Hello Team,
There is a phenomenon that appears as a function of sensitivity of the oscilloscope.
I’ve done numérous tests and it is always the same.
For measurements below, the probe remained connected, but nothing has been modified texcept the calibration AC 20mV versus AC 50mV.
So in clear I measure the exact same thing but as you will see the results are quite different.
First snap shot
Measurement done using the 50mv (mode AC) sensibility with the probe factor set to x1.
We can see a 125 KHz oscillation (14mV) on the oscilloscope view.
On the FFT View, we the the exact same thing . (that is cool !)
Please note that the same phenomenon appear when I increase the sensbility 0.1V 0.2V …up to 2V.
Second snapshot :
The probe is still in place, I just click on the 20mV sensibility. The 125Khz Oscillation disapear…… as well on the FTT.
Please note that the same phenomena appear when I used lower sensibility (10mV and 5mV).
Am I Missing something there ?
Is this a bug ?
It seams that there is a special treatment when we are between 20mv and 5mv sensibility….
Thanks in advance for your help and support.
Brgds Gilles.
NOTE : I’ve the V4.02 version of the PcLAB2000SE
It seems that I can’t reproduce the issue.
This is the screenshot with the same settings as you used:
The Volt/Div setting is changed from 20mV to 50mV just before plotting this screenshot.
The probe tip is connected to the probe ground lead.
Does the problem occur if you disconnect the probe from the circuit and connect the probe tip to the probe ground lead?
Sorry I was not clear. 
In my first post I did not mention that I was measuring something .
In fact i was measuring a 50V SMPS Power supply, and wanted to see the noise of this power supply. The Oscillation we see are normal - MOSFET are working at 125Khz approximativelly and sur-oscillation of the SMPS … in my case around 11MhZ.
What is weird is the fact that in one case I’ve 14mV of oscillation (like a square signal) … and not in the other case.
the strange thing is that when I click on the 20mV or on the 50mV (with the exact same configuration) I got differents results. That should not be the case. right ?
Ok I will try to see what’s going on in the same configuration as you mention.
thank you. Gilles.
[quote]the strange thing is that when I click on the 20mV or on the 50mV (with the exact same configuration) I got differents results. That should not be the case. right ?[/quote]You are right. This shouldn’t happen.
Do you have the same problem with CH2 ?
Do you have the same problem if you use the probe x10 position?
[quote=“VEL255”][quote]the strange thing is that when I click on the 20mV or on the 50mV (with the exact same configuration) I got differents results. That should not be the case. right ?[/quote]You are right. This shouldn’t happen.
Do you have the same problem with CH2 ?
Do you have the same problem if you use the probe x10 position?[/quote]
I will try that tonight and will tell you.
Brgds gilles.
I have been pondering this and keep coming back to thinking that the YT trace in the original message (see below) looks as though there is a triggering issue–as though that at 50mV/div the input signal amplitude is too low for the internal trigger circuit to get a good lock???
I could be way off base on this, however the trace looks a lot like what happens when a DSO cannot get a good internal trigger lock…
