If you’re reading this, you are probably new to the idea or considering buying your first oscilloscope. So what is an oscilloscope? The name has it all: oscillo – to oscillate (vary up and down) and scope – to see. In other words, it’s a tool that allows you to see electrical signals which change in amplitude with time.
The Right Oscilloscope for You
There are many types of oscilloscopes – analog, digital, cathode ray tube (CRT)-based, computer controlled. Oscilloscopes are ancient by electronic timeline standards and first appeared in WW2 to help develop the first radar system, but they have stayed the course and have come a long way since then. But for this article, we will stick to an analog, CRT-based oscilloscope because if you are considering getting one, this will be your most cost-effective route.
There are also many second-hand units out there that are perfectly functional. Excellent second-hand buys would include Tektronix, HP (Agilent) Kikosui, and National. The higher the oscilloscope’s bandwidth (the maximum frequency it will display accurately), the more you will pay. For general purpose use, 20MHz will suffice, but 100MHz is better if you want to work on RF stuff. Get the highest you can afford. But if you are a beginner, a very complex, expensive scope will just frustrate you. Don’t worry about a delayed timebase—in my 40 years of using oscilloscope, I may have needed this three or four times.
Shown below are three oscilloscopes I have in my lab. They each have their own characteristics and usefulness. From left to right are a digital Tektronix, an analog Kikosui, and an analog Philips oscilloscope with delayed timebase.
In general, digital oscilloscopes are great for displaying measured values but trigger poorly or alias when looking at complex RF wave-forms like modulated signals, whereas analog oscilloscopes excel here.
Why use an oscilloscope?
Why would you want to use an oscilloscope rather than a multimeter? It’s because the scope can see beyond the numbers on a multimeter LCD. An oscilloscope virtually enables you to “see” electric signals, both DC and AC. In a nutshell, it shows changes in voltage against time. So if you connect it to a sine wave source like a signal generator, you will see a sine wave on the screen and the relationships of different waveforms to each other, such as phase and frequency.
Common applications for oscilloscopes include the following:
- Observing signals at various points in an audio amplifier to look for distortion, clipping, and offset
- Looking at digital circuits to check the shape and level of pulses, perhaps unwanted glitches, verifying that oscillators are running cleanly and at the right frequency
- Looking at the output levels of transducers, guitars, and microphones
How to use an Oscilloscope
When you are first confronted with an unfamiliar oscilloscope, it may take you several minutes to get the display to show anything. Follow the steps below to get your signal to show up on the oscilloscope monitor:
- Make sure that the oscilloscope is turned on—either a led, pilot lamp, or the graticule should light.
- Turn all the knobs to the central position.
- Turn the intensity knob to about 80%.
- Select ‘auto’ in the sweep selection.
- Select ‘internal’ in the sweep source.
- Set triggering to channel 1 internal.
- Vertical mode selects channel 1 and selects AC coupling.
- Vertical set to 0.1V/div.
- Horizontal set to 1mS/div.
Turn the vertical and horizontal position knobs to see if you can find the trace. The oscilloscope should now be showing a horizontal line. If you have a trace, adjust the focus and intensity.
Connect an oscilloscope probe and touch the tip. You should see a rough mains 50Hz waveform. Adjust the vertical and horizontal controls to get a display that fits in the screen.
If your oscilloscope has a calibrator, connect the probe to that, and you should get a nice clean square wave, usually 1V or 5V, at 1kHz. If it is over or under shooting, adjust the small trimmer on the probe for a nice clean square wave.
If possible, use the 10X setting on the probe (if it has one) for wider bandwidth. There are many options to look at now, which channel to trigger on (the one your probe is connected to), and you may need to adjust a hold-off knob if there is one to get a stable display. If you have a dual-channel oscilloscope, there are some options like ALT CHOP and ADD. ADD displays a single trace, which is the arithmetic sum of both channels. ALT displays a full sweep of channel 1 then a full sweep of channel 2. CHOP does something similar but displays a bit of each trace alternately. You will have to play with ALT and CHOP to get the best display for the sweep frequency and triggering you are using. We never mentioned DC measurements. If you set the vertical coupling to DC, you can use the vertical position to set a baseline and then do DC measurements. I highly recommend this as you may get unexpected results like an excessive ripple on a power supply, an AC signal at a point where you expected DC, something oscillating, stray spikes and pulses. If I have a fault or something just does not work, I check the DC conditions, and I always do that with an oscilloscope.
Images used with permission from Tektronix.
Some real measurements
The signal shown on the oscilloscope monitor below has a nice clean sine wave. Now let’s see how to calculate the amplitude and frequency of this sine wave.
To find the amplitude, you need to know 3 things: the probe gain (X1 or X10), the vertical amplifier volts/div, and the size of the wave-form on the screen. So in the image above, probe gain is X1, volts/div is 50mV/div and the wave form is 6 divisions. Given that, the peak to peak voltage (Vpp) is 50*6 = 300mVpp. The peak value is half of that—150mV and the RMS value (what you would read on a meter) is 0.707*150 = 106mV (0.707 is 1/√2).
To find the frequency (F), we only need the time/div and the number of divisions between any two repeating wave-form peaks. In the example below, time/div is 0.5mS.div and there are 2.2 div 0.5*10-3 = 0.0011. Now, this is the period, and F is 1/period = 1/0.0011 = 909Hz.
Rise time is defined as the time required for a pulse to rise from 10 percent to 90 percent of its steady value. The rise time shown is 4.55uS. Doing this manually in the trace, the horizontal time/div is 2.5uS/div, and it looks like it took about 1.8 divisions, i.e., 1.8*2.5uS = 4.5uS.
Fall time is defined as the time required for a pulse to fall from 90 percent to 10 percent of its steady value. In the trace shown, the horizontal time/div is 1uS, and if you didn’t have the fancy oscilloscope (shows 1.08uS), you could estimate it at about 1 div, i.e., 1uS.
The duty cycle is the ratio of pulse duration or pulse width (PW) to the waveform’s total period, expressed as a percentage. In the example, the frequency is 1.3kHz, so the period is 1/f = 786uS, and the pulse high time is 79.7. The duty cycle is 79.7/786 = 10.1%. You could also estimate this as follows: the horizontal is 100uS/div and about 8.8 divs, and the pulse looks about 90uS wide, so the duty cycle is 90/(8.8*100) = 0.1023, 10.2%, which checks.
Phase measurement. Here is a 50Hz trace (the rough one) and a signal generator set to almost the same frequency, but offset in phase (the clean sine-wave). Note that the signal generator is leading the 50Hz by about 1 division ie 5mS. Phase θ = td * F * 360° =5mS*50*360 = 90° leading.
You can do a great deal with oscilloscopes – considerably more than what a multimeter can do. They are very powerful and sophisticated tools, and it will be worth your while to try using one. get/download its manual and study it carefully.
Hope this article has helped you to understand how to use and setup an oscilloscope! Be sure to let us know in the comments below if you have questions about anything.