Oscilloscope techniques

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An input coupling switch in the "AC" position connects a capacitor in series with the input. This passes only the changes provided they are not too slow "slow" would mean visible [ citation needed ]. However, when the signal has a fixed offset of interest, or changes quite slowly, the user will usually prefer "DC" coupling, which bypasses any such capacitor. Most oscilloscopes offer the DC input option.

For convenience, to see where zero volts input currently shows on the screen, many oscilloscopes have a third switch position usually labeled "GND" for ground that disconnects the input and grounds it. Often, in this case, the user centers the trace with the vertical position control. Better oscilloscopes have a polarity selector. Normally, a positive input moves the trace upward; the polarity selector offers an "inverting" option, in which a positive-going signal deflects the trace downward. This control is found only on more elaborate oscilloscopes; it offers adjustable sensitivity for external horizontal inputs.

It is only active when the instrument is in X-Y mode, i. The vertical position control moves the whole displayed trace up and down. It is used to set the no-input trace exactly on the center line of the graticule, but also permits offsetting vertically by a limited amount. With direct coupling, adjustment of this control can compensate for a limited DC component of an input. The horizontal position control moves the display sidewise. It usually sets the left end of the trace at the left edge of the graticule, but it can displace the whole trace when desired.

This control also moves the X-Y mode traces sidewise in some instruments, and can compensate for a limited DC component as for vertical position. Each input channel usually has its own set of sensitivity, coupling, and position controls, though some four-trace oscilloscopes have only minimal controls for their third and fourth channels. When both channels are displayed, the type of channel switching can be selected on some oscilloscopes; on others, the type depends upon timebase setting. If manually selectable, channel switching can be free-running asynchronous , or between consecutive sweeps.

Some Philips dual-trace analog oscilloscopes had a fast analog multiplier, and provided a display of the product of the input channels. Multiple-trace oscilloscopes have a switch for each channel to enable or disable display of the channel's trace. These include controls for the delayed-sweep timebase, which is calibrated, and often also variable.

The slowest speed is several steps faster than the slowest main sweep speed, though the fastest is generally the same. A calibrated multiturn delay time control offers wide range, high resolution delay settings; it spans the full duration of the main sweep, and its reading corresponds to graticule divisions but with much finer precision. Its accuracy is also superior to that of the display. A switch selects display modes: Main sweep only, with a brightened region showing when the delayed sweep is advancing, delayed sweep only, or on some a combination mode.

Good CRT oscilloscopes include a delayed-sweep intensity control, to allow for the dimmer trace of a much-faster delayed sweep which nevertheless occurs only once per main sweep. Such oscilloscopes also are likely to have a trace separation control for multiplexed display of both the main and delayed sweeps together. A switch selects the trigger source.

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It can be an external input, one of the vertical channels of a dual or multiple-trace oscilloscope, or the AC line mains frequency. Another switch enables or disables auto trigger mode, or selects single sweep, if provided in the oscilloscope. Either a spring-return switch position or a pushbutton arms single sweeps.

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A trigger level control varies the voltage required to generate a trigger, and the slope switch selects positive-going or negative-going polarity at the selected trigger level. To display events with unchanging or slowly visibly changing waveforms, but occurring at times that may not be evenly spaced, modern oscilloscopes have triggered sweeps.

Compared to older, simpler oscilloscopes with continuously-running sweep oscillators, triggered-sweep oscilloscopes are markedly more versatile. A triggered sweep starts at a selected point on the signal, providing a stable display. In this way, triggering allows the display of periodic signals such as sine waves and square waves, as well as nonperiodic signals such as single pulses, or pulses that do not recur at a fixed rate. With triggered sweeps, the scope blanks the beam and starts to reset the sweep circuit each time the beam reaches the extreme right side of the screen.

For a period of time, called holdoff , extendable by a front-panel control on some better oscilloscopes , the sweep circuit resets completely and ignores triggers. Once holdoff expires, the next trigger starts a sweep. The trigger event is usually the input waveform reaching some user-specified threshold voltage trigger level in the specified direction going positive or going negative—trigger polarity. In some cases, variable holdoff time can be useful to make the sweep ignore interfering triggers that occur before the events to be observed.

In the case of repetitive, but complex waveforms, variable holdoff can provide a stable display that could not otherwise be achieved. Trigger holdoff defines a certain period following a trigger during which the sweep cannot be triggered again. This makes it easier to establish a stable view of a waveform with multiple edges, which would otherwise cause additional triggers. Imagine the following repeating waveform: The green line is the waveform, the red vertical partial line represents the location of the trigger, and the yellow line represents the trigger level.

If the scope was simply set to trigger on every rising edge, this waveform would cause three triggers for each cycle: Assuming the signal is fairly high frequency , the scope would probably look something like this: On an actual scope, each trigger would be the same channel, so all would be the same color. It is desirable for the scope to only trigger on one edge per cycle, so it is necessary to set the holdoff at slightly less than the period of the waveform. This prevents triggering from occurring more than once per cycle, but still lets it trigger on the first edge of the next cycle.

Triggered sweeps can display a blank screen if there are no triggers. To avoid this, these sweeps include a timing circuit that generates free-running triggers so a trace is always visible. This is referred to as "auto sweep" or "automatic sweep" in the controls. Once triggers arrive, the timer stops providing pseudo-triggers. The user will usually disable automatic sweep when observing low repetition rates. If the input signal is periodic, the sweep repetition rate can be adjusted to display a few cycles of the waveform.

Early tube oscilloscopes and lowest-cost oscilloscopes have sweep oscillators that run continuously, and are uncalibrated. Such oscilloscopes are very simple, comparatively inexpensive, and were useful in radio servicing and some TV servicing. Measuring voltage or time is possible, but only with extra equipment, and is quite inconvenient. They are primarily qualitative instruments. They have a few widely spaced frequency ranges, and relatively wide-range continuous frequency control within a given range.

In use, the sweep frequency is set to slightly lower than some submultiple of the input frequency, to display typically at least two cycles of the input signal so all details are visible. A very simple control feeds an adjustable amount of the vertical signal or possibly, a related external signal to the sweep oscillator. The signal triggers beam blanking and a sweep retrace sooner than it would occur free-running, and the display becomes stable. Some oscilloscopes offer these. The user manually arms the sweep circuit typically by a pushbutton or equivalent.

Once the sweep completes, it resets, and does not sweep again until re-armed. This mode, combined with an oscilloscope camera, captures single-shot events. Some recent designs of oscilloscopes include more sophisticated triggering schemes; these are described toward the end of this article. More sophisticated analog oscilloscopes contain a second timebase for a delayed sweep. A delayed sweep provides a very detailed look at some small selected portion of the main timebase. The main timebase serves as a controllable delay, after which the delayed timebase starts.

This can start when the delay expires, or can be triggered only after the delay expires. Ordinarily, the delayed timebase is set for a faster sweep, sometimes much faster, such as At extreme ratios, jitter in the delays on consecutive main sweeps degrades the display, but delayed-sweep triggers can overcome this.

The display shows the vertical signal in one of several modes: the main timebase, or the delayed timebase only, or a combination thereof. When the delayed sweep is active, the main sweep trace brightens while the delayed sweep is advancing. In one combination mode, provided only on some oscilloscopes, the trace changes from the main sweep to the delayed sweep once the delayed sweep starts, though less of the delayed fast sweep is visible for longer delays. Another combination mode multiplexes alternates the main and delayed sweeps so that both appear at once; a trace separation control displaces them.

DSOs can display waveforms this way, without offering a delayed timebase as such. Oscilloscopes with two vertical inputs, referred to as dual-trace oscilloscopes, are extremely useful and commonplace. Using a single-beam CRT, they multiplex the inputs, usually switching between them fast enough to display two traces apparently at once. Less common are oscilloscopes with more traces; four inputs are common among these, but a few Kikusui, for one offered a display of the sweep trigger signal if desired. Some multi-trace oscilloscopes use the external trigger input as an optional vertical input, and some have third and fourth channels with only minimal controls.

In all cases, the inputs, when independently displayed, are time-multiplexed, but dual-trace oscilloscopes often can add their inputs to display a real-time analog sum. Inverting one channel while adding them together results in a display of the differences between them, provided neither channel is overloaded. This difference mode can provide a moderate-performance differential input. Switching channels can be asynchronous, i.

Asynchronous switching is usually designated "Chopped", while sweep-synchronized is designated "Alt[ernate]". A given channel is alternately connected and disconnected, leading to the term "chopped". Multi-trace oscilloscopes also switch channels either in chopped or alternate modes. In general, chopped mode is better for slower sweeps. It is possible for the internal chopping rate to be a multiple of the sweep repetition rate, creating blanks in the traces, but in practice this is rarely a problem.

The gaps in one trace are overwritten by traces of the following sweep. A few oscilloscopes had a modulated chopping rate to avoid this occasional problem. Alternate mode, however, is better for faster sweeps. True dual-beam CRT oscilloscopes did exist, but were not common. One type Cossor, U. Others had two complete electron guns, requiring tight control of axial rotational mechanical alignment in manufacturing the CRT. Beam-splitter types had horizontal deflection common to both vertical channels, but dual-gun oscilloscopes could have separate time bases, or use one time base for both channels.

Multiple-gun CRTs up to ten guns were made in past decades. With ten guns, the envelope bulb was cylindrical throughout its length. In an analog oscilloscope, the vertical amplifier acquires the signal[s] to be displayed and provides a signal large enough to deflect the CRT's beam. In better oscilloscopes, it delays the signal by a fraction of a microsecond. The maximum deflection is at least somewhat beyond the edges of the graticule, and more typically some distance off-screen. The amplifier has to have low distortion to display its input accurately it must be linear , and it has to recover quickly from overloads.

As well, its time-domain response has to represent transients accurately—minimal overshoot, rounding, and tilt of a flat pulse top.

A vertical input goes to a frequency-compensated step attenuator to reduce large signals to prevent overload. The attenuator feeds one or more low-level stages, which in turn feed gain stages and a delay-line driver if there is a delay. Subsequent gain stages lead to the final output stage, which develops a large signal swing tens of volts, sometimes over volts for CRT electrostatic deflection. In dual and multiple-trace oscilloscopes, an internal electronic switch selects the relatively low-level output of one channel's early-stage amplifier and sends it to the following stages of the vertical amplifier.

In free-running "chopped" mode, the oscillator which may be simply a different operating mode of the switch driver blanks the beam before switching, and unblanks it only after the switching transients have settled. Part way through the amplifier is a feed to the sweep trigger circuits, for internal triggering from the signal. This feed would be from an individual channel's amplifier in a dual or multi-trace oscilloscope, the channel depending upon the setting of the trigger source selector. This feed precedes the delay if there is one , which allows the sweep circuit to unblank the CRT and start the forward sweep, so the CRT can show the triggering event.

High-quality analog delays add a modest cost to an oscilloscope, and are omitted in cost-sensitive oscilloscopes. The delay, itself, comes from a special cable with a pair of conductors wound around a flexible, magnetically soft core. The coiling provides distributed inductance, while a conductive layer close to the wires provides distributed capacitance. The combination is a wideband transmission line with considerable delay per unit length. Both ends of the delay cable require matched impedances to avoid reflections.

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. Historically, stable Lissajous figures were used to show that two sine waves had a relatively simple frequency relationship, a numerically-small ratio.

They also indicated phase difference between two sine waves of the same frequency. The X-Y mode also lets the oscilloscope serve as a vector monitor to display images or user interfaces. Many early games, such as Tennis for Two , used an oscilloscope as an output device. Complete loss of signal in an X-Y CRT display means that the beam is stationary, striking a small spot.

This risks burning the phosphor if the brightness is too high. Such damage was more common in older scopes as the phosphors previously used burned more easily. Some dedicated X-Y displays reduce beam current greatly, or blank the display entirely, if there are no inputs present. As with all practical instruments, oscilloscopes do not respond equally to all possible input frequencies. The range of frequencies an oscilloscope can usefully display is referred to as its bandwidth. Bandwidth applies primarily to the Y-axis, though the X-axis sweeps must be fast enough to show the highest-frequency waveforms.

The bandwidth is defined as the frequency at which the sensitivity is 0. One source [13] says there is a noticeable effect on the accuracy of voltage measurements at only 20 percent of the stated bandwidth. Some oscilloscopes' specifications do include a narrower tolerance range within the stated bandwidth. Probes also have bandwidth limits and must be chosen and used to properly handle the frequencies of interest.

To achieve the flattest response, most probes must be "compensated" an adjustment performed using a test signal from the oscilloscope to allow for the reactance of the probe's cable. Another related specification is rise time. This is the duration of the fastest pulse that can be resolved by the scope.

It is related to the bandwidth approximately by:. In analog instruments, the bandwidth of the oscilloscope is limited by the vertical amplifiers and the CRT or other display subsystem. In digital instruments, the sampling rate of the analog to digital converter ADC is a factor, but the stated analog bandwidth and therefore the overall bandwidth of the instrument is usually less than the ADC's Nyquist frequency.

This is due to limitations in the analog signal amplifier, deliberate design of the anti-aliasing filter that precedes the ADC, or both. A sampling oscilloscope can display signals of considerably higher frequency than the sampling rate if the signals are exactly, or nearly, repetitive. It does this by taking one sample from each successive repetition of the input waveform, each sample being at an increased time interval from the trigger event.

The waveform is then displayed from these collected samples. This mechanism is referred to as "equivalent-time sampling". Some oscilloscopes have cursors. These are lines that can be moved about the screen to measure the time interval between two points, or the difference between two voltages.

A few older oscilloscopes simply brightened the trace at movable locations. These cursors are more accurate than visual estimates referring to graticule lines. Some better oscilloscopes also have a squared-off loop for checking and adjusting current probes. Sometimes a user wants to see an event that happens only occasionally. To catch these events, some oscilloscopes—called storage scopes —preserve the most recent sweep on the screen. This was originally achieved with a special CRT, a " storage tube ", which retained 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. All but the simplest models of current oscilloscopes more often use digital signal sampling. Samples feed fast analog-to-digital converters, following which all signal processing and storage is digital.

Many oscilloscopes accommodate plug-in modules for different purposes, e. One of the most frequent uses of scopes is troubleshooting malfunctioning electronic equipment. For example, where a voltmeter may show a totally unexpected voltage, a scope may reveal that the circuit is oscillating. In other cases the precise shape or timing of a pulse is important. In a piece of electronic equipment, for example, the connections between stages e.

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 show that some 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.

Often, a newly designed circuit misbehaves because of design errors, bad voltage levels, electrical noise etc. Digital electronics usually operate from a clock, so a dual-trace scope showing 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.

First appearing in the s for ignition system analysis, automotive oscilloscopes are becoming an important workshop tool for testing sensors and output signals on electronic engine management systems, braking and stability systems. Some oscilloscopes can trigger and decode serial bus messages, such as the CAN bus commonly used in automotive applications.

For work at high frequencies and with fast digital signals, the bandwidth of the vertical amplifiers and sampling rate must be high enough. A much lower bandwidth is sufficient for audio-frequency applications only. A well-designed, stable trigger circuit is required for a steady display. The chief benefit of a quality oscilloscope is the quality of the trigger circuit. Key selection criteria of a DSO apart from input bandwidth are the sample memory depth and sample rate.

This is adequate for basic waveform display, but does not allow detailed examination of the waveform or inspection of long data packets for example. At the highest sample rates, the memory may be limited to a few tens of KB. Analog oscilloscopes have been almost totally displaced by digital storage scopes except for use exclusively at lower frequencies. Greatly increased sample rates have largely eliminated the display of incorrect signals, known as "aliasing", which was sometimes present in the first generation of digital scopes. The problem can still occur when, for example, viewing a short section of a repetitive waveform that repeats at intervals thousands of times longer than the section viewed for example a short synchronization pulse at the beginning of a particular television line , with an oscilloscope that cannot store the extremely large number of samples between one instance of the short section and the next.

The used test equipment market, particularly on-line auction venues, typically has 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. Used instruments are usually out of calibration, and recalibration by companies with the equipment and expertise usually costs more than the second-hand value of the instrument.

These often have limited bandwidth and other facilities, but fulfill the basic functions of an oscilloscope. Many oscilloscopes today provide one or more external interfaces to allow remote instrument control by external software. The following section is a brief summary of various types and models available. For a detailed discussion, refer to the other article. The earliest and simplest type of oscilloscope consisted of a cathode ray tube , a vertical amplifier , a timebase, a horizontal amplifier and a power supply.

These are now called "analog" scopes to distinguish them from the "digital" scopes that became common in the s and later. Analog scopes do not necessarily include a calibrated reference grid for size measurement of waves, and they may not display waves in the traditional sense of a line segment sweeping from left to right.

Oscilloscope techniques Oscilloscope techniques
Oscilloscope techniques Oscilloscope techniques
Oscilloscope techniques Oscilloscope techniques
Oscilloscope techniques Oscilloscope techniques
Oscilloscope techniques Oscilloscope techniques
Oscilloscope techniques Oscilloscope techniques

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