Time domain adding of multiple DDC transients from PXI5142 (Tclock) plus other questions about the OSP

[blakney]

I have some questions about the OSP features of the PXI-5142.

I have previously used an Agilent VXI E1437A that allowed one to sample at full rate and then split the sampled data into two data sets. The second data set was mixed with a user defined digital local oscillator (LO) with a 90 degree phase shift. Both sets of data were then digitally filtered and decimated to give the two datasets required to perform digital hetrodyning of the original signal at any frequency from near DC to the antialiasing filter cutoff. I was not ever able to sum (coadd) stimulus response when combined with the VXI AWG I had due to a phasing error between the AWG and the LO on the digitizer.

I am curious if this same heterodyning procedure is allowed in the 5142 under mode 1 of DDC operation (p.21 of NI 5142 spec pdf). If this is the case, it looks as if you have a 14 bit signal mixed with a 18 bit (user defined frequency) oscillator and then stored as a 16 bit integer on board, just curious about specifics here. If this mode of operation is allowed, does Tclock correct for the phase of the digital oscilator and the digitizer decimation as well as in my case a 5421 decimation factor . For example,. if one were  to do stimulus response with a 5421 stimulus followed a response detected at the 5142  in digital heterodyne mode as described. Would multiple transients from such an exercise remain phase locked and allow multiple scans to be coadded?

Next, if one wanted to replace a 5122 with the 5142 in an application are there any considerations or incompatibilities or is the 5142 completely compatible plus extra goodies?

Finally, I note from the specs that you can do "real downconversion on Channel 0". Does this mean that if you want to aliased protected decimation on both channels simultaneously that this is not allowed?

If there is a white paper or similar on the workings of the OSP on the 5142 is available I would appreciate the link.

Thanks in advance,

Greg

[HSD]

Greg,

General Information:

The 5142 is almost the same device as the 5122 with a larger FPGA and additional firmware.  The main incompatibility between the two is that the maximum record length of the 5142 is about 200 samples less than the 5122.

The ADC on the 5142/5122 is 14 bits.  The 14 bits is processed in the DDC using 18 bits of precision.  After all the processing is complete only 16 bits are actually stored in memory.  The digital oscillator used by the DDC is reset with the “start trigger”, so if you supply a digital start trigger the digital oscillator in the DDC will get reset to phase 0 when the first point is stored into memory.

As of niScope 3.2 we added new triggering capabilities to the 5142.  You can now trigger off the “complex magnitude” of the DDC data (I squared plus Q squared).  There is also an option to trigger on bursts.  This complex magnitude trigger can only be configured on the reference trigger and currently can not be exported.

Another feature we added in niScope 3.2 is to allow the user to download custom FIR coefficients to channel 0.  The FIR filter processes data before the data reaches the DDC.  The FIR can take 35 coefficients.

Your questions:

1.      There is a configuration that will allow you to do aliased protected decimation on both channels simultaneously.  There is an example in the driver called “niScope EX OSP Baseband Decimation” that illustrates how.

2.      Unfortunately TClk is not currently supported on the 5142 with the DDC or any OSP feature enabled.  TClk is only supported with the DDC disabled.  This is a feature on our roadmap, but currently there is no target date for the feature.  However, since the digital oscillator is reset with the start trigger, you can still share a trigger between the 5421 and 5142.  You can also lock the PLL of both boards to a common clock.  This should get you synchronized to with in +/- 1 sample clock (~20 ns).  I will let others know about your need for TClk with the DDC enabled.

Please let me know if you have any other questions or if you have any suggestions for future features on our OSP products,

Kunal