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ADC 212/100 Spectrum Analyzer question

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ADC 212/100 Spectrum Analyzer question

Postby Mark_H » Sat Jul 26, 2003 6:11 pm


I'm new to the world of oscilloscopes and electronics in general, so I'm on a real steep learning curve and probably making a very simpl mistake. Please be gentle! :D

I'm trying to use the spectrum analyzer to create a frequency response curve from a CD player's analogue output. I'm using a test CD to do a frequency sweep from 20Hz to 20Khz and have the analyzer set to show peak dB as the sweep passes.

However, I am not getting the results I expected.

On the scope, the input is reading from -3 to 3V instead of the expected +/-2V as per the player's spec. And on the spectrum analyzer, the readings are all at +10dB, even though the sweep is recorded at 0dB. I assuming some sort of termination problem as I'm simply connecting the player's output directly to the scope.

On a related note, I've been measuring video output from a DVD player. I am getting the correct readings from the scope (1V pp on Y output) - this time using 75ohm termination, but again, all spectrum analyzer readings are at +10dB.

Finally, using the scope's signal generator to generate a 2.5Khz square wave at 5V, ie indeed see the wave in the scope view, but again it is at 10dB in the spectrum view when I would expect the 2.5Khz peak to be 0dB?

Any insights into where I'm screwing up would be much appreciated.

Many thanks,


ADC 212/100 Spectrum Analyzer question

Postby wheissinger » Tue Jul 29, 2003 1:11 am

0 db record level on a cd just means it is the maximum unclipped digital level. Most players will output around 2 volt rms from a 0 db test tone. Picoscopes 0 db reference in the spectrum analyzer is 0.775 volt rms by default. +10 db is a gain of 3.162, so (0.775 x 3.162) = 2.45 volt rms, a little on the high side of normal but not much. Picoscopes 0 db reference level can be changed as explained in help/technical info/ini file settings. Decibels are only ratios to a reference level which can be anything you want it to be. I hope that helps on that question, as to the others someone smarter than me will have to jump in. W.H.

Postby Mark_H » Tue Jul 29, 2003 10:56 am

Thanks for that.

By setting the ZeroDb setting in the ini file I was able to then plot the results I expected.

Editing the .ini file is a little cumbersome, so I tried setting a custom range which scaled the CD player's AC RMS output to the default ZeroDb setting, ie

0 0
2138 776

Unfortunately, and this must be a software bug(?), when the custom range is used, the Y axis display starts from -200dB and only goes lower... I cannot see anything above this and cannot therefore see the readings. However, if I add a max amplitude measurement, I can see that the max amplitude is being scaled to close to 0dB. This suggests that the scaling is working but that there is an bug in the spectrum Y axis display routine?

It would be great to be able to specify the 0dB reference voltage in the UI.

Anyway, thanks for the original answer - it helped!



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