Measure in and output of a Mosfet

[Apfelsaftbaum]

Hi there,
first of all, i am a beginner when it comes to oscilloscopes, so please don`t be too hard with me  
I am working on a little PWM based led dimmer, using a n-channel mosfet (IRLB8721) to dim some 12V led strips. It seams to work as it should.
Now i want to measure and compare the 3.3V logic signal with the 12V Output signal, to see if the mosfet switches all the way and to check the timing...

Now my question: How can i measure the in and output at the same time with only one 2 channel oscilloscope (Siglent SDS1102)? Is that even possible?
As far as i understand, i need a reference for the scope, but for the logic signal it would be ground and for the Mosfet +12V right?
But the clips from both channels of the scope are directly connected...

Anyway, i hope someone can help me out, thanks in advance!

Greetings from Germany

[uncle_bob]

Hi

Simple approach:

Both scope probe clips go to ground. That's the reference for both signals.

Trigger on the 3.3V logic input.

The 12V signal will be swinging from +12 down to "something". It might be zero it might be another level. Since you are referenced to ground, you can directly read out what that voltage is. Since you are triggering on the logic input, any delay will show up as a shift of the 12V signal on the scope display.

Hopefully that makes sense !!!

Bob

[alsetalokin4017]

In other words: Both probe references to Mosfet Source pin, which is normally tied to the circuit's ground or negative power rail. One probe to Mosfet Gate pin, and trigger on this channel. Other probe to mosfet Drain pin.

While this mosfet has a very low Gate threshold voltage it might be happier with at least 4.5 volts to the Gate rather than the 3.3 volts.

If the mosfet is turning all the way on, the Drain-Source voltage will be essentially zero.

[Apfelsaftbaum]

A big thanks to both of you for your help!
The method with the logic level being the trigger input worked out perfectly.
I also put in a level shifter to 5V, with that the output signal now looks much cleaner than with 3.3V Thanks for the hint there.