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Is PXA's average speed dependent on power level?

Question asked by midam001 on Feb 1, 2017
Latest reply on Oct 25, 2017 by bchen94
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Is PXA's (9030A) averaging speed in FFT mode dependent on signal power level?

I am seeing slower average measurement when power is closer to the Reference level but it gets much faster once the power level decreases and goes further below Ref level.

Thanks.

Steve

Edit: An E-band external smart mixer was used if that makes any difference.

bchen94
bchen94 Employee Oct 25, 2017 1:23 PM
Correct Answer

Hi midam001!

 

An analyzer was tested with FFT sweeps and Trace Averaging turned on. The input signal was -5 dBm. Long story short, there is a measurement speed slowdown, here's why:

 

First of all, the external smart mixer does not impact the measurement speed. Instead, the analyzer has a setting called FFT IF Gain. The three states are below:

 

  • High Gain - this provides the best noise level for the analyzer
  • Low Gain - this is better for large signals
  • Auto Range - this 'follows' the signal and attempts both the high gain and low gain setting, then chooses the appropriate one.

 

The automatic setting in the analyzer is Auto Range. You can already infer that in this mode it'll be slower because it has a few criteria to meet to determine the final setting.

 

It first tries High Gain, and checks if the ADC is overloaded. If not, then the measurement is fast and it sticks with this mode.

 

If the ADC is overloaded, the Auto Range mode switches to Low Gain and does averaging again. High amplitude signals are very close to the reference level, so in your case the instrument had to check twice, hence why it appears that the measurement speed was power dependent.

 

Hope this helps!

Bertie

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