RFSA

[Markus_Kuhn]

The niRFSA Fetch IQ VI provides me access to the absolute time at which the first sample of the IQ recording was required, as well as the IQ samples.

I have two requirements for the data types involved:

1. I need to access the absolute timestamp t0 using LabView's 128-bit fixed-point timestamp format, because a 64-bit floating point format simply does not have enough mantissa bits to uniquely identify each sample-clock edge accurately across the potential lifetime of the application, and because using floating-point numbers for timestamps is generally a pretty bad idea (as their absolute resolution continuously decreases as time increases, causing difficult to test problems late in product life).
2. I also need the IQ samples in unscaled I16 format, which I assume is the most compact, native format that my NI PXIe-5622 down-converter records internally. (I want to do the scaling myself much later, in off-line processing)

Unfortunately, at present, I can only have one or the other (high-res 128-bit timestamps or native, unscaled 16-bit integer samples), but not both simultaneously. This is because the polymorphic niRFSA Fetch IQ VI offers me either unscaled I16 IQ data along with a wfm info cluster that contains the absolute timestamp in the inappropriate DBL format, or it offers me complex WDT records with nice 128-bit timestamps, but then the IQ data comes in inappropriate scaled complex single or double format, which are not the compact native unscaled integer data format I would prefer, and which is tedious to scale back into I16 (leading to an unnecessary I16->float->I16 conversion roundtrip).

Feature request: Could you please provide in the next RFSA release a variant of the unscaled I16 niRFSA Fetch IQ VIs that outputs the absolute timestamp in a fixed-point type (either scaled in LabView's128-bit timestamp type, or unscaled as a simple sample-clock integer count)?

Application: I'm acquiring IQ data in multiple frequency bands, and I need to know exactly (with <1 sample accuracy) the relative timing between these acquisitions. As my NI PXIe-5667 acquires IQ values with 75 megasamples per second, the required timestamp resolution is at least 13.3 nanoseconds, or 0.0000000133 seconds. But as explained here, absolute timestamps in DBL have only 5 decimal digits resolution left. Therefore I can only determine the relative timing between multiple recordings with an accuracy of slightly better than a millisecond.

Generally, I would recommend that event timestamps should always be provided in APIs in fixed-point timestamp format, to guarantee uniform resolution. They can always easily be converted into floating-point representation later. Floating-point timestamps are a pretty dangerous engineering practice and should be discouraged by APIs.