert whom I had to consult to get more information on this topic was out of the office last week. At any rate, here is how he describes the various DFT type settings in the SA Channel:
DFT types:

POW2 - Sets the DFT record size to the next power of 2 greater than or equal to the current ADC recordsize. This is the fastest DFT processing available, the power of 2 recordsize allows for very efficient computation shortcuts (the aka FFT algorithm)

RADix - Sets the DFT to the nearest equal or larger integer that can be decomposed with 2, 3, 5, 7, 11 ,13 radixes (aka 13-smooth numbers). The Intel CPUs have shard-coded trigonometric values for 2, 3, 5, 7,11, 13 fractional angles, the DFT code takes benefits of that for still efficient DFT processing.

ARBitrary - Sets DFT record size equal to the ADC record size. If the current ADC record size is a large prime number, then the DFT can be very slow.

FASTest - Sets the DFT record size as close as possible to the ADC record size (larger or equal) while optimizing processing speed. Sometimes, the recordsize will be increased more than the minimum number required to match the 13-smooth condition, if the whole processing of the sweep is faster with a recordsize that has a faster DFT time. There is a trade-off here: increasing the recordsize to speed up the DFT will increase the amount of data to process for the further steps of the SA processing: image rejection, detection.



Additional comments:

There is a given ADC recordsize that gives a given RBW (for a given window type), the RBW evolves as 1/ADC recordsize. If the DFT recordsize must be greater, depending on the DFT mode some zeros will be added to the ADC recordsize. This is the difference that can be noted between the ADC recordsize and the DFT recordsize on Advanced Processing page.

When running the coherent mode of SA, the ARBitrary mode will always be selected, to make sure the DFT bins frequencies exactly land on the coherent signal tones.

The POW2, RADix and FASTest mode have the same behavior regarding the RBW setting: These 3 modes are increasing the ADC recordsize to the next best match. This is more sensitive with POW2 mode, as the density of available ADC recordsize is sparse.

The POW2 mode makes the SA computation behaving exactly like some tools based on FFT processing, this is a use case of this mode. The other use case is to get the fastest processing time, that can be useful for intensive spurious search measurements. The penalty is the small number of RBW values available.

The RADix mode is currently the default mode, this is the one that gives the most accurate RBW setting. The FASTest is to be tried if speed matters for a given non-coherent SA setting.

Example: running 100 KHz RBW with Gaussian filter, the ADC recordsize must be 1988 samples. Here are the DFT lengths for the different algorithms:


Power of 2: 2048

Fastest: 2048

Radix: 2000

Arbitrary: 1988
Here is my own summary of these settings:

Point #1: if the SA channel is in coherent mode, then the DFT type is automatically set to ARBitrary.

Point #2: the other DFT type selections trade processing speed versus available resBW selections or processing speed versus resBW accuracy (the typical speed improvement that results from changing the default settings is commonly 2-5%)

Point #3: RADix is the default mode unless the channel is in Coherent mode.

Conclusion: Unless you are doing something very specialized and fully understand the SA data acquisition and processing algorithms, it is best to just use the default settings under the Advanced or Advance/Advanced portion of the SA setup dialog.
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How to choose DFT Type with SA option in PNA-X？

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