Oscilloscope
An oscilloscope is an electronic display instrument for graphically representing electrical signals that can vary over time. It is widely used in signal electronics, often in conjunction with a spectrum analyzer. It allows to see the temporal evolution of different signals present in the electronic circuits. These devices have some switches that allow the adjustment of the time and voltage scale. The frequencies at which an oscilloscope can be used can be from a signal that does not vary with time (such as a direct current) to the order of 10 MHz or more depending on the model used.
Presents the values of the electrical signals in the form of coordinates on a screen, in which the x axis (horizontal) normally represents time and the y axis (vertical) represents time.) represents stresses. The image thus obtained is called an oscillogram. They usually include another input, called “THRASHER axis” or “Wehnelt cylinder” that controls the luminosity of the beam, allowing some segments of the trace to be highlighted or turned off.
Oscilloscopes, classified according to their internal functioning; They can be both analog and digital, with the result shown being identical in either case (in theory).
History
In 1897, German physicist Karl Ferdinand Braun developed the first oscilloscope by adapting a cathode ray tube—created by William Crookes in 1875—so that the tube's stream of electrons would be directed toward a fluorescent screen by means of magnetic fields generated by alternating current. In 1897 and in 1899, Jonathan Zenneck fitted it with beamforming plates and a magnetic field to sweep the trace. Early cathode ray tubes were applied experimentally to laboratory measurements from the 1920s, but suffered from poor stability. vacuum and cathode emitters. Vladimir K. Zworykin described a permanently sealed, high-vacuum cathode ray tube with a thermionic emitter in 1931. This stable and reproducible component enabled General Radio to manufacture an oscilloscope that was usable outside of a laboratory setting.
After World War II, surplus electronic components became the basis for the revival of the Heathkit Corporation, and a $50 kit to build an oscilloscope out of those parts was an early success on the market.
Usage
In an oscilloscope there are basically two types of controls that are used as regulators that adjust the input signal and consequently allow to measure on the screen and in this way you can see the shape of the signal measured by the oscilloscope, this called in a technical way, it can be said that the oscilloscope is used to observe the signal that you want to measure.
To measure it can be compared with the Cartesian plane.
The first control regulates the x axis and appreciates fractions of time (seconds, milliseconds, microseconds, etc., depending on the resolution of the device). The second regulates the y axis, controlling the input voltage (in volts, millivolts, microvolts, etc., depending on the resolution of the device).
These adjustments determine the value of the grid scale that divides the screen, allowing to know how much each square of it represents to, consequently, know the value of the signal to be measured, both in voltage and in frequency (actually it is measured the period of a wave of a signal, and then the frequency is calculated).
Other functions
Some oscilloscopes have cursors. These are lines that can be moved across the screen to measure the time interval between two points, or the difference between two voltages. Some older oscilloscopes simply illuminated the trace at moving locations. These cursors are more accurate than visual estimates that refer to grid lines.
Higher quality general purpose oscilloscopes include a calibration signal to set the test probes offset; this is (often) a 1 kHz square wave signal of a defined peak-to-peak voltage available from a front panel test terminal. Some advanced oscilloscopes also have a square loop for checking and adjusting current probes.
Sometimes a user wants to see an event that happens only occasionally. To capture these events, some oscilloscopes, called "storage oscilloscopes", keep 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 rates as slow as once an hour, emulating a strip chart recorder. That is, the signal scrolls across the screen from right to left. Most oscilloscopes with this feature switch from a sweep mode to a strip graph approximately one sweep every ten seconds. This is because otherwise the scope looks broken - it's collecting data, but you can't see the point.
All but the simplest oscilloscope models today make more frequent use of digital signal sampling. The samples feed fast analog-to-digital converters, after which all signal processing (and storage) is digital.
Many oscilloscopes support plug-in modules for different purposes, for example, relatively narrow-bandwidth high-sensitivity amplifiers, differential amplifiers, amplifiers with four or more channels, sampling plugins for very high-frequency repetitive signals, and purpose plugins. including audio/ultrasonic spectrum analyzers and stable voltage compensated direct coupled channels with relatively high gain.
Analog Oscilloscope
The voltage to be measured is applied to the oscillating vertical deflection plates of a cathode ray tube (using an amplifier with high input impedance and adjustable gain) while a tooth voltage is applied to the horizontal deflection plates. sierra (so called because it repeatedly rises gently and then falls sharply). This voltage is produced by means of an appropriate oscillator circuit and its frequency can be adjusted within a wide range of values, which makes it possible to adapt to the frequency of the signal to be measured. This is what is called time base.
In Figure 1 you can see a schematic representation of an oscilloscope with indication of the fundamental minimum stages. The operation is as follows:
In the cathode ray tube, the electron beam generated by the cathode and accelerated by the anode reaches the screen, covered on the inside with a fluorescent layer that lights up due to the impact of the electrons.
If a potential difference is applied to either of the two pairs of deflection plates, a deflection of the electron beam occurs due to the electric field created by the applied voltage. Thus, the sawtooth voltage, which is applied to the horizontal deflection plates, causes the beam to move from left to right and during this time, in the absence of signal on the vertical deflection plates, draw a line horizontal line on the screen and then return to the starting point to start a new sweep. This return is not perceived by the human eye due to the speed at which it is carried out and to the fact that, additionally, during it a partial shutdown (erasing) or a deviation of the beam occurs.
If in these conditions the signal to be measured is applied to the vertical deviation plates (through the adjustable gain amplifier), the beam, in addition to moving from left to right, will move up or down, depending on the signal polarity, and with greater or lesser amplitude depending on the applied voltage.
Since the coordinate axes are divided by marks, it is possible to establish a relationship between these divisions and the period of the sawtooth as regards the x axis and the voltage as regards the x. With this, each horizontal division will correspond to a specific time, in the same way that each vertical division will correspond to a specific voltage. In this way, in the case of periodic signals, both their period and their amplitude can be determined.
The typical scaling range of microvolts to a few volts and microseconds to several seconds makes this instrument highly versatile for studying a wide variety of signals.
Limitations of the Analog Oscilloscope
The analog oscilloscope has a series of limitations inherent to its operation:
- Signs must be periodic. To see a stable trace, the signal must be periodic as it is the periodicity of the signal that refreshes the trace on the screen. To solve this problem synchronism signals are used with the input signal to shoot the horizontal sweep (trigger level) or oscilloscopes are used with a triggered time base.
- Very fast signals reduce brightness. When part of the signal period is observed, the brightness is reduced due to the low phosphoric persistence of the screen. This is solved by placing a post-accelerator potential in the cathodic ray tube.
- Slow signals do not form a trace. The signs of low frequencies produce a very slow sweep that does not allow the retina to integrate the trace. This is solvent with high persistence tubes. There were also Polaroid cameras specially adapted to photograph oscilloscope screens. Keeping the exhibition for a period you get a picture of the trace. Another way to solve the problem is to give different slopes to the horizontal sweeping tooth. This allows it to take longer to sweep the entire screen, and therefore low-frequency signals can be displayed but you will see a turning point through the screen because the phosphoric persistence is not high.
- They can only be seen transient if these are repetitive; but a oscilloscope can be used with a triggered time base. This type of oscilloscope has a function mode called "unique device". When a transient comes the oscilloscope will show this and only this, leaving to sweep once the signal was already printed on the screen.
Digital Oscilloscope
In the digital oscilloscope the signal is previously digitized by an analog-to-digital converter. As the reliability of the display depends on the quality of this component, it must be taken care of to the maximum.
The characteristics and procedures indicated for analog oscilloscopes are applicable to digital ones. However, these have additional possibilities, such as early triggering (pre-triggering) for viewing short-term events, or memorizing the oscillogram by transferring the data to a PC. This allows comparing measurements made at the same point of a circuit or element. There are also equipment that combines analog and digital stages.
The main characteristic of a digital oscilloscope is the sampling rate; it will determine the maximum bandwidth that the instrument can measure based on the Nyquist theorem. It is expressed in MS/s (millions of samples 'samples' per second).
Most digital oscilloscopes today are based on FPGA (Field Programmable Gate Array) control, which is the controller element of the device's high-speed analog-to-digital converter. and other internal circuitry, such as memory, buffers, among others.
These oscilloscopes add features and user convenience that are impossible to obtain with analog circuitry, such as the following:
- Automatic measurement of peak values, maximums and minimum signals. True effective value.
- Measure of signal flanks and other intervals.
- Catch of transients.
- Advanced calculations, such as the FFT to calculate the signal spectrum. It also serves to measure tension signals.
Within the digital oscilloscope there are two types that are clearly differentiated:
- Bank: more powerful than those that precede it, made to have in a single location for longer.
- Portable: with less power but more compact to take from one place to another.
Digital Phosphor Oscilloscope
The Digital Phosphor Oscilloscope (DPO) offers a new take on oscilloscope architecture by combining the best features of an analog oscilloscope with those of a digital oscilloscope. Like the analog oscilloscope, the first stage is the vertical amplifier, and like the digital oscilloscope, the second stage is an ADC converter. But after analog to digital conversion, digital phosphor oscilloscope is a little different from digital. This has special functions designed to recreate the degree of intensity of a cathode ray tube. Instead of using chemical phosphor, like an analog oscilloscope, the DPO has digital phosphor which is a constantly updated database. This database has a separate cell of information for each of the pixels on the screen. Every time a waveform is captured (in other words, every time the oscilloscope is triggered) it is stored in the database cells. Each cell that stores the waveform information is then inserted with the intensity information. Finally all the information is displayed on the LCD screen or stored by the oscilloscope.