Oscilloscope VARIABLE knobs in normal use (not in calibration)

[fezder]

Title says it all. I've NEVER used these variable volts/div and time/div (like vernier you could say) in normal use with oscilloscope. Now then; are there any practical uses for them?

 

[Nigel Goodwin]

Generally you don't - however, you 'can' use them to make a 'nicer' display, so it fits exactly between the grids, or to match two waves exactly on top of each other so you can better see any differences (and having done so subtract them to leave only the difference).

 

[JimB]

As Nigel says, only when it is convenient to have the trace sitting exactly on major graticule lines.

A couple of examples from my distant past:

Measuring risetimes of pulse edges
Measuring modulation depth of an amplitude modulated RF signal.

JimB

 

[ronsimpson]

I need to measure rise and fall time of a signal. Time for the signal to get from 10% to 90%.
I use the knobs to set the signal to exactly fit 0% to 100%. Then masure the time from 10% to 90%.

 

[ronsimpson]

Also good to measure percent ring or overshoot.

 

[fezder]

Ah! But, since variable knobs effect reading, how do I know what is real value? Since it attenuates, I know signal is bigger, but how much compared to what I see on screen? For example, I see on your picture that It's bigger than 50mV, but not accurately how much.

 

[fezder]

Hmm, then again; 100% of say 5V is still 5V....

 

[ronsimpson]

variable knobs effect reading

Another example for the use of the variable gain knob:
I built a amplifier that has a gain of 3.59. (strange number) In "cal. mode" I can measure the voltage and gain. But I want to see distortion. Put one probe on the input and one probe on the output. Play with "cal." until the two signals are the same size. Put the two traces on top of each other and see the difference. (might have to invert one channel) You can also use the add/substrate function where the scope will compare the two signals and show the difference. In these modes I don't care the voltage. I want to see if signal-1 has the same shape as signal-2 or did the amplifier distort the signal.

 

[Nigel Goodwin]

Another example for the use of the variable gain knob:
I built a amplifier that has a gain of 3.59. (strange number) In "cal. mode" I can measure the voltage and gain. But I want to see distortion. Put one probe on the input and one probe on the output. Play with "cal." until the two signals are the same size. Put the two traces on top of each other and see the difference. (might have to invert one channel) You can also use the add/substrate function where the scope will compare the two signals and show the difference. In these modes I don't care the voltage. I want to see if signal-1 has the same shape as signal-2 or did the amplifier distort the signal.

You mean like I said in post #2?