# Square wave distortion at low frequencies, high-pass filtering, AC coupling

This is an update on my previous question.

As Hearth suggeste, I changed to DC coupling on my scope.

This is what I got:

That's a clear change. Not a perfect square waves but I guess it is because of the 200MKF capacitors at the end of those transistors.

I was assigned to fix the generator (square wave function didn't work before.)

I had to change two 200MKF capacitors. The lowest I could get was 220MKF (I guess that's the reason behind the not so perfect squares,) and I had to connect the square wave amplifier to the output.

IWhen I thought I had finished the job, the professor told me about not getting square waves at 1V 1Hz (first picture.) I can't really understand if he didn't know that scope was set to AC coupling or he wanted it to be on AC coupling.

Is it possible to get square wave at that voltage and frequency with AC coupling? If yes, how? (Best guess using compensator.)

Is it really that distorted because I used 220MKF instead of 200MKF, or ar there any other possible reasons? How can I improve it?

What is the difference between AC and DC coupling?

What I understood from the internet is that DC coupling lets the signal through to the scope directly whilst AC coupling uses a high-pass filter - a capacitor, which removes some frequencies, and at low frequencies I get s distorted square wave (picture 1), but at higher frequencies that capacitor acts like resistor, thus increasing frequency, makes wave more like square. Is this thinking anywhere near true? If not let me know how it really ist.

When and why do I have to use AC or DC coupling?

I really want to understand science behind all of this.

• "and when and why i have to use AC or DC coupling" AC coupling is used when there is a small AC signal over a larger DC one (typical example: the output of a switched mode power supply, set to 10 Vdc with 20mV ripple). If the signal of interest is the AC one, the DC part should be removed otherwise it would mean a very poor use of the ADC range. By removing the DC with AC coupling, instead of using, say, 2V/division a 5mV per division can be used. May 6, 2022 at 13:59
• so when i need square wave i need dc coupling. is this right? is there any case when i might need ac coupling for square wave? May 6, 2022 at 14:02