The classic use of a scope is to diagnose a failing piece of electronic equipment. In a radio, for example, one looks at the schematic and tries to locate the connections between stages (e.g. electronic mixers, electronic oscillators, amplifiers).
Then one puts the scope's ground on the circuit's ground, and the probe of the scope on a connection between two of the stages in the middle of the train of stages.
When the expected signal is absent, one knows that some preceding stage of the electronics has failed. 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 failing stage is found, further probing of the defective stage can usually tell a skilled technician exactly which component is broken. Once the technician replaces the component, 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 bad voltage levels, electrical noise or design errors. Digital electronics usually operates from a clock, so a dual-trace scope is needed to check digital circuits. "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.
Oscilloscope - Description
A typical oscilloscope is a rectangular box with a small screen, numerous input connectors and 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. The signal to be measured is fed to one of the input connectors, which is usually a co-axial connector such as a BNC or N type. If the signal source has its own co-axial connector, then a simple co-axial cable is used; otherwise, a specialised cable called a scope probe, supplied with the oscilloscope, is used.
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 most recent past to the left, the less 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.
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:
* external trigger, a pulse from an external source connected to a dedicated input on the scope.
* edge trigger, an edge-detector that generates a pulse when the input signal crosses a specified threshold voltage in a specified direction.
* video trigger, a circuit that extracts synchronising pulses from video formats such as PAL and NTSC and triggers the timebase on every line, a specified line, every field, or every frame. This circuit is typically found in a waveform monitor device.
* delayed trigger, which waits a specified time after an edge trigger before starting the sweep. No trigger circuit acts instantaneously, so there is always a certain delay, but a trigger delay circuit extends this delay to a known and adjustable interval. In this way, the operator can examine a particular pulse in a long train of pulses.
Most oscilloscopes also allow you to bypass the timebase and feed an external signal into the horizontal amplifier. This is called X-Y mode, and is useful for viewing the phase relationship between two signals, which is commonly done in radio and television engineering. When the two signals are sinusoids of varying frequency and phase, the resulting trace is called a Lissajous curve.
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.
Most oscilloscopes 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.
A dual-timebase oscilloscope has two triggering systems so that two signals can be viewed on different time axes. This is also known as a "magnification" mode. The user traps the desired, complex signal using a suitable trigger setting. Then he enables the "magnification", "zoom" or "dual timebase" feature, and can move a window to look at details of the complex signal.
Sometimes the event that the user wants to see may only happen occasionally. To catch these events, some oscilloscopes are "storage scopes" that preserve the most recent sweep on the screen.
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 fancy oscilloscopes 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.
Oscilloscope - 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 inaccurately display high speed signals. If the signal looks distorted, that is, if it shows unusual spikes ("ringing") or weird humps, try adjusting the scope probe's capacitance. Many scope probes (voltage divider types, 10x for example) have a small adjustment screw on the probe. Most oscilloscopes provide a test output that produces a square wave for adjusting the probe. Adjust the probe so that the corners of the square wave appear square, exhibiting no overshoot or undershoot.
The bandwidth of the test probes should equal or exceed the bandwidth of the oscilloscope's input amplifiers.
In general, the ground connection of the oscilloscope should be attached to the ground of the circuit under test. 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 oscilloscope via an isolation transformer.
"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, although large in value, can affect how low frequency signals appear.
"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.
Oscilloscope - 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.
As of 2004, a 150 MHz dual-channel storage scope costs about US$1200 new, and is good enough for general use. The current bandwidth record, as of February 2005, is held by the Tektronix TDS6000C oscilloscope family with a digitally enhanced bandwidth of up to 15 GHz and costing about US$150,000.
