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N9030A Amplitude/Span issue

Question asked by andyTPE on Mar 17, 2011
Latest reply on Mar 31, 2011 by tabbott

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I have an N9030A with f/ware A.07.04

I am trying to make some IM measurements, so the input signal to the spectrum analyzer are two tones separated by 1 MHz with a centre frequency of 1 GHz (vary centre frequency 1- 6 GHz in 1 GHz steps).

The signal generation 10 MHz is used as the reference to the N9030A (i.e. the source and receivers are locked together)

I have my front end attenuator set to 20 dB. Given that my IM products are low down, I have the RBW=1 Hz and the span set to ~5 kHz. Under this configuration the SA is using the FFT sweep mode. I am using a Pk detector. There are 1001 pts in the frequency sweep.

Now, with the span set to 5 kHz, my IM level measures -98 dBm. If I change the span (LEAVING ALL OTHER SETTINGS ALONE), the measured IM level now measures -108 dBm. I left the sweep time autocoupled, so the sweep time changes slightly between the two span settings.

I would have thought that the amplitude of my IM tone would be independent of the span.

There is no mention in the A.07.06 firmware release of this being a known bug or that it has been fixed.

Can someone explain to me why when I change the span my tone changes by 10 dB? Is this a bug, is it a measurement limitation or do I have some conflicting settings that is causing this?

Thank you,

Andy

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andyTPE Mar 17, 2011 10:47 AM

Sorry, I forgot to mention that the 2nd span is 2 kHz....

Span=5 kHz I measure -98 dBm
Span=2 kHz I measure -108 dBm

Thanks

Andy

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tabbott

Mar 31, 2011 10:57 AM

Hi Andy -

I realize you have already received support for this issue elsewhere - but thought I'd post the support response for our Forum viewers...

I believe that probably the default IF prefilter setting (based on the FFT width) is too wide for your particular IM measurement where the tone spacing is 1 MHz and the IM product you are measuring is about 1 MHz away from one of the tones. With the default FFT width of <411.9 kHz, this yields a default IF prefilter BW of 966 kHz. This wide prefilter BW setting is very close to the tone spacing, therefore, instead of allowing only one tone ‘through”, the filter allows part of the second tone to be present in the measurement.

Why do I think this?
Look at the PXA specs guide under the TOI specification. Somewhere around page 41. The TOI specification mentions that the “Tone separation >5 times IF Prefilter bandwidth” and if you follow the foot note, you are referred to the IF Prefilter bandwidth spec which shows FFT width verses IF prefilter 3 dB BW.
If we follow the Tone separation >5 times IF Prefilter BW, our IF Prefilter needs to be about 200 kHz (and not 966 kHz).

So, when I set up the same measurement that you sent your screen shots for, I then pressed: Sweep Control, Sweep Set up, and with the FFT Width in Auto, I see that the FFT Width is indeed <411.9 kHz, which should yield the IF prefilter of 966 kHz.

So, please modify the FFT width by pressing measurement by pressing: Sweep Control, Sweep Setup, FFT Width, <28.81 kHz. This provides an IF Prefilter BW of 79 kHz. The measurement speed will increase, but we will know if the problem goes away or not.

Regards -

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andyTPE Mar 17, 2011 10:47 AM
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Sorry, I forgot to mention that the 2nd span is 2 kHz....

Span=5 kHz I measure -98 dBm
Span=2 kHz I measure -108 dBm

Thanks

Andy
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tabbott Mar 31, 2011 10:57 AM
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Hi Andy -

I realize you have already received support for this issue elsewhere - but thought I'd post the support response for our Forum viewers...

I believe that probably the default IF prefilter setting (based on the FFT width) is too wide for your particular IM measurement where the tone spacing is 1 MHz and the IM product you are measuring is about 1 MHz away from one of the tones. With the default FFT width of <411.9 kHz, this yields a default IF prefilter BW of 966 kHz. This wide prefilter BW setting is very close to the tone spacing, therefore, instead of allowing only one tone ‘through”, the filter allows part of the second tone to be present in the measurement.

Why do I think this?
Look at the PXA specs guide under the TOI specification. Somewhere around page 41. The TOI specification mentions that the “Tone separation >5 times IF Prefilter bandwidth” and if you follow the foot note, you are referred to the IF Prefilter bandwidth spec which shows FFT width verses IF prefilter 3 dB BW.
If we follow the Tone separation >5 times IF Prefilter BW, our IF Prefilter needs to be about 200 kHz (and not 966 kHz).

So, when I set up the same measurement that you sent your screen shots for, I then pressed: Sweep Control, Sweep Set up, and with the FFT Width in Auto, I see that the FFT Width is indeed <411.9 kHz, which should yield the IF prefilter of 966 kHz.

So, please modify the FFT width by pressing measurement by pressing: Sweep Control, Sweep Setup, FFT Width, <28.81 kHz. This provides an IF Prefilter BW of 79 kHz. The measurement speed will increase, but we will know if the problem goes away or not.

Regards -
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N9030A Amplitude/Span issue

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