Oscilloscope Triggering DC coupling and AC coupling Distorts waveform's

[danny davis]

Why do you have to use DC coupling on the Oscilloscope Triggering when measuring Low frequency's?

What does DC coupling do for the triggering? for low frequency's?

Why does AC coupling Distorts waveform's? I know the AC coupling RC network causes the waveform distortion

When measuring Low frequency squarewaveforms, the AC coupling will distort the waveform or round , slant the squarewaveform

When do you use AC coupling? for what kind of waveforms, signals, frequencys than?

 

[jiripolivka]

Why do you have to use DC coupling on the Oscilloscope Triggering when measuring Low frequency's?

What does DC coupling do for the triggering? for low frequency's?

Why does AC coupling Distorts waveform's? I know the AC coupling RC network causes the waveform distortion

When measuring Low frequency squarewaveforms, the AC coupling will distort the waveform or round , slant the squarewaveform

When do you use AC coupling? for what kind of waveforms, signals, frequencys than?

To trigger oscilloscope time base, you can use the signal source directly, so the source is connected to the vertical input as well as to the trigger input. Then it depends on signal source internal impedance how the signal shape is affected.
At low frequencies, below say 100 Hz signal frequency, the oscilloscope inputs are either DC (directly) or AC (through a capacitor) coupled. This affects signal amplitude (again source impedance is important) and any signal DC component is amplified. If you need to see small AC variations on a larger DC component, at a high gain the trace may shift off the screen.

Learn to use your oscilloscope at various situations. Experience may be needed quite often.
Many scopes have the input impedance of 1 MOhm in parallel with 20 pF which limits the high-frequency observation. Probes have an adjustable capacitor to correct displaying pulse shape, with an internal test pulse output. Measuring signals from sources with 50 Ohm impedance needs that scope input is terminated in 50 OHms to prevent signal distortion.

 

[chuckey]

First of all consider an AC coupled CRO, it will typically have a 1 Mohm input resistance with a .1 MF capacitor in series. If you apply a low frequency square wave to the input of the capacitor, the waveform across the resistor will not have a "flat" top, it will have have a slope. This slope will will go negative after a positive excursion and positive after a negative going edge. As the frequency of the squarewave gets lower, the slope gets worse and finally ends up as a spike where the signal returns to zero before the other edge come along.
The 1M ohm/.1MF input applies to the trigger input, so if the CRO was triggering on the slope bit of the wave form, any very slight variation of the actual triggering level would slide the trigger point point along the slope by a long way and the timebase would then then try and sychronise to a different part of the waveform and the waveform as view would be seen as multiple images sliding along the horizontal axis. if you then switched the triggering to DC, the top part of the squarewave would be flat and the trigger circuit would not be able to trigger from it at all, so you would naturally adjust the trigger level to halfway up the edge of the square wave and the CRO trace would lock up perfectly.
it is best to use AC coupling all the time except for low frequency waveforms. If you use DC coupling, the DC bias on you signal may fire the signal right of the screen, i.e. if you are trying to look at a 100mV signal sitting on a 12V DC level, you would set your sensitivity to 50 mV/CM to get a 2 CM high display but the DC level would deflect the display by 240 CMs and the Y shift will not be able to bring the trace back to the centre of the screen.
Frank