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MXA N9020A - intermodulation behavior issue with narrow and wide span

Question asked by mirek on Mar 4, 2016
Latest reply on May 31, 2016 by mirek

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I see an issue with the intermodulation measurement on MXA Spectrum Analyzer when you expand the span and you keep the RBW and VBW narrow.
Normal way to measure intermodulation on MXA is to set Spectrum Analyzer to narrow span (1 kHz) with RBW and VBW to Auto (it show 9.1Hz for both RBW & VBW). Also the FFT gain will be set to low. With this setting I can get system intermod to around 110dBc.

I prepare some demo to show intermodulation performance, so I set the MXA to wide Span, however I kept the RBW and VBW at 9.1Hz
I will assume that with wide Span and narrow RBW I should get the same Intermodulation performance as narrow span. I am getting around 90dBc with 5MHz span.
It look like some IF filter is disabled in the receiver path as you expand Span. I think that it is not correct behavior.
If you keep the RBW low and you expand the span the only penalty you should see is long sweep time, not dynamic range of intermodulation.

I wonder if there is any way to force the same setting as you have on narrow Span to get sufficient dynamic range for Intermod?

The firmware on the unin that I run is A.13.12

Regards.

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Im3 measurement 1KHz span 9_1Hz RBW.png70.2 KB
Im3 measurement 5MHz span 9_1Hz RBW.png75.2 KB

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am95405 Mar 29, 2016 3:31 PM
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I'm not an MXA expert but I do have a PXA at my desk and was trying to see if I could replicate your question.

What are you measuring in the 1 kHz span case? You have a different Center frequency and the delta marker shows 2 MHz, but the span is 1 kHz.

Also, your phase noise setting is different, though I am not sure if that would have any impact on your question.

Afsi
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- mirek @ am95405 on Apr 12, 2016 1:22 PM
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  Thank you for the response.
  
  On the 1 KHz Span I am measuring system upper sideband IM3 product where the fundamental F1 & F2 are 1 MHz apart. The 2 MHz offset is because it refer to F1 frequency that is 2 MHz below upper IM3 product.
  I never change manually the phase noise setting...I think that automatically done when I change the Span.
  
  ps. I should really make plot that is less confusing where I not moving the center frequency, just change the span with fix RBW.
  Here it is:
  
  Edited by: mirek on Apr 12, 2016 2:18 PM
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  - 5MHz span 9_1RBW manual.png79.3 KB
  - 1kHz span 9_1RBW manual.png75.4 KB
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  - am95405 @ mirek on Apr 13, 2016 1:43 PM
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    Attached are four plots, two with my PXA and two with a colleague's MXA. Do they help? Also, this is MXA Service Guide, which has block diagrams.
    http://literature.cdn.keysight.com/litweb/pdf/N9020-90218.pdf
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    MXA_1kHz.PNG60.3 KB
    MXA_5MHz.PNG61.1 KB
    PXA_1kHz.PNG189.7 KB
    PXA_5MHz.PNG190.0 KB
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    - mirek @ am95405 on Apr 18, 2016 9:57 AM
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      Appreciate for the effort.
      
      There is not much telling without the RF1 & RF 2 apply. Without this two signals there is nothing to destabilize the receiver. Can you get some 2 RF generator and combiner (coupler) and feed the two signals into both spectrum analyzer and see if there is any difference when span is set wide and narrow with 9.1Hz RBW?
      The sideband I see are intermodulation products so you need to apply real signal to see them.
      
      I see this behavior on all our MXAs N9020A
      
      I do not have in our lab PXA, however I do have the old PSA. Look like it do not have that behavior. Almost no difference between 5MHz (probably 100 time slower sweep time then on MXA) span and 1 KHz Span. The reading for with RF1 & RF2 at -20dB to input (internal attenuation set to16dB) show around -130dBm. The only adjustment I had to do was to change default 601 point to 1001 points.
      My belief is that MXA might have some software bug where it force the signal for the wide band path even so the RBW is set to narrow band.
      
      Regards,
      
      Mirek
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      - am95405 @ mirek on Apr 21, 2016 9:15 AM
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        I have an EXG that can generate a two-tone signal, but I don't know how clean that is. But I don't see a difference in the marker reading when I change the span. My PXA screenshots attached. The MXA I was using is gone, so I can't try the same experiment on the MXA. Anyway, I'll try and locate an MXA engineer and forward this thread on to them to see what they think.
        
        Afsi
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        5MHz.png115.3 KB
        1kHz.png113.7 KB
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        mirek @ am95405 on Apr 22, 2016 6:29 AM
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        Yes the EXG might not be best choise. I think on EXG when generate two tone it will get IM3 max 90dBc. That far cry what you can get from two independent RF generator and good quality combiner. If you have (any) 2 generators and some 86205A coupler/bridge around that might be perfect. The 86205A is 16 dB coupler, so I usually balance the ports adding 13-16dB attenuator. That also improve the isolation to get even better intermod from the setup.
        The 90MHz is not critical. I see that behavior on all frequencies, so if you have any other coupler that work at different frequency and have good directivity is should be good for the setup..
        You setup at narrow band (1KHz Span) should deliver better 100dBc (it will be more like 110-115dBc). Otherwise you will not see any difference.
        
        regards,
        
        mirek
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        coupler_configuration_.jpg177.6 KB
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        am95405 @ mirek on Apr 22, 2016 11:02 AM
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        I have sent you a private message.
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        am95405 @ mirek on May 2, 2016 8:22 AM
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        So we moved to a new system. I think this is the third or fourth iteration of the forum. :-)
        
        Did you get my private message? If you replied, I do not have it. My forum inbox does not seem to have migrated to the new platform.
        
        Afsi Moaveni
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mirek May 3, 2016 8:15 AM
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No the system did not show the message during the update...now I am not sure where they are located after the update.
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- am95405 @ mirek on May 3, 2016 8:54 AM
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  I can't figure out the direct messaging here (looks like we have to be friends or something and I don't have time to fiddle with the software today), so can you just send me an email. My email is:
  
  afsi underscore moaveni @keysight.com (remove the spaces).
  
  Afsi
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Dr_joel

May 3, 2016 2:10 PM

You might check to see if the FFT width is set to auto or to a fixed width. Default is to an auto width, and the width is adjusted according to the span. If you go to your narrow span case, and look at the width selected, and then choose the same width for your wideband case, I guess it might make the two the same. I also guess it will make a slow sweep.

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mirek @ Dr_joel on May 31, 2016 5:43 AM

Dr_joel

That you for the response.

I follow you advice at try to adjust the FFT width. Here are results for 1K & 5M span:

SPAN 1 KHz IFBW 9.1Hz
FFT width		IM3 (dBc)
4.01	KHz	110
28.81	KHz	109
167.4	KHz	108
411.9	KHz	110 (Set Auto)
7.99	MHz	110
10	MHz	109
SPAN 5MHz IIFBW 9.1Hz
FFT		IM3 (dBc)
4.01	KHz	106
28.81	KHz	108
167.4	KHz	90
411.9	KHz	90 (Set Auto)
7.99	MHz	112
10	MHz	111

At 1 KHz Span look like no difference what FFT is set, however when you go to 5MHz Span it totally different story.

You can see that 411.9 KHz & 167.4 KHz FFT width show upper IM3 only 90 dBc. Interesting that the 7.99 MHz and 10 MHz show better reading than 4.01KHz & 28.81 KHz.

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MXA N9020A - intermodulation behavior issue with narrow and wide span

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