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

Question

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.

Answer 1

In DC mode a scope will show whatever the voltage on the input is, if it's 1 VDC you will see a flat trace at 1 V (assuming the range is set accordingly).

In AC mode a capacitor is put in series with the signal. This will block DC voltages but allow AC voltages to pass. The reason for doing this is that you don't always want to see the DC component of a waveform, for example you could have a 10 mV peak sine wave riding on a 100 VDC voltage, so the sine wave is going from 99.09 V to 100.01 V. If you set the scope to display a 100 V signal the 10 mV waveform is going to be very small. So you switch to AC mode, which removes the DC component and you can set the vertical range to something that shows the AC signal well.

When displaying a square wave in AC mode, the capacitor will pass the leading and falling edges, because they are basically a very fast AC signal. But the top and bottom of the waveform are DC, so it gets blocked. The curve you see is the exponential discharge curve of the capacitor. The smaller the capacitor, the faster it discharges, and the more pronounced this curve will be. With a larger capacitor, or a narrower square wave, you will only see the beginning of the discharge curves, so it appears to be flatter.

In DC mode you may still see some tilting of the waveform due to the frequency response of the scope's amplifiers and impedances in the probe and cable. There are usually calibration controls to compensate for this, both in the scope and in the probe. Most scopes have a reference output, you connect the probe to it and adjust a small variable capacitor in the probe to get the top and bottom of the reference waveform as flat as possible. If this is adjusted properly and you still see tilting when looking at a signal the signal may be too fast for the scope. There will be a bandwidth specification for the scope. They will not work perfectly up to that frequency, a scope specced at 100 MHz might start to degrade the waveform above 20 MHz.