SINAD Measurement with CCITT Filter

[Julie46825]

Hi,

I am doing some audio measurements (SINAD) with the NI 4461 Data Aq Card and using LabView to make the measurement. Normally the measurement is done with a Agilent 8903 Audio Analyzer which is obsolete and we are now using the NI 4461 card as a replacement in newer test stations. I am trying to correlate the measurement to a 8903B and I am 3 dB different between the NI 4461 card (SINAD=13dB) and the 8903B (SINAD=16dB).

I am using the CCITT weighting filter and a 30KHZ low pass filter in LabView which is what I have set on the 8903B Analyzer. Do you know of anyone familiar with using the tool kit or maybe anyone who has modeled a 8903 in LabView?

Thanks,

[MickeyD]

Hi,

Thank you for brining your issue to the National Instruments Forumns!

I am assuming the Sound and Vibration Toolkit you are using is the SVT Telecomunications Weighting Filter vi.  If you are using a different VI, then please let me know, because that is the one that meets the CCITT weighting specification that I can find.  I have not modeled the behavior of an 8903B Analyzer in LabVIEW, but will be glad to help you in understanding what the particular VIs in LabVIEW are doing.  One thing that will help me in making sure we are on the same page is knowing what version of LabVIEW you have installed and what version of the Sound and Vibration Toolkit you are using.  A screenshot of the section of code you have placed the filter in is often very helpful as well.  A 3dB difference seems to imply we might just have an input on the filter VI you are using set incorrectly, but if you can let me know what versions of the software you have I will be able to give the appropriate VIs a look and help get your issue resolved.

Have a great morning,

Michael D
Applications Engineering
National Instruments

[Julie46825]

Hi,

Thanks for the reply and appreciate any help you can give as I am very stuck.

I am using LabView 8.2 with SVT version 4.0. Attached is my VI and I am comparing my reading to a 8903B Audio Analyzer with the CCITT filter and the 30KHz low pass filters switched on.

Thanks,

Julie

[Julie46825]

Michael D,

Here is the same CCITT  Audio VI that I copied and pasted into Word Document in case you don't have the same version software at hand.

Julie

[Julie46825]

Well this fixed my SINAD measurement with the CCITT filter.  The NI 4461 and Labview now track the 8903B Audio Analyzer.  See attached file.

[MickeyD]

Hi Julie,

First of all congratulations on finding a fix.  I am still looking into possible reasons the low pass filter was not behaving correctly.  It seems that you simply removed that filter and then took an array subset of your data.  Applying the SINAD measurement to that.  With the original problem were you seeing the problem in that the filtered data did not appear as the filtered data on your scope, or was it simply the SINAD values at the end that were off? 

It sounds like the actual filtered signal looked different, and it is possible the low pass filter express VI since it is a software filter was cutting off data that you would not see cut off if you used a hardware filter, which would be what you would see by activating it on the oscilloscope.  If you increased the filter cut off frequency on your original program I suspect you would have seen more of the signal you expected.  From what I understand of the function the signal conditioning on the 4461 must just be removing the noise that you were trying to filter out, because otherwise the data should not be causing these troubles.  I was also wondering why you were using array subset, could you not just convert straight from the filtered signal to the dynamic data type and input that to the Distortion measurements VI?  I have a different version of the toolkit installed on my machine, but I am close to having recreated your code, so maybe I can take a look at how the behavior changes from what you described.  Let me know if I can help you further and if you want feel free to answer the questions I asked.  I will try and find an explanation and post it, but we always appreciate your contributions to the forums.

Have a great day,

Michael D
Applications Engineering
National Instruments

[jsimmo04]

I am having a similar issue trying to match a SINAD measurement of an agilient 8903b to a PXI 4461. What was the sampling rate used?

[Finnabon]

Unfortunately, given the documents provided in this forum there isn't a record of what sampling rate they used. Since they worked on this issue 4 years ago, I doubt they will respond with that information. 

As good practice, your sampling rate should be at least 10x faster than the rate of your signal to acquire a decent signal.

[jsimmo04]

I was wondering if there is a type of filtering recommended to use to make the SINAD measurement on a NI- PXI 4461 more compatible to an Agilent 8903b audio analyzer? I noticed in this code the user is using the CCITT weighting filter is this recommended? I currently I am using two separate stations one with the Agilent 8903B and one with the NI-PXI 4461 I am not using any filtering at all and the 8903B consistently measures 1.5 dB higher SINAD? I also noticed in the code from this thread the user is using 10,000 samples instead of 200,000 was this done to make measurements more compatible?