Multiple digitizers with one downconverter

[TimS.]

Hello,

I will soon be attempting to use one PXI-5600 downconverter and two digitizers. I have a PXI-5142 and a PXIe-5122. The RF from the downconverter will be split to the two digitizers. Does anyone know of any limitations that would prevent this from working effectively? Has anyone tried this before and are there any hurdles that I should keep an eye out for?

The reason I will be doing this is to be able to continuously acquire 20 MHz of spectrum for display purposes while simultaneously tuning the other digitizer over small frequency ranges for audio demodulation and recording. Any advice, comments, suggestions, and alternative solutions are certainly welcome.

Thank You,

Tim S.
Systems Engineer

[samantham]

Hello,

The first concern would be making sure the output of the 5600 is properly split by using a 6 dB splitter.  This way the SNR and thus the demodulation is not negatively influenced.  Second, I am curious which digitizer will be used for displaying the spectrum and which one for the demodulation and recording.  Please give me some feedback and I will make sure there is nothing else to consider.

[TimS.]

Hi Samantha,

That was one my concerns as well. I know that the cable which comes with the PXI-5661 has been length optimized to be efficient for the entire 9 kHz to 2.7 GHz range. I would be introducing the insertion loss of the splitter and additional cable lengths.

I will be using the PXI-5661 (PXI-5600 and PXI-5142) for the spectrum display and the PXIe-5122 for keying up audio demodulation from the spectrum display. I already have an entire spectrum application written around using the RFSA drivers with the PXI-5661 as well as the spectral measurements toolkit which acts on the IQ data. It would be a major code re-work to control all three devices independently. I don't expect it to be that difficult to control the PXIe-5122 for audio demodulation, which is basically narrowband compared to 20MHz of spectrum. I would just tune the PXIe-5122 around the IF center frequency based on where a peak occured in relation to the spectrum's center frequency (please advise if I'm wrong on this).

Thank you,

Tim Sileo

[samantham]

Hello,

What type of audio demodulation are you referring to … Amplitude or Frequency Modulation (AM or FM)?  My concern is that the 5122 does not have the On-Board Signal Processing (OSP) to take the IF signal to IQ Baseband in hardware.  The 5142 has OSP and would reduce the processor time and RAM needed to do the Digital Downconverting.  Plus, the 5124 is supported by the NI-RFSA driver and the 5122 is not. 

Please consider reversing the cards, where the 5122 is displaying the spectrum and the 5124 as part of the 5661 is demodulating the audio signal.  This would involve using NI-RFSA for the 5661 and NI-Scope for the 5122.  The way originally noted will use NI-Tuner for the 5600 and two sessions of NI-Scope for each individual digitizer.  This sounds like a bit more work.  Please give me some feedback or insight on some reasons that I might not be aware of that might make audio demodulation on the 5122 better for this specific application. 

I noted that you have that Spectral Meausurement Toolkit.  Do you have the Demodulation Toolkit?

[TimS.]

Yes, I was going to use the 5122 for performing AM and FM demodulation on a narrowband signal within the 20 MHz IF from the 5600. I just completed trying this last week and determined that it wouldn't work due to what you mention about not having the OSP on the 5142. I knew that reversing the cards would have made more sense but I was under tight time constraints and I already had code in place for a spectrum viewing application using the RFSA drivers. If, in the future, I have more time (...budget) I would like to try this again using the method you suggested. However, I believe that the management team will more readily buy another PXI-5661 for simultaneous spectrum viewing while demodulating audio streams than continuing to investigate this method. I was just trying to come up with a way to save on slot space in the chassis because of the downconverter taking up three.

My current solution still doesn't give simultaneous spectrum viewing and audio demodulation but the application I've created can accomplish both. When audio is set to demodulate a peak that was seen, the spectrum will pause while the audio is playing and resume when an operator stops the audio stream. Essentially, when the audio stream is of interest I configure the PXI-5661 to return narrowband IQ data and when spectrum is of interest I configure it to return the full 20 MHz of IQ data.

Yes, I have been using the Modulation toolkit for AM and FM demod to accomplish this.

Thank you for the help and insight.

-Tim Sileo