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Different values spectrum mode

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Different values spectrum mode

Postby klc » Mon Mar 30, 2009 9:16 am

I have a nice clean sinus 133Khz showing 2.23 Vrms in scope mode
In spectrum mode when scale is set to dBV it meassures 6.86 dBV (2.2Vrms). If i set it to arbitrary (1 dBuv) i read 123.9 dBuv (1.56 Vrms).
Why do i see this difference.

Lennert
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Postby Robin » Tue Mar 31, 2009 9:24 am

Hi

I just want to be clear what you are doing, so I can check this out.

Are you using the "amplitude at peak" measurement in spectrum mode?

What value are you entering for the arbitrary dB, is it 1 uV?

Robin
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Postby klc » Thu Jun 18, 2009 9:16 am

So sorry for late responce :oops:

Freq: 133Khz
Voltage: 2.01 Vrms clean sinus
Probe 1:1

Spectrum setup

Sample interval: 3,200uS
Sample rate: 312,5 kS/s
No. samples: 32,768
Window: Blackman
No. Bins: 16384

Meassurements:

Range +- 20V: 8,8 dbV
Range +- 10V: 8,8 dbV
Range +- 5V: 8,8 dbV
Range +- 2V: 7,1 dbV ????

Arbitrary 1uV

Range +- 20V: 126 dbuV
Range +- 10V: 126 dbuV
Range +- 5V: 126 dbuV
Range +- 2V: 124 dbuV ????
Range +- 1V: 118 dbuV ????

I can see it's because we're out of range but nothing tells me that. In scope mode the signal just passes the display but in spectrum mode the signal seems ok, it is still within the display, just reduced. I know it is up to the user but some kind of overload indication would be nice.

Lennert
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klc
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Postby klc » Thu Jun 18, 2009 9:23 am

In my first post it seems that voltage range was +- 2V so that might be the explanation :)

But still no overload varning in spectrum mode !!

Lennert
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Postby Robin » Thu Jun 18, 2009 11:55 am

Hi

You should still get the little red box with the exclamation mark in spectrum mode. I have just double checked this.

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Postby klc » Thu Jun 18, 2009 12:13 pm

WOW - it's there :oops: I didn't know and i haven't noticed that little mark.
Perfect!!
Thank you.

Lennert
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