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Andras

WaveForms 3.8.22 Spectrum Analyzer scaling problem

Question

Hi,

I'm using WaveForms 3.8.22's Spectrum Analyzer with Analog Discovery 2.

When I run the analyzer I use FFT with 129 BINs in the 1 Hz to 100 Hz range on both channels.

The magnitude settings of the Spectrum Analyzer are the follows: Vrms, top: 10 mV, bottom: 0 V, reference: 1V,  range: 5V.

I selected the exponential dB Average on both channels.

If I run the spectrum analyzer and while it's running I change the top value from 10 mV to let's say 20 mV it instantly changes the chart correctly, but after a new measurement completes the chart changes drastically which is not the expected behavior, I think.

Here is a screenshot of my workspace (just before the change):

image.thumb.png.174a52a08b9dd885923f802ef9346774.png

And another one just after the change from 10 mV to 20 mV:

image.thumb.png.097d29759105389fbb4a8132b537e2f9.png

 

As you can see the chart changes significantly while the source signal remained somewhat the same.

Interestingly when I tried to reproduce the problem by connecting the signal generator with the scope while generating a steady sine signal, the problem didn't show.
It did show though when I set the signal generator to sweep mode.
It also work fine if I change the type of the trace from the exponential dB average to sample.

Is there a reason why this could happen? Does the averaging algorithm's buffer get reset when we change any of the parameters of the spectrum analyzer?

 

Thank you,

Andras

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Szia András,

In WF 3.8.2 Spectrum the channel range (by default) is adjusted automatically based on the Magnitude Top.
In WF 3.8.12 a Main Range option was added, since setting Top to 5dB or higher the AD2 input channels switched to 50V range which provides lower resolution, higher noise floor.
Under channel options the range can be adjusted individually for each channel.

However, regardless of the above settings the capture (averaging, hold...) are restarted when changing the Top parameter. 
I have corrected this for the software release.

image.png.41452bf409a1534f942fdc59dbc84976.png

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9 minutes ago, Andras said:

Is there a reason why this could happen? Does the averaging algorithm's buffer get reset when we change any of the parameters of the spectrum analyzer?

Ah, never mind, I have just realized that the dB average counter on the bottom of the chart shows that is get reset.

In this case it's more like a feature request: could you keep the average function's buffer if the user changes a non-critical parameter of the measurement like the vertical scale?
Do I understand it correctly that changing the top and bottom settings of the spectrum analyzer doesn't change the measurement parameters if the range remains the same?

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