Oscilloscope types

This is a subdivision of the Oscilloscope article, discussing the various types and models of oscilloscopes in greater detail.

Digital oscilloscopes

While analog devices make use of continually varying voltages, digital devices employ binary numbers which correspond to samples of the voltage. In the case of digital oscilloscopes, an analog-to-digital converter (ADC) is used to change the measured voltages into digital information. Waveforms are taken as a series of samples. The samples are stored, accumulating until enough are taken in order to describe the waveform, which are then reassembled for display. Digital technology allows the information to be displayed with brightness, clarity, and stability. There are, however, limitations as with the performance of any oscilloscope. The highest frequency at which the oscilloscope can operate is determined by the analog bandwidth of the front-end components of the instrument and the sampling rate.

Digital oscilloscopes can be classified into three primary categories: digital storage oscilloscopes, digital phosphor oscilloscopes, and digital sampling oscilloscopes.[1][2] Newer variants include PC-based oscilloscopes (which attach to a PC for data processing and display) and mixed-signal oscilloscopes (which employ other functions in addition to voltage measurement).

Digital storage oscilloscope

Screen of a digital oscilloscope from HP that uses a cathode-ray tube display

The digital storage oscilloscope, or DSO for short, is now the preferred type for most industrial applications. Instead of storage-type cathode ray tubes, DSOs use digital memory, which can store data as long as required without degradation. A digital storage oscilloscope also allows complex processing of the signal by high-speed digital signal processing circuits.

The vertical input is digitized by an analog to digital converter to create a data set that is stored in the memory of a microprocessor. The data set is processed and then sent to the display, which in early DSOs was a cathode ray tube, but is now more likely to be an LCD flat panel. DSOs with color LCD displays are common. The data set can be sent over a LAN or a WAN for processing or archiving. The screen image can be directly recorded on paper by means of an attached printer or plotter, without the need for an oscilloscope camera. The oscilloscope's own signal analysis software can extract many useful time-domain features (e.g., rise time, pulse width, amplitude), frequency spectra, histograms and statistics, persistence maps, and a large number of parameters meaningful to engineers in specialized fields such as telecommunications, disk drive analysis and power electronics.

Digital storage also makes possible another type of oscilloscope, the equivalent-time sample oscilloscope. Instead of taking consecutive samples after the trigger event, only one sample is taken. However, the oscilloscope is able to vary its timebase to precisely time its sample, thus building up the picture of the signal over the subsequent repeats of the signal. This requires that either a clock or repeating pattern be provided. This type of oscilloscope is frequently used for very high speed communication because it allows for a very high "sample rate" and low amplitude noise compared to traditional real-time oscilloscopes.

Digital oscilloscopes are limited principally by the performance of the analog input circuitry, the duration of the sample window, and resolution of the sample rate. When not using equivalent-time sampling, the sampling frequency should be at least the Nyquist rate, double the frequency of the highest-frequency component of the observed signal, otherwise aliasing occurs.

Advantages over the analog oscilloscope are:

A disadvantage of digital oscilloscopes is the limited refresh rate of the screen. On an analog oscilloscope, the user can get an intuitive sense of the trigger rate simply by looking at the steadiness of the CRT trace. For a digital oscilloscope, the screen looks exactly the same for any signal rate which exceeds the screen's refresh rate. Additionally, it is sometimes difficult to spot "glitches" or other rare phenomena on the black-and-white screens of standard digital oscilloscopes; the slight persistence of CRT phosphors on analog oscilloscopes makes glitches visible even if many subsequent triggers overwrite them. Both of these difficulties have been overcome recently by "digital phosphor oscilloscopes", which store data at a very high refresh rate and display it with variable intensity, to simulate the trace persistence of a CRT oscilloscope.

Digital sampling oscilloscopes

Digital sampling oscilloscopes operate on the same principle as analog sampling oscilloscopes and, like their analog counterparts, are of great use when analyzing high-frequency signals; that is, repetitive signals whose frequencies are higher than the oscilloscope's sampling rate. For measuring repetitive signals, this type can have bandwidth and high-speed timing up to ten times greater than any real-time oscilloscope.

A real-time oscilloscope, sometimes called a “single-shot” scope, captures an entire waveform on each trigger event. This requires the scope to capture a large number of data points in one continuous record. A sequential equivalent-time sampling oscilloscope, sometimes simply called a “sampling scope,” measures the input signal only once per trigger. The next time the scope is triggered, a small delay is added and another sample is taken. Thus a large number of trigger events must occur in order to collect enough samples to build a picture of the waveform. The measurement bandwidth is determined by the frequency response of the sampler which currently can extend beyond 90 GHz.[3]

An alternative to sequential equivalent-time sampling is called random equivalent-time sampling. Samples are synchronised not with trigger events but with the scope's internal sampling clock. This causes them to occur at apparently random times relative to the trigger event. The scope measures the time interval between the trigger and each sample, and uses this to locate the sample correctly on the x-axis. This process continues until enough samples have been collected to build up a picture of the waveform. The advantage of this technique over sequential equivalent-time sampling is that the scope can collect data from before the trigger event as well as after it, in a similar way to the pre-trigger function of most real-time digital storage scopes. Random equivalent-time sampling can be integrated into a standard DSO without requiring special sampling hardware, but has the disadvantage of poorer timing precision than the sequential sampling method.[4]

Handheld oscilloscopes

Handheld oscilloscopes (also called scopemeters) are useful for many test and field service applications. Today, a hand held oscilloscope is usually a digital sampling oscilloscope, using a liquid crystal display. Typically, a hand held oscilloscope has two analog input channels, but four input channel versions are also available. Some instruments combine the functions of a digital multimeter with the oscilloscope. These usually are lightweight and have good accuracy.

PC-based oscilloscopes

A new type of "oscilloscope" is emerging that consists of a specialized signal acquisition board (which can be an external USB or Parallel port device, or an internal add-on PCI or ISA card). The hardware itself usually consists of an electrical interface providing isolation and automatic gain controls, several high-speed analog-to-digital converters and some buffer memory, or even on-board Digital Signal Processor (DSPs). Depending on the exact hardware configuration, the hardware could be best described as a digitizer, a data logger or as a part of a specialized automatic control system.

The PC provides the display, control interface, disc storage, networking and often the electrical power for the acquisition hardware. The viability of PC-based oscilloscopes depends on the current widespread use and low cost of standardized PCs. Since prices can range from as little as US$100 to as much as US$3000 depending on their capabilities, such instruments are particularly suitable for the educational market, where PCs are commonplace but equipment budgets are often low.

PCO acquisition hardware, in certain cases, may only consist of a standard sound card or even a game port, if only audio and low-frequency signals are involved, though in many cases it will be considerably more robust. The PCO can transfer data to the computer in two main ways — streaming, and block mode. In streaming mode the data is transferred to the PC in a continuous flow without any loss of data. The way in which the PCO is connected to the PC (e.g., IEEE1394, Ethernet, USB etc.) will dictate the maximum achievable speed and thereby frequency and resolution using this method. Block mode utilizes the on-board memory of the PCO to collect a block of data which is then transferred to the PC after the block has been recorded. The PCO hardware then resets and records another block of data. This process happens very quickly, but the time taken will vary according to the size of the block of data and the speed at which it can be transferred. This method enables a much higher sampling speed, but in many cases the hardware will not record data whilst it is transferring the existing block, meaning that some data loss will occur.

The advantages of PC-based oscilloscopes include:

There are also some disadvantages, which include:

As more processing power and data storage is included in oscilloscopes, the distinction is becoming blurred. Mainstream oscilloscope vendors manufacture large-screen, PC-based oscilloscopes, with very fast (multi-GHz) input digitizers and highly customized user interfaces.

Software for a PC may use the sound card or game port to acquire analog signals, instead of dedicated signal acquisition hardware. However, these devices have very restricted input voltage ranges, limited precision/resolution, and very restricted frequency ranges. The ground reference for these inputs is the same as the ground for the PC logic and power supply; this may inject unacceptable amounts of noise into the circuit under test. However, these devices can be useful for demonstration, hobby use, or specific setups where these factors won't interfere. Ground reference can also be eliminated with capacitor AC coupling or a signal transformer.

If a sound card is used, frequency response is usually limited to the audio range, and DC signals cannot be measured without hardware modification. The number of inputs is limited by the number of recording channels and the inputs can handle only audio line-level voltages (usually ~1 Vpp) without the risk of damage.

If the game port is used as the acquisition hardware, the possible sampling frequency is very low, typically below 1 kHz, and the input voltages can only vary over a range of a couple of volts. In addition, the game port cannot easily be programmed for a specific sampling rate, nor can it be easily assigned a precise quantization step. The analog to digital conversion is accomplished by triggering the discharge of a capacitor and then measuring how long it takes to charge it to a fixed threshold that is seen as a "0" to "1" transition on the PC ISA bus. This means a huge resistance at the input takes longer to measure than a low resistance, which results in asymmetrical sampling intervals.[5] These limitations only make it suitable for low-precision visualization of low frequency signals.

Mixed-signal oscilloscopes

A mixed-signal oscilloscope (or MSO) has two kinds of inputs, a small number (typically two or four) of analog channels, and a larger number (typically sixteen) of digital channels. These measurements are acquired with a single time base, they are viewed on a single display, and any combination of these signals can be used to trigger the oscilloscope.

An MSO combines all the measurement capabilities and the use model of a Digital Storage Oscilloscope (DSO) with some of the measurement capabilities of a logic analyzer. MSOs typically lack the advanced digital measurement capabilities and the large number of digital acquisition channels of full-fledged logic analyzers,[6] but they are also much less complex to use. Typical mixed-signal measurement uses include the characterization and debugging of hybrid analog/digital circuits like: embedded systems, Analog-to-digital converters (ADCs), Digital-to-analog converters (DACs), and control systems.

Cathode-ray oscilloscope

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" oscilloscopes to distinguish them from the "digital" oscilloscopes that became common in the 1990s and 2000s.

Before the introduction of the CRO in its current form, the cathode ray tube had already been in use as a measuring device. The cathode ray tube is an evacuated glass envelope, similar to that in a black-and-white television set, with its flat face covered in a fluorescent material (the phosphor). The screen is typically less than 20 cm in diameter, much smaller than the one in a television set. Older CROs had round screens or faceplates, while newer CRTs in better CROs have rectangular faceplates.

In the neck of the tube is an electron gun, which is a small heated metal cylinder with a flat end coated with electron-emitting oxides. Close to it is a much-larger-diameter cylinder carrying a disc at its cathode end with a round hole in it; it's called a "grid" (G1), by historic analogy with amplifier vacuum-tube grids. A small negative grid potential (referred to the cathode) is used to block electrons from passing through the hole when the electron beam needs to be turned off, as during sweep retrace or when no trigger events occur.

However, when G1 becomes less negative with respect to the cathode, another cylindrical electrode designated G2, which is hundreds of volts positive referred to the cathode, attracts electrons through the hole. Their trajectories converge as they pass through the hole, creating quite-small diameter "pinch" called the crossover. Following electrodes ("grids"), electrostatic lenses, focus this crossover onto the screen; the spot is an image of the crossover.

Typically, the CRT runs at roughly -2 kV or so, and various methods are used to correspondingly offset the G1 voltage. Proceeding along the electron gun, the beam passes through the imaging lenses and first anode, emerging with an energy in electron-volts equal to that of the cathode. The beam passes through one set of deflection plates , then the other, where it is deflected as required to the phosphor screen.

The average voltage of the deflection plates is relatively close to ground, because they have to be directly connected to the vertical output stage.

By itself, once the beam leaves the deflection region, it can produce a usefully bright trace. However, for higher bandwidth CROs where the trace may move more rapidly across the phosphor screen, a positive post-deflection acceleration ("PDA") voltage of over 10,000 volts is often used, increasing the energy (speed) of the electrons that strike the phosphor. The kinetic energy of the electrons is converted by the phosphor into visible light at the point of impact.

When switched on, a CRT normally displays a single bright dot in the center of the screen, but the dot can be moved about electrostatically or magnetically. The CRT in an oscilloscope always uses electrostatic deflection. Ordinary electrostatic deflection plates can typically move the beam roughly only 15 degrees or so off-axis, which means that oscilloscope CRTs have long, narrow funnels, and for their screen size, are usually quite long. It's the CRT length that makes CROs "deep", from front to back. Modern flat-panel oscilloscopes have no need for such rather-extreme dimensions; their shapes tend to be more like one kind of rectangular lunchbox.

Between the electron gun and the screen are two opposed pairs of metal plates called the deflection plates. The vertical amplifier generates a potential difference across one pair of plates, giving rise to a vertical electric field through which the electron beam passes. When the plate potentials are the same, the beam is not deflected. When the top plate is positive with respect to the bottom plate, the beam is deflected upwards; when the field is reversed, the beam is deflected downwards. The horizontal amplifier does a similar job with the other pair of deflection plates, causing the beam to move left or right. This deflection system is called electrostatic deflection, and is different from the electromagnetic deflection system used in television tubes. In comparison to magnetic deflection, electrostatic deflection can more readily follow random and fast changes in potential, but is limited to small deflection angles.

Common representations of deflection plates are misleading. For one, the plates for one deflection axis are closer to the screen than the plates for the other. Plates that are closer together provide better sensitivity, but they also need to be extend far enough along the CRT's axis to obtain adequate sensitivity. (The longer the time a given electron spends in the field, the farther it's deflected.) However, closely spaced long plates would cause the beam to contact them before full amplitude deflection occurs, so the compromise shape has them relatively close together toward the cathode, and flared apart in a shallow vee toward the screen. They are not flat in any but quite-old CRTs!

The timebase is an electronic circuit that generates a ramp voltage. This is a voltage that changes continuously and linearly with time. When it reaches a predefined value the ramp is reset and settles to its starting value. When a trigger event is recognized, provided the reset process (holdoff) is complete, the ramp starts again. The timebase voltage usually drives the horizontal amplifier. Its effect is to sweep the screen end of the electron beam at a constant speed from left to right across the screen, then blank the beam and return its deflection voltages to the left, so to speak, in time to begin the next sweep. Typical sweep circuits can take significant time to reset; in some CROs, fast sweeps required more time to retrace than to sweep.

Meanwhile, the vertical amplifier is driven by an external voltage (the vertical input) that is taken from the circuit or experiment that is being measured. The amplifier has a very high input impedance, typically one megohm, so that it draws only a tiny current from the signal source. Attenuator probes reduce the current drawn even more. The amplifier drives the vertical deflection plates with a voltage that is proportional to the vertical input. Because the electrons have already been accelerated by typically 2kV (roughly), this amplifier also has to deliver almost a hundred volts, and this with a very wide bandwidth. The gain of the vertical amplifier can be adjusted to suit the amplitude of the input voltage. A positive input voltage bends the electron beam upwards, and a negative voltage bends it downwards, so that the vertical deflection at any part of the trace shows the value of the input at that time.[7]

The response of any oscilloscope is much faster than that of mechanical measuring devices such as the multimeter, where the inertia of the pointer (and perhaps damping) slows down its response to the input.

Observing high speed signals, especially non-repetitive signals, with a conventional CRO is difficult, due to non-stable or changing triggering threshold which makes it hard to "freeze" the waveform on the screen. This often requires the room to be darkened or a special viewing hood to be placed over the face of the display tube. To aid in viewing such signals, special oscilloscopes have borrowed from night vision technology, employing a microchannel plate electron multiplier behind the tube face to amplify faint beam currents.

Tektronix Model C-5A Oscilloscope Camera with Polaroid instant film pack back.

Although a CRO allows one to view a signal, in its basic form it has no means of recording that signal on paper for the purpose of documentation. Therefore, special oscilloscope cameras were developed to photograph the screen directly. Early cameras used roll or plate film, while in the 1970s Polaroid instant cameras became popular. A P11 CRT phosphor (visually blue) was especially effective in exposing film. Cameras (sometimes using single sweeps) were used to capture faint traces.

The power supply is an important component of the oscilloscope. It provides low voltages to power the cathode heater in the tube (isolated for high voltage!), and the vertical and horizontal amplifiers as well as the trigger and sweep circuits. Higher voltages are needed to drive the electrostatic deflection plates, which means that the output stage of the vertical deflection amplifier has to develop large signal swings. These voltages must be very stable, and amplifier gain must be correspondingly stable. Any significant variations will cause errors in the size of the trace, making the oscilloscope inaccurate.

Later analog oscilloscopes added digital processing to the standard design. The same basic architecture — cathode ray tube, vertical and horizontal amplifiers — was retained, but the electron beam was controlled by digital circuitry that could display graphics and text mixed with the analog waveforms. Display time for those was interleaved — multiplexed — with waveform display in basically much the same way that a dual/multitrace oscilloscope displays its channels. The extra features that this system provides include:

Dual-beam oscilloscope

A dual-beam oscilloscope was a type of oscilloscope once used to compare one signal with another. There were two beams produced in a special type of CRT.

Unlike an ordinary "dual-trace" oscilloscope (which time-shared a single electron beam, thus losing about 50% of each signal), a dual-beam oscilloscope simultaneously produced two separate electron beams, capturing the entirety of both signals. One type (Cossor, UK) had a beam-splitter plate in its CRT, and single-ended vertical deflection following the splitter. (There is more about this type of oscilloscope near the end of this article.)

Other dual-beam oscilloscopes had two complete electron guns, requiring tight control of axial (rotational) mechanical alignment in manufacturing the CRT. In the latter type, two independent pairs of vertical plates deflect the beams. Vertical plates for channel A had no effect on channel B's beam. Similarly for channel B, separate vertical plates existed which deflected the B beam only.

On some dual-beam oscilloscopes the time base, horizontal plates and horizontal amplifier were common to both beams (the beam-splitter CRT worked this way). More elaborate oscilloscopes like the Tektronix 556 and 7844 could employ two independent time bases and two sets of horizontal plates and horizontal amplifiers. Thus one could look at a very fast signal on one beam and a slow signal on another beam.

Most multichannel oscilloscopes do not have multiple electron beams. Instead, they display only one trace at a time, but switch the later stages of the vertical amplifier between one channel and the other either on alternate sweeps (ALT mode) or many times per sweep (CHOP mode). Very few true dual-beam oscilloscopes were built.

With the advent of digital signal capture, true dual-beam oscilloscopes became obsolete, as it was then possible to display two truly simultaneous signals from memory using either the ALT or CHOP display technique, or even possibly a raster display mode.

Analog storage oscilloscope

Trace storage is an extra feature available on some analog oscilloscopes; they used direct-view storage CRTs. Storage allows the trace pattern that normally decays in a fraction of a second to remain on the screen for several minutes or longer. An electrical circuit can then be deliberately activated to store and erase the trace on the screen.

The storage is accomplished using the principle of secondary emission. When the ordinary writing electron beam passes a point on the phosphor surface, not only does it momentarily cause the phosphor to illuminate, but the kinetic energy of the electron beam knocks other electrons loose from the phosphor surface. This can leave a net positive charge. Storage oscilloscopes then provide one or more secondary electron guns (called the "flood guns") that provide a steady flood of low-energy electrons traveling towards the phosphor screen. Flood guns cover the entire screen, ideally uniformly. The electrons from the flood guns are more strongly drawn to the areas of the phosphor screen where the writing gun has left a net positive charge; in this way, the electrons from the flood guns re-illuminate the phosphor in these positively charged areas of the phosphor screen.

If the energy of the flood gun electrons is properly balanced, each impinging flood gun electron knocks out one secondary electron from the phosphor screen, thus preserving the net positive charge in the illuminated areas of the phosphor screen. In this way, the image originally written by the writing gun can be maintained for a long time — many seconds to a few minutes. Eventually, small imbalances in the secondary emission ratio cause the entire screen to "fade positive" (light up) or cause the originally written trace to "fade negative" (extinguish). It is these imbalances that limit the ultimate storage time possible.

Storage oscilloscopes (and large-screen storage CRT displays) of this type, with storage at the phosphor, were made by Tektronix. Other companies, notably Hughes, earlier made storage oscilloscopes with a more-elaborate and costly internal storage structure.

Some oscilloscopes used a strictly binary (on/off) form of storage known as "bistable storage". Others permitted a constant series of short, incomplete erasure cycles which created the impression of a phosphor with "variable persistence". Certain oscilloscopes also allowed the partial or complete shutdown of the flood guns, allowing the preservation (albeit invisibly) of the latent stored image for later viewing. (Fading positive or fading negative only occurs when the flood guns are "on"; with the flood guns off, only leakage of the charges on the phosphor screen degrades the stored image.

Analog sampling oscilloscope

The principle of sampling was developed during the 1930s in Bell Laboratories by Nyquist, after whom the sampling theorem is named. The first sampling oscilloscope was, however, developed in the late 1950s at the Atomic Energy Research Establishment at Harwell in England by G.B.B. Chaplin, A.R. Owens and A.J. Cole. ["A Sensitive Transistor Oscillograph With DC to 300 Mc/s Response", Proc I.E.E. (London) Vol.106, Part B. Suppl., No. 16, 1959].

The first sampling oscilloscope was an analog instrument, originally developed as a front-end unit for a conventional oscilloscope. The need for this instrument grew out of the requirement of nuclear scientists at Harwell to capture the waveform of very fast repetitive pulses. The current state-of-the-art oscilloscopes — with bandwidths of typically 20 MHz — were not able to do this and the 300 MHz effective bandwidth of their analog sampling oscilloscope represented a considerable advance.

A short series of these "front-ends" was made at Harwell and found much use and Chaplin, et al. patented the invention. Commercial exploitation of this patent was ultimately done by the Hewlett-Packard Company (later Agilent Technologies).

Sampling oscilloscopes achieve their large bandwidths by not taking the entire signal at a time. Instead, only a sample of the signal is taken. The samples are then assembled to create the waveform. This method can only work for repetitive signals, not transient events. The idea of sampling can be thought of as a stroboscopic technique. When using a strobe light, only pieces of the motion are seen, but when enough of these images are taken, the overall motion can be captured[8]

Related instruments

A large number of instruments used in a variety of technical fields are really oscilloscopes with inputs, calibration, controls, display calibration, etc., specialized and optimized for a particular application. In some cases additional functions such as a signal generator are built into the instrument to facilitate measurements that would otherwise require one or more additional instruments.

The waveform monitor in television broadcast engineering is very close to a standard oscilloscope, but it includes triggering circuits and controls that allow a stable display of a composite video frame, field, or even a selected line out of a field. Robert Hartwig explains the waveform monitor as "providing a graphic display of the black-and-white portion of the picture."[9] The black-and-white portion of the video signal is called the "luminance" due to its fluorescent complexion. The waveform monitor's display of black vs. white levels allows the engineer to troubleshoot the quality of the picture and be certain that it is within the required standards. For convenience, the vertical scale of the waveform monitor is calibrated in IRE units.

See also

References

  1. ↑ "Oscilloscope Types" http://www.radio-electronics.com/info/t_and_m/oscilloscope/oscilloscope_types.php
  2. ↑ "XYZs of Oscilloscopes Primer" www.tektronix.com 03W_8605_3.pdf
  3. ↑ "What is the difference between an equivalent time sampling oscilloscope and a real-time oscilloscope?" (PDF). Agilent Technologies. Retrieved 10 June 2013.
  4. ↑ [Sampling Oscilloscope Techniques, http://www.cbtricks.com/miscellaneous/tech_publications/scope/sampling.pdf], Tek Technique Primer 47W-7209, Tektronix Inc., 1989, accessed 2013-09-25
  5. ↑ "PC joystick interface". 090907 epanorama.net
  6. ↑ "When your MSO needs help". Byte Paradigm. Retrieved 13 August 2014.
  7. ↑ Special purpose oscilloscopes called modulation monitors may directly apply a relatively large-voltage radio-frequency signal to the deflection plates with no intervening amplifier stage. In such instances, the waveform of the applied RF could generally not be shown, because the frequency was much too high. In such monitors, the CRT's bandwidth, which is typically a few hundred MHz, permits the envelope of the high-frequency RF to be displayed. The display is not a trace, but a solid triangle of light. Some bench-top oscilloscopes brought out terminals for the deflection plates for such uses. (Edited; basically from D. S. Evans and G. R. Jessup (ed), VHF-UHF Manual (3rd Edition), Radio Society of Great Britain, London, 1976 page 10.15)
  8. ↑ Hicman, Ian. Oscilloscopes: How to Use Them, How They Work, 5th ed., Newness, 2001 p.88-91.
  9. ↑ Robert Hartwig, Basic TV Technology, Focal Press, Boston, 1995, ISBN 0-240-80228-4 pg. 28
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