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Nov 15th, 2019, 3:27pm
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Power average vs Log average on Spectrum analyzer (Read 630 times)
iVenky
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Power average vs Log average on Spectrum analyzer
Dec 18th, 2018, 3:58pm
 
Hi,

I have a question regarding the average feature on spectrum analyzer.

First thing, why do we need average? What's that that's causing the measurement error to change? if you look at the figure below, we can see that the noise power changes by about 10 dB (between -50 dBm and -60 dBm). Why does this happen? Is it just because the mean square value at the output of IF filter is changing with time because of finite time averaging?

Second question. Which one is accurate or right? Log average or Power average? I can understand Power average. Because log average can give a lower value, I would want to know the reason behind using log average or when it is useful?

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Horror Vacui
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Re:  Power average vs Log average on Spectrum analyzer
Reply #1 - Sep 18th, 2019, 4:56am
 
iVenky wrote on Dec 18th, 2018, 3:58pm:
Hi,
What's that that's causing the measurement error to change? if you look at the figure below, we can see that the noise power changes by about 10 dB (between -50 dBm and -60 dBm). Why does this happen?


Noise.

iVenky wrote on Dec 18th, 2018, 3:58pm:
Hi,
Which one is accurate or right?


Averaging the power - i.e. normal averaging - is equivalent of making multiple measurements (N), which in general reduces your noise (by 1/N). I believe reducing the resolution bandwidth would have a similar effect. Narrower bandwidth means longer integration, which is just kind of an extreme version of averaging.
Averaging the log would give you a nicer curve, with less noise. It would make sense to me, if there would be an exponential characteristic somewhere in the processing chain.

If you have a strong enough signal, than it does not matter which one you choose.
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