RFSG Generation & RFSA Acqusition Precise Timing

[RF_Buzzer]

Hi NI RF Guys,

I am tyring to find the time difference between RFSG Generation and RFSA Acquisition.

I have initialized the RFSG to transmit Complex IQ Signal and RFSA to acquire that Complex IQ Signal soon after RFSG generates it.

I am pasting a snapshot of this VI built using NI RFSG and RFSA examples...

I find that the time difference between RFSG Generation and RFSA Acquisition never goes below 2ms ( two milliseconds ).

Why is there so much of a delay in the RFSA Acquisition, even though the RFSA has been initialised and is ready for acquiring data ?

Is it due to the fact that it takes a finite amount of time for transferring the IQ data from the PXI Digitizer via the PXI Bus to the PXI Embedded Controller... If that time is finite, I can substract that time from my calculations (i.e., Time difference between RFSG Generation and RFSA Acquisition ).

But I cannot figure out how to calculate this Data Transfer time for transferring the IQ data from the PXI Digitizer via the PXI Bus to the PXI Embedded Controller. ( If at all 2ms delay is due to this ??? )

Any thoughts on this would be appreciated ..

Thanks in advance.

Dharmendra

[John_P1]

Hi Dharmendra,

I am a bit confused about what you need to measure?  The amount of time it takes the OS to make the software calls could easily be a few milliseconds.  Keep in mind that the OS is not deterministic (unless using a real time OS), so using the clock ticks as a timebase for measuring short delays is not common.  Even on a real time system, the resolution of the clock might not be fast enough to make a meaningful measurement.  You are currently essentially measuring the amount of time it takes for LabVIEW to call the functions in between the two Tick Count vis using the OS clock as a reference.

The software calls are used to configure the hardware, and once the tasks initiate they are based off of hardware-timed clocks and triggers.  Measuring the time between software calls is not necessarily beneficial, but we should rather be more concerned with the delay between hardware signals used to begin the acquisition.

Since you are triggering the RFSA task off of the power level of its input which is coming from the RFSG, you should Initiate the RFSA task first.  After initiating the RFSA task, the digitizer at this point is ready to accept the trigger signal.  Presumably this trigger won't come until after the RFSG task has started as well, so once the RFSG task begins it will trigger the RFSA task as well.  Currently, the RFSG task begins, and the RFSA task must wait that ~2ms before it is ready to accept the trigger.

Depending on which RF device you are using, you could also use the RFSG Export Signal.vi to output the start trigger as soon as the generation begins, and use this signal as a digital trigger for your digitizer.  The amount of time it takes the computer to read back the data is again non-deterministic, but if you want the tasks to start together then this is possible to accomplish in hardware.  The samples that you acquire are all based off of hardware-timed signals and are brought over in chunks back into LabVIEW with the Fetch command.

-John

Message Edited by John P on 04-16-2009 06:46 PM