Exterior
A typical oscilloscope has a display screen, numerous input connectors, and control knobs and buttons on the front panel. Portable instruments are small enough to carry to a work site and may even be battery operated. Laboratory grade 'scopes, especially old instruments using vacuum tubes, are bench-top devices. Special purpose 'scopes may be permanently mounted in a rack. 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. On old low-cost CRT 'scopes the graticule was a printed piece of plastic; higher-cost instruments have the graticule printed on the face of the CRT, to eliminate parallax errors. Digital 'scopes generate the graticule markings on the display in the same way as the trace.
Large bench-top oscilloscopes were sometimes mounted on carts to allow sharing one expensive instrument by several work areas. Miniaturized oscilloscopes were of great value for field service equipment repair. Today even a very capable laboratory instrument can be lifted by a single person, and hand-held digital oscilloscopes are made by several manufacturers.
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. Binding posts or banana plugs may be used for lower frequencies. 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.
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 plot of voltage against time, with the more distant past on the left and the more recent past on 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. Since the calibrated oscilloscope timebase may not exactly match the period of the input signal, the trace will drift across the screen making measurements difficult. If the time base is adjusted to stabilize the trace, the time per horizontal division is altered, and usually uncalibrated.
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 resynchronize the timebase to the input signal, preventing horiz
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 synchronizing 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.
Bandwidth
Bandwidth is a measure of the range of frequencies that can be displayed. The bandwidth of the 'scope is limited by the vertical amplifiers and CRT (in analog instruments) or by the sampling rate of the analog to digital converter in digital instruments. The bandwidth is defined as the frequency at which the sensitivity is 0.707 of the sensitivity at lower frequency (a drop of 3 dB). The rise time of the fastest pulse that can be resolved by the scope is related to its bandwidth approximately:
Bandwidth in Hz x rise time in seconds = 0.35 [2]
For example, a 'scope intended to resolve pulses with a rise time of 1 nanosecond would have a bandwidth of 350 MHz.
For a digital oscilloscope, a rule of thumb is that the continuous sampling rate should be ten times the highest frequency desired to resolve; for example a 20 megasample/second rate would be applicable for measuring signals up to about 2 megahertz.
X-Y mode
Most modern oscilloscopes have several inputs for voltages, and thus can be used to plot one varying voltage versus another. This is especially useful for graphing I-V curves (current versus voltage characteristics) for components such as diodes, as well as Lissajous patterns. Lissajous figures are an example of how an oscilloscope can be used to track phase differences between multiple input signals. This is very frequently used in broadcast engineering to plot the left and right stereophonic channels, to ensure that the stereo generator is calibrated properly.
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.
Better quality general purpose oscilloscopes include a calibration signal for setting up the compensation of test probes; this is (often) a 1 kHz square-wave signal available at a test terminal on the front panel.
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 at about 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.
Pictures of use
Heterodyne
AC hum on sound.
AM signal.
Bad filter on sine.
Dual trace, showing different time bases on each trace.
Selection
 Heterodyne |  AC hum on sound. |  AM signal. |  Bad filter on sine. |
 |
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.
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.
Analog oscilloscopes have been almost totally displaced by digital storage scopes except for the low bandwidth (< 60 MHz) segment of the market. Greatly increased sample rates have eliminated the display of incorrect signals, known as "aliasing", that was sometimes present in the first generation of digital scopes. The used test equipment market, particularly on-line auction venues, typically have a wide selection of older analog scopes available. However it is becoming more difficult to obtain replacement parts for these instruments and repair services are generally unavailable from the original manufacturer.
As of 2007[update], a 350 MHz bandwidth (BW), 2.5 giga-samples per second (GS/s), dual-channel digital storage scope costs about US$7000 new. The current real-time analog bandwidth record, as of February 2007[update], is held by the Tektronix DPO70000 and DSA70000 oscilloscope families with a 20 GHz BW (non-interleaved) and a sample rate of 50 GHz. The current equivalent time sampling bandwidth record for sampling digital storage oscilloscopes, as of June 2006[update], is held by the LeCroy WaveExpert series with a 100 GHz bandwidth.
Software
Many oscilloscopes today provide one or more external interfaces to allow remote instrument control by external software. These interfaces (or buses) include GPIB, Ethernet, serial port, and USB.
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