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An oscilloscope (sometimes abbreviated CRO, for cathode-ray oscilloscope, or commonly just scope or O-scope) is a piece of electronic test equipment that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis).
Features and uses
Exterior
A typical oscilloscope is usually box shaped with a display screen, numerous input connectors, control knobs and buttons on the front panel. To aid measurement, a grid called the graticule is drawn on the face of the screen. Each square in the graticule is known as a division.
Inputs
The signal to be measured is fed to one of the input connectors, which is usually a coaxial connector such as a BNC or N type. If the signal source has its own coaxial connector, then a simple coaxial cable is used; otherwise, a specialised cable called a 'scope probe', supplied with the oscilloscope, is used. General-purpose oscilloscopes have a standardised input resistance of 1 megohm in parallel with a capacitance of around 20 picofarads. This allows the use of standard oscilloscope probes. Scopes for use with very high frequencies may have 50-ohm inputs, which must be either connected directly to a 50-ohm signal source or used with Z0 or active probes. It is used for measuring voltage.
The trace
In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. One of the controls, the timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division. If the input voltage departs from zero, the trace is deflected either upwards or downwards. Another control, the vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time (the present plotted at a varying position, the less recent past to the left, the most recent past to the right).
If the input signal is periodic, then a nearly stable trace can be obtained just by setting the timebase to match the frequency of the input signal. For example, if the input signal is a 50 Hz sine wave, then its period is 20 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 20 ms. This mode is called continual sweep. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.
Trigger
To provide a more stable trace, modern oscilloscopes have a function called the trigger. When using triggering, the scope will pause each time the sweep reaches the extreme right side of the screen. The scope then waits for a specified event before drawing the next trace. The trigger event is usually the input waveform reaching some user-specified threshold voltage in the specified direction (going positive or going negative).
The effect is to resynchronise the timebase to the input signal, preventing horizontal drift of the trace. In this way, triggering allows the display of periodic signals such as sine waves and square waves. Trigger circuits also allow the display of nonperiodic signals such as single pulses or pulses that don't recur at a fixed rate.
Types of trigger include:
X-Y mode
Other features
Some oscilloscopes have cursors, which are lines that can be moved about the screen to measure the time interval between two points, or the difference between two voltages.
Oscilloscopes may have two or more input channels, allowing them to display more than one input signal on the screen. Usually the oscilloscope has a separate set of vertical controls for each channel, but only one triggering system and timebase.
Sometimes the event that the user wants to see may only happen occasionally. To catch these events, some oscilloscopes, known as "storage scopes", preserve the most recent sweep on the screen. This was originally achieved by using a special CRT, a "storage tube", which would retain the image of even a very brief event for a long time.
Some digital oscilloscopes can sweep at speeds as slow as once per hour, emulating a strip chart recorder. That is, the signal scrolls across the screen from right to left. Most oscilloscopes with this facility switch from a sweep to a strip-chart mode right around one sweep per ten seconds. This is because otherwise, the scope looks broken: it's collecting data, but the dot cannot be seen.
Oscilloscopes were originally analog devices. In more recent times digital signal sampling is more often used for all but the simplest models.
Many oscilloscopes have different plug-in modules for different purposes, e.g., high-sensitivity amplifiers of relatively narrow bandwidth, differential amplifiers, amplifiers with 4 or more channels, sampling plugins for repetitive signals of very high frequency, and special-purpose plugins.
Examples of use
One of the most frequent uses of scopes is troubleshooting malfunctioning electronic equipment. One of the advantages of a scope is that it can graphically show signals: where a voltmeter may show a totally unexpected voltage, a scope may reveal that the circuit is oscillating. In other cases the precise shape of a pulse is important.
In a piece of electronic equipment, for example, the connections between stages (e.g. electronic mixers, electronic oscillators, amplifiers) may be 'probed' for the expected signal, using the scope as a simple signal tracer. If the expected signal is absent or incorrect, some preceding stage of the electronics is not operating correctly. Since most failures occur because of a single faulty component, each measurement can prove that half of the stages of a complex piece of equipment either work, or probably did not cause the fault.
Once the faulty stage is found, further probing can usually tell a skilled technician exactly which component has failed. Once the component is replaced, the unit can be restored to service, or at least the next fault can be isolated.
Another use is to check newly designed circuitry. Very often a newly designed circuit will misbehave because of design errors, bad voltage levels, electrical noise etc. Digital electronics usually operate from a clock, so a dual-trace scope which shows both the clock signal and a test signal dependent upon the clock is useful. "Storage scopes" are helpful for "capturing" rare electronic events that cause defective operation.
Another use is for software engineers who must program electronics. Often a scope is the only way to see if the software is running the electronics properly.
Tips for use
The most typical problem encountered when approaching an unfamiliar scope is that the trace is not visible. Many newer scopes have a "reset options" or "auto set up" button. Use it when you get confused, or when you first approach an unfamiliar scope. Some scopes have a "beamfinder" button. It limits the size of the scan so the trace will appear on the screen.
Make sure that at first you set the options of a channel to "DC" coupling, with automatic triggering. Increase the channel's volts per division (effectively dividing down the line height) until a line appears. Set the sweep time per division near the speed of the desired event, and then adjust the volts per division until the event appears at a useful size.
Oscilloscopes almost always have a test output that one can measure to assure that a channel and probe are working. When approaching an unfamiliar oscilloscope, it's wise to measure this signal first.
The capacitance of the wire in the test probe can cause an oscilloscope to display high-speed signals inaccurately. For example, the leading and trailing edges of a square wave may seem to be over- or under-shooting, due to an incorrectly compensated probe. Many probes (voltage divider types, such as x10 and x100 probes) allow this to be compensated for by adjusting a trimmer capacitor. Probes should always be set to show square waves properly; most scopes have a square wave test-signal output for this purpose.
The bandwidth of the test probes should exceed the bandwidth of the oscilloscope's input amplifiers.
The signal ground connection of the oscilloscope should be attached to the ground of the circuit under test, although this is clearly not possible when measuring the difference between two voltages neither of which is ground. Most test leads for oscilloscopes have the ground clip built into their end. To accurately probe high-speed signals, the ground lead must be kept as short as possible; at frequencies above 100 MHz, the flying ground lead should be removed and replaced with a small ground pin which slips over the ground ring at the tip of the probe.
If the oscilloscope has connection to mains earth, it is likely that the test lead ground is also attached to mains earth (via the oscilloscope chassis). If the circuit under test is also referenced to mains earth, then attaching the probe ground to any signal will effectively act like a short circuit to earth, possibly causing damage to the circuit under test or the oscilloscope itself. This can be alleviated by supplying power to the circuit under test via an isolation transformer. A common but dangerous mistake is to use the isolation transformer to power the oscilloscope ("float" it) instead of the circuit under test. This allows dangerous voltages to be present on the metal parts of the oscilloscope, which is an unacceptable electrocution risk. Removing the ground connection of the power cord is of course not acceptable either. Alternatively an oscilloscope isolation amplifier can be used to isolate the probe grounds from the oscilloscope earth ground, this is also a good solution if the DUT can not be powered via an isolation transformer. Differential amplifiers or probes are another solution to the problem.
"AC coupling" blocks any DC in the signal. This is useful when measuring a small signal riding on a DC offset. Note that the AC coupling mode simply adds an internal series capacitor, which will affect low-frequency response.
"DC coupling" must be used when measuring a DC voltage.
Make sure you are triggering from the correct channel. Set the trigger delay to zero. Adjust the trigger level until the desired event triggers. Last of all, adjust the trigger delay until the desired signal feature appears.
Scope probes are both expensive and fragile. To reduce capacitance, the conductor in a scope probe's wire is sometimes narrower than a human hair. The plastic "pen" part of the probe is often easy to break. Never leave a probe on the floor where one can walk on it. If you must share a scope, consider having and protecting your own set of probes.
Selection
Oscilloscopes generally have a checklist of some set of the above features. The basic measure of virtue is the bandwidth of its vertical amplifiers. Typical scopes for general purpose use should have a bandwidth of at least 100 MHz, although much lower bandwidths are acceptable for audio-frequency applications. A useful sweep range is from one second to 100 nanoseconds, with triggering and delayed sweep. For work on digital signals, dual channels are necessary, and a storage scope with a sweep speed of at least 1/5 your system's maximum frequency is recommended.
The chief benefit of a quality oscilloscope is the quality of the trigger circuit. If the trigger is unstable, the display will always be fuzzy. The quality improves roughly as the frequency response and voltage stability of the trigger increase.
Digital storage scopes (almost the only kind now available at the higher end of the market) used to display misleading signals at low sample rates, but this "aliasing" problem is now much rarer due to increased memory length. It's worth asking about in the used market, though.
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