oscilloscope question

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lokeycmos

Joined Apr 3, 2009
431
quick question about osciloscope. on the front of my old school tektronix scope is a switch that lets me choose between "ac", "dc", and "ground".
could someone please explain what this switch is for and when i would use each positon?. i attached a pic of my scope. i also connected a simple 555 circuit to my scope and took a pic of each waveform in each switch position. i get different results in each mode. thank you
 

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Sparky49

Joined Jul 16, 2011
833
I believe that DC allows both DC and AC signals, whilst AC only allows AC signals.

Posh name - AC and DC Coupling.

EDIT: In some cases AC is used when you want to know where 0V is - it'll be in the centre of the waveform. But it's better to use DC for low-frequency (a few Hz).
 

JMac3108

Joined Aug 16, 2010
348
This is the "coupling" for your input.

(1) DC coupling shows the signal including any DC level.

(2) AC coupling removes any DC level in your signal.

(3) GND simply connects the input of the scope to ground.

DC coupling is used the most. Use AC coupling when you need to look at a small AC signal on top of a large DC level. For example looking at 50mVAC of noise on a 12VDC power supply line.

The ony thing I've ever used GND for is to locate my trace.
 

Adjuster

Joined Dec 26, 2010
2,148
The "AC" setting typically provides a capacitive input coupling. This eliminates DC, and also imposes an AC low-frequency cutoff, typically -3dB at 10Hz.

If you look carefully at your waveform with the "AC" setting, you can see a tilt in the tops and bottoms of what should be a rectangular waveform. Essentially, what you are seeing are the beginnings of exponential capacitor discharge waveforms.
If you looked a a lower frequency square wave, the tilt would become more pronounced.

The "GND" setting can be useful to set the beam level to a graticule line, particularly when making DC measurements. This makes it easier to see what absolute voltages are displayed.
 

w2aew

Joined Jan 3, 2012
219
quick question about osciloscope. on the front of my old school tektronix scope is a switch that lets me choose between "ac", "dc", and "ground".
could someone please explain what this switch is for and when i would use each positon?. i attached a pic of my scope. i also connected a simple 555 circuit to my scope and took a pic of each waveform in each switch position. i get different results in each mode. thank you
Here is a link to a YouTube video that I did a few months ago on the AC and DC Coupling switch on the oscilloscope.
 
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