Electronics > Beginners
Oscilloscope: Square wave view while in FFT mode?
Wimberleytech:
--- Quote from: tinkerbotstl on January 29, 2020, 05:44:24 pm ---Not getting into the issues with using an oscilloscope on AC mains, I have seen examples of AC sine waves that show the exact same view on an oscilloscope, but the AC waveform is not a square wave. So how do they get this square wave view above with the fft mode, on a AC sine wave - which is what I'm really interested in?
--- End quote ---
No, a sine wave (a pure one, or even mostly pure one) will not have the same FFT results as a square wave. That is a fundamental fact.
What is the big picture of what you are trying to solve/understand. We are here to help, but need more information.
Wimberleytech:
--- Quote from: tinkerbotstl on January 29, 2020, 05:59:53 pm ---Maybe my AC wave does not have the harmonics I thought. I will have to look at one I know does.
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Do not confuse the terms "AC" with "sine wave." Do not use them synonymously.
tinkerbotstl:
Thanks, I am trying see the harmonics on an AC line.
rstofer:
--- Quote from: tinkerbotstl on January 29, 2020, 06:11:52 pm ---Thanks, I'm primarly trying see the harmonics on a AC line. Then I want to see if that passes on to a computer power supply, and ultimately to the USB ouput.
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I don't probe mains... Even if it is full of trash, there's probably nothing you can do about it.
You can safely look at the power supply output if it is a normal PC because the low voltages are referenced to the chassis ground which is earth ground which matches the earth ground on the scope. Same with the USB power, one side will be earth ground. There is an exception if the PC only has a 2 conductor power cord or it uses a power brick with a 2 conductor plug.
You need to think very carefully about how you probe stuff. If is often better to check things with a DMM. For example, check the voltage between what you think is ground on the PC and an actual earth ground like the BNC connector on your grounded scope. It should be 0V or very very close. If it isn't, you can't hook the probe ground lead to that point in the circuit.
Dave did a video on "How Not To Blow Up Your Oscilloscope". Watch it a few times:
https://youtu.be/xaELqAo4kkQ
Before you start probing power systems, you need to ask yourself "why". What do you hope to learn? What can you do if the display has trash? Is making this measurement worth a) my life (in the case of mains) b) my scope and c) my power source. If you mess up on the USB connector, you can take out the entire motherboard (in the worst case).
Why do you think the USB power is noisy? It probably isn't, you know.
I look at signals on PCBs and breadboards and they are usually powered by a wall wart or floating power supply. The voltages are low and I like it that way. My idea of high voltage these days is 12V. No more of the high voltage, high current, industrial systems. I'm retired!
Wimberleytech:
--- Quote from: rstofer on January 29, 2020, 06:28:08 pm ---
I look at signals on PCBs and breadboards and they are usually powered by a wall wart or floating power supply. The voltages are low and I like it that way. My idea of high voltage these days is 12V. No more of the high voltage, high current, industrial systems. I'm retired!
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AMEN and ditto (but I splurge from time to time and go to 24 volts--automatic gates are sometimes 24 volt systems).
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