Spectrum display peak values. Log vs linear

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Apno
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Joined: Mon Jun 05, 2017 12:38 pm

Spectrum display peak values. Log vs linear

Post by Apno »

I use the spectrum display frequently (excuse the pun) and like it. However I have noticed something odd in the way it displays peak values.
Have a look at a log sweep vs a linear sweep. You'll see that the peak values of the linear sweep are all the same value across the bandwidth, as they should be. But not on the log sweep. Flat trace up to a point, then falling.

I'm sure there is a calculation reason for this, but it's misleading. The peak values of the log sweep do not actually fall as frequency rises.
White noise, pink noise and brownian noise all display as they should, just not the log sweep.

GoldWave Inc.
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Re: Spectrum display peak values. Log vs linear

Post by GoldWave Inc. »

That could be a side-effect of the finite length of the peak detection. Zooming in should give a more accurate display of the peak levels.

What equation are you using to generate the log sweep?

Apno
Posts: 8
Joined: Mon Jun 05, 2017 12:38 pm

Re: Spectrum display peak values. Log vs linear

Post by Apno »

Sorry to be so long in getting back. I'll have a look at zooming.
The sweeps were make in GW, REW and HOLMImpulse for comparison. Linear vs log for sure show different peaks levels.

GoldWave Inc.
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Re: Spectrum display peak values. Log vs linear

Post by GoldWave Inc. »

Please post the expression you used for the log sweep.

The log sweep may be generating frequencies that straddle a frequency bin in the FFT calculation. That would give a drop in the peak because half the signal is in one bin and the other half is in the other bin. Try viewing the graph in the Spectrum Filter and adjust the FFT size to see what that gives. Note that a slow changing frequency may straddle a bin at any FFT size.

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