Test and Measurement Forum

Hi,

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!

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,

Mark.

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.

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!

Cheers,

Mark