PXI-5660 / 5620. What is lowest reasonable voltage when using Analog Edge Trig?

[GlennD]

Presently, I am using Analog Edge Ref Triggering on the PXI-5660.  The lowest voltage that I can set the trigger level to is 0.200 volts without noise causing the trigger to occur.  Is this level reasonable?  Is there a way to reduce the noise so that the trigger level can be set lower?

I graphed the waveform coming out of the niScope MultiRead Cluster.vi in the ni5660 Read Average Power Spectrum.vi.  The noise on the graph was as high as 0.190 volts.  I thought that I saw the noise down to 0.010 volts at one time.

Thanks.

[Travis G.]

Hello Test Tiger,

That noise level of 0.200V does not sound very reasonable to me.  The first thing that I would check is to make sure that you are using the latest version of the driver, NI-RFSA 2.0.1.  I tried this out with a PXI-5600 and PXI-5621, and was able to trigger off signals as low as 5mV without noise interfering with the trigger signal.  One thing I have noticed is that the noise level will depend on the reference level you set with the ni5660 Configure.vi.  This reference level determines how the attenuators in the PXI-5600 downconverter are set, which ultimately determines the magnitude of the signal being passed from the downconverter into the PXI-562x digitizer.  What reference level are you using?

One good test to try would be to view the raw power spectrum acquired by the PXI-562x digitizer with the niScope SMT Power in Band.vi example program, found under C:\Program Files\National Instruments\LabVIEW 8.x\examples\Spectral Measurements Toolset\niScope.  On the front panel of this example program, under the Configuration tab, enter the Device Name for your PXI-5620 (i.e. DAQ::1), enter a center frequency of 16M and span of 32M.  Under the Averaging tab, select RMS averaging for the type, and continuous for the linear weighting mode.  Under the Units tab, place a checkmark in the box for PSD.  With these setting, run the example program with and without the output of the PXI-5600 connected to the PXI-5620.  This will give you a good idea of the noise floor of the PXI-5620.  The peaks in the lower frequency ranges (0-5MHz) are due to dithering being enabled on the PXI-5620.  You can disable this dithering by configuring the niScope Property Node in the SMT Config niScope for Spectrum.vi.  With dithering disabled, I was able to see a noise floor of around -125dBm with the downconverter attached, and around -125dBm without the downconverter attached.  If you see a drastically different noise floor, it might be helpful to attach a screenshot of your power spectrum graph in a reply to this post.

I hope this helps,

Travis G.
Applications Engineering
National Instruments
www.ni.com/support

[GlennD]

Thank you for your response.

The reference level I am using is 0 dBm.

I ran the ‘niScope SMT Power in Band.vi’ example with dithering enabled and disabled.  I could see the noise when dithering was enabled and the reduced noise or no noise when dithering was disabled.  See attached .jpg’s.  I added ‘Waveform Graph 2’ to display the output of ‘niScope Multi Read Cluster.vi’.

In my application, I disabled dithering and put the trigger level lower.  It now works.

Disabling dithering needs to be set after ‘ni5660 Configure for spectrum.vi’.  Inside the ‘ni5660 Configure for Spectrum.vi’, a vi called ‘SMT Configure RFSA for Spectrum.vi’ enables dithering.  ‘SMT Config niScope for Spectrum.vi’ is another vi that enables dithering.

Thank you for your help.

[Erik_J]

Hi Test Tiger,

We're glad everything is working now.  Thank you for letting us know what was causing the problem and what your solution was.  This forum will likely help others in the future.

Please let us know if you need anything else, and thanks again!

Best Regards,
Erik J.
National Instruments