O-scopes are an important tool in any electrical engineer's lab. They allow you to see electric signals as they vary over time, which can be critical in diagnosing why your 555 timer circuit isn't blinking correctly, or why your noise maker isn't reaching maximum annoyance levels.
Basics of O-Scopes – An introduction to what, exactly, oscilloscopes are, what they measure, and why we use them.
Oscilloscope Lexicon – A glossary covering some of the more common oscilloscope characteristics.
Anatomy of an O-Scope – An overview of the most critical systems on an oscilloscope – the screen, horizontal and vertical controls, triggers, and probes.
Using an Oscilloscope – Tips and tricks for someone using an oscilloscope for the first time.
We'll be using the Gratten GA1102CAL – a handy, mid-level, digital oscilloscope – as the basis for our scope discussion. Other o-scopes may look different, but they should all share a similar set of control and interface mechanisms.
Controls surrounding the scope's screen allow you to adjust the scale of the graph, both vertically and horizontally – allowing you to zoom in and out on a signal. There are also controls to set the trigger on the scope, which helps focus and stabilize the display.
Frequency and period – Frequency is defined as the number of times per second a waveform repeats. And the period is the reciprocal of that (number of seconds each repeating waveform takes). The maximum frequency a scope can measure varies, but it's often in the 100's of MHz (1E6 Hz) range.
Duty cycle – The percentage of a period that a wave is either positive or negative (there are both positive and negative duty cycles). The duty cycle is a ratio that tells you how long a signal is "on" versus how long it's "off" each period.
Rise and fall time – Signals can't instantaneously go from 0V to 5V, they have to smoothly rise. The duration of a wave going from a low point to a high point is called the rise time, and fall time measures the opposite. These characteristics are important when considering how fast a circuit can respond to signals.
Amplitude – Amplitude is a measure of the magnitude of a signal. There are a variety of amplitude measurements including peak-to-peak amplitude, which measures the absoluted difference between a high and low voltage point of a signal. Peak amplitude, on the other hand, only measures how high or low a signal is past 0V.
Maximum and minimum voltages – The scope can tell you exactly how high and low the voltage of your signal gets.
Mean and average voltages – Oscilloscopes can calculate the average or mean of your signal, and it can also tell you the average of your signal's minimum and maximum voltage.
Determining the frequency and amplitude of a signal, which can be critical in debugging a circuit's input, output, or internal systems. From this, you can tell if a component in your circuit has malfunctioned.
Identifying the shape of a wave – sine, square, triangle, sawtooth, complex, etc.
Learning how to use an oscilloscope means being introduced to an entire lexicon of terms. On this page we'll introduce some of the important o-scope buzzwords you should be familiar with before turning one on.
Bandwidth – Oscilloscopes are most commonly used to measure waveforms which have a defined frequency. No scope is perfect though: they all have limits as to how fast they can see a signal change. The bandwidth of a scope specifies the range of frequencies it can reliably measure.
Digital vs. Analog – As with most everything electronic, o-scopes can either be analog or digital. Analog scopes use an electron beam to directly map the input voltage to a display. Digital scopes incorporate microcontrollers, which sample the input signal with an analog-to-digital converter and map that reading to the display. Generally analog scopes are older, have a lower bandwidth, and less features, but they may have a faster response (and look much cooler).
Channel Amount – Many scopes can read more than one signal at a time, displaying them all on the screen simultaneously. Each signal read by a scope is fed into a separate channel. Two to four channel scopes are very common.
Sampling Rate – This characteristic is unique to digital scopes, it defines how many times per second a signal is read. For scopes that have more than one channel, this value may decrease if multiple channels are in use.
Rise Time – The specified rise time of a scope defines the fastest rising pulse it can measure. The rise time of a scope is very closely related to the bandwidth. It can be calculated as Rise Time = 0.35 / Bandwidth.
Maximum Input Voltage – Every piece of electronics has its limits when it comes to high voltage. Scopes should all be rated with a maximum input voltage. If your signal exceeds that voltage, there's a good chance the scope will be damaged.
Resolution – The resolution of a scope represents how precisely it can measure the input voltage. This value can change as the vertical scale is adjusted.
Vertical Sensitivity – This value represents the minimum and maximum values of your vertical, voltage scale. This value is listed in volts per div.
Time Base – Time base usually indicates the range of sensitivities on the horizontal, time axis. This value is listed in seconds per div.
Input Impedance – When signal frequencies get very high, even a small impedance (resistance, capacitance, or inductance) added to a circuit can affect the signal. Every oscilloscope will add a certain impedance to a circuit it's reading, called the input impedance. Input impedances are generally represented as a large resistive impedance (>1 MΩ) in parallel (||) with small capacitance (in the pF range). The impact of input impedance is more apparent when measuring very high frequency signals, and the probe you use may have to help compensate for it.
Understanding these characteristics, you should be able to pick out an oscilloscope that'll best fit your needs. But you still have to know how to use it…onto the next page!
While no scopes are created exactly equal, they should all share a few similarities that make them function similarly. On this page we'll discuss a few of the more common systems of an oscilloscope: the display, horizontal, vertical, trigger, and inputs.
Many scope displays are situated next to a set of about five buttons – either to the side or below the display. These buttons can be used to navigate menus and control settings of the scope.
The infinite variety of signals out there means you'll never operate an oscilloscope the same way twice. But there are some steps you can count on performing just about every time you test a circuit. On this page we'll show an example signal, and the steps required to measure it.
First off, you'll need to select a probe. For most signals, the simple passive probe included with your scope will work perfectly fine.
Next, before connecting it to your scope, set the attenuation on your probe. 10X – the most common attenuation factor – is usually the most well-rounded choice. If you're trying to measure a very low-voltage signal though, you may need to use 1X.
Set the trigger source to channel 1 – no external source or alternate channel triggering.
For help making these adjustments, consult your scope's user's manual (as an example, here's the GA1102CAL manual).
Let's connect that channel up to a meaningful signal. Most scopes will have a built-in frequency generator that emits a reliable, set-frequency wave – on the GA1102CAL there is a 1kHz square wave output at the bottom-right of the front panel. The frequency generator output has two separate conductors – one for the signal and one for ground. Connect your probe's ground clip to the ground, and the probe tip to the signal output.
As soon as you connect both parts of the probe, you should see a signal begin to dance around your screen. Try fiddling with the horizontal and vertical system knobs to maneuver the waveform around the screen. Rotating the scale knobs clockwise will "zoom into" your waveform, and counter-clockwise zooms out. You can also use the position knob to further locate your waveform.
If your wave is still unstable, try rotating the trigger position knob. Make sure the trigger isn't higher than the tallest peak of your waveform. By default, the trigger type should be set to edge, which is usually a good choice for square waves like this.
If your probe is set to 10X, and you don't have a perfectly square waveform as shown above, you may need to compensate your probe. Most probes have a recessed screw head, which you can rotate to adjust the shunt capacitance of the probe.
Adjust the trimming cap on the probe handle until you have a straight-edged square wave. Compensation is only necessary if your probe is attenuated (e.g. 10X), in which case it's critical (especially if you don't know who used your scope last!).
Once you've compensated your probe, it's time to measure a real signal! Go find a signal source (frequency generator?, Terror-Min?) and come back.
The first key to probing a signal is finding a solid, reliable grounding point. Clasp your ground clip to a known ground, sometimes you may have to use a small wire to intermediate between the ground clip and your circuit's ground point. Then connect your probe tip to the signal under test. Probe tips exist in a variety of form factors – the spring-loaded clip, fine point, hooks, etc. – try to find one that doesn't require you to hold it in place all the time.
Once your signal is on the screen, you may want to begin by adjusting the horizontal and vertical scales into at least the "ballpark" of your signal. If you're probing a 5V 1kHz square wave, you'll probably want the volts/div somewhere around 0.5-1V, and set the seconds/div to around 100µs (14 divisions would show about one and a half periods).
Once you have the scales ballparked, your waveform may need some triggering. Edge triggering – where the scope tries to begin its scan when it sees voltage rise (or fall) past a set point – is the easiest type to use. Using an edge trigger, try to set the trigger level to a point on your waveform that only sees a rising edge once per period.
Now just scale, position, trigger and repeat until you're looking at exactly what you need.
With a signal scoped, triggered, and scaled, it comes time to measure transients, periods, and other waveform properties. Some scopes have more measurement tools than others, but they'll all at least have divisions, from which you should be able to at least estimate the amplitude and frequency.
Many scopes support a variety of automatic measurement tools, they may even constantly display the most relevant information, like frequency. To get the most out of your scope, you'll want to explore all of the measure functions it supports. Most scopes will calculate frequency, amplitude, duty cycle, mean voltage, and a variety of other wave characteristics for you automatically.
Using the scope's measure tools to find VPP, VMax, frequency, period, and duty cycle.
Once you've measured the quantity you were looking for, you can begin to make adjustments to your circuit and measure some more! Some scopes also support saving, printing, or storing a waveform, so you can recall it and remember those good ol' times when you scoped that signal.
To find out more about what your scope can do, consult its user's manual!
Now that you've learned all about this handy tool's features and benefits, it's time to put an oscilloscope on the workbench.