Hello there,
I'm experiencing unexpected numeric results for the library function block "Fractional Interpolator" used in the default personality of the PXIe-5785 board. This is the only documentation I've found online (fragmentary and outdated, since is not referring to the 4 SPC version used in the above-mentioned design). The design make use of this VI to raise the sample rate of incoming samples from arbitrary to a known value of 800 MSPS (4 SPC @ 200 MHz clock)
As you can see in the attached image, the resulting interpolation (in BLACK) is not as the expected results (in BLUE). In RED, the input samples that refer to a simple 25MHz sinewave with oversampling factor 8 (200 MHz sample frequency).
The block seems to work as expected if the oversampling factor is 4 (in case of sinewaves) or if the incoming samples (of any waveform, I've tried QPSK as well) are already at a 800 MSPS rate (i.e., interpolating actually nothing).
The reference design, is feeding the block with data retrieved from the DDR memory (4 samples at a time) and using the Fractional Interpolator block own "ready for input" boolean output terminal as feedback to pace the DDR reading and thus operate flow control (I suppose taking advantage of the internal NCO to achieve the correct input data-rate).
Any help is appreciated. Ty!
