Spectrum dropping

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jcclq
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Spectrum dropping

Post by jcclq »

Hi

I am acquiring a batch of spectra to calculate the RIN spectra, and I noticed that sometimes, this goes very low. I checked in oscilloscope mode, and I did not notice any drop to 0. I only noticed that sometimes I have a drop of 1%, but not sure it can explain.

I used AC coupling, but of course I still have a peak at 0-frequency. I noticed that a drop on the spectrum is also associated to an increase of the 0-frequency signal. I thought that this 0-frequency increase could be due to the 1% drops, but I don't see how it could explain the drop of the other frequencies in the spectrum?

I am attaching 2 typical spectra: a common one, and one with a drop.

Thanks in advance for any help

JC
Attachments
Common spectrum.PNG
Common spectrum.PNG (10.99 KiB) Viewed 4065 times
Spectrum with a drop.PNG
Spectrum with a drop.PNG (9.72 KiB) Viewed 4065 times

bennog
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Re: Spectrum dropping

Post by bennog »

can you post a .psdata of both examples.

Benno

jcclq
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Re: Spectrum dropping

Post by jcclq »

Hi bennog

Thanks for your reply. I don't have some .psdata files right now but I'll try to record some.

Best regards
JC

jcclq
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Re: Spectrum dropping

Post by jcclq »

Hi

I recorded a batch of spectra in .psdata format. I attached 3 successives spectra. The middle one (#565) is the strange one, and I also attached some 'usual' spectra, the one jsut before (#564), and the one just after (#566).

Thanks for your help!
JC
Attachments
Usual-#564.psdata
(58.13 KiB) Downloaded 142 times
Low-#565.psdata
(15.81 KiB) Downloaded 143 times
Usual-#566.psdata
(65.42 KiB) Downloaded 140 times

bennog
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Re: Spectrum dropping

Post by bennog »

If you had added the scope view you could have seen the measured values are out of range.
So the low spectrum is because you have almost no signal because all signal is clipping to the top or bottom of the range.

Probably because you had 50/60 Hz (looks like 60) hum on the signal.
Attachments
2021-08-27_17-58-18.png
2021-08-27_17-58-12.png

jcclq
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Re: Spectrum dropping

Post by jcclq »

Thanks a lot. Yes it is clear now. In my case it is 50Hz (I am based in UK). I probably have a bad contact on my BNC line. I will also make sure it is all well grounded.

This is really helpful. I didn't know I could look at the scope view as well, now I will check this!

Best regards, thanks again
JC

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