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This is an update on my previous question.

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

This is what I got:

enter image description here

enter image description here

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.

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  • \$\begingroup\$ "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. \$\endgroup\$
    – devnull
    Commented May 6, 2022 at 13:59
  • \$\begingroup\$ 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? \$\endgroup\$ Commented May 6, 2022 at 14:02

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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.

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  • \$\begingroup\$ thanks really made it clear \$\endgroup\$ Commented May 6, 2022 at 13:59
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    \$\begingroup\$ @kawasaki997 I added a little bit at the end. It's important to know the limitations of your test equipment, read the manual if you have one, that can tell you a lot of things you wouldn't think of. \$\endgroup\$
    – GodJihyo
    Commented May 6, 2022 at 14:09
  • \$\begingroup\$ thanks i found manual but its in russian so i would have a quite hard time understanding it. but doesn't my professor know this or maybe he didn't pay attention as well. what concerns me is what if he asks me to make square waves at ac coupling don't know what to answer or do then \$\endgroup\$ Commented May 6, 2022 at 15:12
  • \$\begingroup\$ @kawasaki997 That's hard to say without knowing why the professor is asking that. They may just be trying to teach you what happens when you use AC coupling with a square wave. \$\endgroup\$
    – GodJihyo
    Commented May 6, 2022 at 15:46

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