cathode ray oscilloscope part 3 (types of oscilloscopes)
Types and models[edit]
The following section is a brief summary of various types and models available. For a detailed discussion, refer to the other article.
Cathode-ray oscilloscope (CRO)[edit]
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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 1990s and 2000s.
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. Instead, they could be used for signal analysis by feeding a reference signal into one axis and the signal to measure into the other axis. For an oscillating reference and measurement signal, this results in a complex looping pattern referred to as a Lissajous curve. The shape of the curve can be interpreted to identify properties of the measurement signal in relation to the reference signal, and is useful across a wide range of oscillation frequencies.
Dual-beam oscilloscope[edit]
The dual-beam analog oscilloscope can display two signals simultaneously. A special dual-beam CRT generates and deflects two separate beams. Although multi-trace analog oscilloscopes can simulate a dual-beam display with chop and alternate sweeps, those features do not provide simultaneous displays. (Real time digital oscilloscopes offer the same benefits of a dual-beam oscilloscope, but they do not require a dual-beam display.) The disadvantages of the dual trace oscilloscope are that it cannot switch quickly between the traces and it cannot capture two fast transient events. In order to avoid this problems a dual beam oscilloscope is used.
Analog storage oscilloscope[edit]
Trace storage is an extra feature available on some analog scopes; 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.
Digital oscilloscopes[edit]
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.
The digital storage oscilloscope, or DSO for short, is now the preferred type for most industrial applications, although simple analog CROs are still used by hobbyists. It replaces the electrostatic storage method used in analog storage scopes with digital memory, which can store data as long as required without degradation and with uniform brightness. It also allows complex processing of the signal by high-speed digital signal processing circuits.[3]
A standard DSO is limited to capturing signals with a bandwidth of less than half the sampling rate of the ADC (called the Nyquist limit). There is a variation of the DSO called the digital sampling oscilloscope that can exceed this limit for certain types of signal, such as high-speed communications signals, where the waveform consists of repeating pulses. This type of DSO deliberately samples at a much lower frequency than the Nyquist limit and then uses signal processing to reconstruct a composite view of a typical pulse. A similar technique, with analog rather than digital samples, was used before the digital era in analog sampling oscilloscopes.[17][18]
A digital phosphor oscilloscope (DPO) uses color information to convey information about a signal. It may, for example, display infrequent signal data in blue to make it stand out. In a conventional analog scope, such a rare trace may not be visible.
Mixed-signal oscilloscopes[edit]
A mixed-signal oscilloscope (or MSO) has two kinds of inputs, a small number of analog channels (typically two or four), and a larger number of digital channels(typically sixteen). It provides the ability to accurately time-correlate analog and digital channels, thus offering a distinct advantage over a separate oscilloscope and logic analyser. Typically, digital channels may be grouped and displayed as a bus with each bus value displayed at the bottom of the display in hex or binary. On most MSOs, the trigger can be set across both analog and digital channels.
Mixed-domain oscilloscopes[edit]
In a mixed-domain oscilloscope (MDO) you have an additional RF input port that goes into a spectrum analyzer part.[dubious ] It links those traditionally separate instruments, so that you can e.g. time correlate events in the time domain (like a specific serial data package) with events happening in the frequency domain (like RF transmissions).
Handheld oscilloscopes[edit]
Handheld oscilloscopes 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.
Many hand-held and bench oscilloscopes have the ground reference voltage common to all input channels. If more than one measurement channel is used at the same time, all the input signals must have the same voltage reference, and the shared default reference is the "earth". If there is no differential preamplifier or external signal isolator, this traditional desktop oscilloscope is not suitable for floating measurements. (Occasionally an oscilloscope user will break the ground pin in the power supply cord of a bench-top oscilloscope in an attempt to isolate the signal common from the earth ground. This practice is unreliable since the entire stray capacitance of the instrument cabinet will be connected into the circuit. Since it is also a hazard to break a safety ground connection, instruction manuals strongly advise against this practice.)
Some models of oscilloscope have isolated inputs, where the signal reference level terminals are not connected together. Each input channel can be used to make a "floating" measurement with an independent signal reference level. Measurements can be made without tying one side of the oscilloscope input to the circuit signal common or ground reference.
The isolation available is categorized as shown below:
| Overvoltage category | Operating voltage (effective value of AC/DC to ground) | Peak instantaneous voltage (repeated 20 times) | Test resistor |
|---|---|---|---|
| CAT I | 600 V | 2500 V | 30 Ω |
| CAT I | 1000 V | 4000 V | 30 Ω |
| CAT II | 600 V | 4000 V | 12 Ω |
| CAT II | 1000 V | 6000 V | 12 Ω |
| CAT III | 600 V | 6000 V | 2 Ω |
PC-based oscilloscopes[edit]
A new type of oscilloscope is emerging that consists of a specialized signal acquisition board (which can be an external USB or parallel portdevice, or an internal add-on PCI or ISA card). The user interface and signal processing software runs on the user's computer, rather than on an embedded computer as in the case of a conventional DSO.
Related instruments[edit]
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. Examples of such oscilloscope-based instruments include waveform monitors for analyzing video levels in television productions and medical devices such as vital function monitors and electrocardiogram and electroencephalogram instruments. In automobile repair, an ignition analyzer is used to show the spark waveforms for each cylinder. All of these are essentially oscilloscopes, performing the basic task of showing the changes in one or more input signals over time in an X‑Ydisplay.
Other instruments convert the results of their measurements to a repetitive electrical signal, and incorporate an oscilloscope as a display element. Such complex measurement systems include spectrum analyzers, transistor analyzers, and time domain reflectometers (TDRs). Unlike an oscilloscope, these instruments automatically generate stimulus or sweep a measurement parameter.
History[edit]
The Braun tube was known in 1897, and in 1899 Jonathan Zenneck equipped it with beam-forming plates and a magnetic field for sweeping the trace.[19] Early cathode ray tubes had been applied experimentally to laboratory measurements as early as the 1920s, but suffered from poor stability of the vacuum and the cathode emitters. V. K. Zworykin described a permanently sealed, high-vacuum cathode ray tube with a thermionic emitter in 1931. This stable and reproducible component allowed General Radio to manufacture an oscilloscope that was usable outside a laboratory setting.[3] After World War II surplus electronic parts became the basis of revival of Heathkit Corporation, and a $50 oscilloscope kit made from such parts was a first market success.
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