# Questions about advanced use of oscilloscope

At my university, I'll be using the ELENCO S-1325 oscilloscope for a instrumentation lab. Since I'm a mechanical engineering student and not an electrical engineering one, I don't know exactly how to use all the functions though I am familiar with the basic use and operation of it. However, I want to know how to use some of the more complex functions of the oscilloscope.

On an oscilloscope, why would anyone move the variable knobs off their “calibrated” positions? the variable knobs (red VAR knob) for voltage measurements? the small gray “var sweep” knob for the time base?

Isn't it a good thing to keep things calibrated rather than use uncalibrated settings?

• The first think I would recommend to a noob in oscilloscopes is this amazing video from Dave Jones (How not to blow your oscilloscope): youtube.com/watch?v=xaELqAo4kkQ Even some novice electrical engineers (like me) have made this mistake. As you are not in the area, this is a must see. – gstorto Feb 17 '15 at 18:36
• If you haven't already read the user manual, do so. There probably isn't a paper one available, but you should be able to find one online. – tcrosley Feb 17 '15 at 19:39

## 3 Answers

If you want to measure the rise or fall time of a digital signal, you use the variable knob for voltage to adjust the amplitude, so that it fits between the horizontal dotted lines (5 divisions). Then you measure the time from 10% to 90% (the middle 4 divisions). Conveniently, there are even percent marks on the screen.

It's less common to use the horizontal variable adjust, maybe it could be used for measuring duty cycle.

Sometimes the relative positions or phases of the signals are more important than the absolute values. E.g. in the photo in your question, you can see that the two traces (i.e. channel 1 and 2) are in phase with each other. If you don't need to know the voltage or exact frequency of the signals, their's no need for exact calibration.

Also, @starblue's answer is spot-on.

Mmmm - I don't have enough reputation to comment, hence the answer.

There are many reasons why you might want to leave the calibrated setting : StarBlue touched on one, here's another.

Imagine you need to do a quick check on the frequency response of an amplifier with a gain of 3. That's usually defined at the -3dB point (amplitude=0.7) It's much easier to see differences between input and output signals if they look the same in the first place, so set up the signal generator on Channel 1 so that the input channel covers 10 of those calibration marks (on the crosshair, i.e. 2 divisions). Now set up the Channel 2 on the output ... the oscilloscope gain is fixed, and none of the calibration settings give you exactly 2 divisions, so use the next largest one and de-calibrate the gain until you have 10 calibration marks.

Now you can adjust the input frequency and observe the frequency response. You need the frequencies where Ch1 covers 10 divisions and Ch2 only covers 7. If the signal generator amplitude changes with frequency, you will see it on Ch1, and adjust the sig gen to compensate.

• You could also adjust the amplitude for an "in-band" frequency to 100%, and then change it until the amplitude is at 70% to find the -3dB point, using only a single channel. – starblue Feb 18 '15 at 21:46