Oscilloscopes vary in form factor, capabilities and purpose but there are elements they have in common, most notably a display where waveforms are made visible. The defining characteristic of a display is that there are two axes, labeled Y and X, which are perpendicular to one another and typically intersect at the center, known as the origin. These axes are multipurpose in the sense that they can be calibrated to show differing parameters of the signal as applied at the oscilloscope input.
An oscilloscope front end begins upstream at the analog inputs, which feed separate channels. Usually there are either two or four channels, although more are possible. Separate signals can be connected to any or all of the channels and the traces shown superimposed, using coinciding or offset axes, or they may be shown in split-screen format.
Generally the signals display simultaneously, or they may be available, to be turned on or off individually by pressing buttons adjacent to the input ports.
The most commonly used and most fundamental oscilloscope mode is the time domain, in which the Y-axis delineates amplitude expressed in volts or parts thereof, and the X-axis delineates time, expressed in seconds or fractions of a second. The Y-axis is generally drawn to cross the X-axis at its zero point, and the X-axis is usually made to cross the Y-axis at its zero point. But these are merely conventions.
In actuality, the axes are everywhere, throughout the display. (There could be additional X and Y axes located anywhere in the display as long as they are parallel to one another and perpendicular to the other axis. The display could be filled with a thick single axis as long as it did not prevent reading the waveform.) However, just one of each is shown, and it is left to the user’s eye to project points on the waveform to the relevant axis so as to determine the instantaneous amplitude or time.
The word oscilloscope implies that it is an instrument for visualizing oscillations. A waveform fluctuates in either periodic or random fashion. It can be repetitive, one-shot or damped, meaning its amplitude diminishes in time due to impedance within the circuit until the waveform goes away entirely.
A signal can be an unchanging dc voltage, displayed as a flat horizontal line. In this case it does not oscillate. It is a limiting case of the usual ac signal.
It must be emphasized that the trace as shown in the display is not the actual signal. It is a graph that depicts the amplitude with respect to time. Nevertheless, it is a convenient way to visualize what is happening regarding changes in air density for an acoustic signal or changes in field strength for an electromagnetic signal.
Other parameter assignments for Y and X axes are used to create different displays, often representations of the same signal, in which case they may be shown simultaneously or offset.
Another widely used axis configuration is for the frequency domain representation. A signal shown in the time domain can instantly be translated into the frequency domain through the miracle of the Fast Fourier Transform (FFT), which had its origins in the early nineteenth century and was finally enhanced in its present form in the 1960s. It is built into the hardware of advanced oscilloscopes such as the Tektronix MDO 3000 series.
To access the frequency domain mode, with the waveform under investigation displayed in the time domain, simply press Math, then FFT. If it doesn’t display, check Source, which can be adjusted using Multipurpose Knob a.
The frequency domain mode shows the same signal that was displayed in the time domain. It is a line graph or bar graph depicting amplitudes of the signal at various frequencies.
In the frequency domain mode, as in the time domain mode, amplitude is shown on the Y-axis. But there are two differences. For one thing, amplitude in the frequency domain is expressed in units of power rather than voltage. Power is more relevant to the frequency domain than is voltage. Second, this power is shown on a logarithmic scale rather than on a lineal scale, as in the time domain. The reason is its large dynamic range would make a lineal frequency representation difficult to draw and read given the constraints of the oscilloscope screen. This logarithmic scale is calibrated in decibels.
An additional oscilloscope mode, often used in conjunction with the frequency domain, is the Spectrogram. To bring up this display in the Tektronix MDO 3000 series instruments with a waveform displayed in the time domain, first press RF. Then press the spectrogram tab in the menu across the bottom of the display. In the menu to the right of the display, press the soft key so as to toggle on spectrogram.
The spectrogram is generated in the upper half of a split-screen format when a different axis configuration is invoked. Here, the Y-axis is employed to represent the passage of time. That fact accounts for the strange appearance of the spectrogram, which begins at the bottom of the window and slowly rises to the top. The X-axis, in the time domain used to represent time, here represents frequency.
Amplitude, left without an axis, must be represented in some other way. The method chosen by engineers who developed the spectrogram is color. A higher amplitude is depicted in warm colors such as orange or yellow and zero amplitude is blue. Notice, however, that the blue background is not a uniform hue, but rather it is speckled. This corresponds to the noise floor of the instrument or to noise that is introduced into the signal from outside the instrument.
Still another oscilloscope capability is the very interesting XY mode. It is used to display Lissajous patterns associated with signals. These patterns, in connection with non-electrical waveforms, were discovered and their properties investigate by Jules Antoine Lissajous, beginning in 1857. Subsequently, Lissajous traveled extensively and lectured on the subject, demonstrating apparatus he had built for the purpose of deriving these patterns from waveforms.
The apparatus consisted of a vibrating tuning fork, to which Lissajous had affixed a mirror. The vibrating mirror reflected light onto a second mirror, similarly affixed to a second tuning fork. From the second mirror, an image was projected onto a wall. This image was the resultant of acoustic waves from the two tuning forks.
Using a modern, advanced oscilloscope, electrical signals can be displayed as Lissajous patterns instantly by making two hookups and pressing front panel controls. In the Tektronix MDO 3104 oscilloscope, here’s how it’s done:
First press Default Setup and Menu Off so as to clear the table. Then press Acquire, which brings up the acquisition menu across the bottom of the screen. The last menu item on the right is XY display, and according to the tab, it is currently off. Press the associated soft key, and the XY display appears to the right of the display. To turn on the XY mode, press Triggered XY. Now, analog input channels One and Two both become active. This is because to view a Lissajous pattern, two signals must connect to the instrument. One is the signal under investigation, the other is the triggering signal. This is in contrast to a time or frequency domain display, where only one channel is required to make a display. One signal is connected and that signal in addition to being displayed also provides the triggering.
There is another tab that permits YT, which means time domain, to be turned on or off. Turning it on, the familiar time domain graph appears at the top, so that there is a split-screen setup.
To see a Lissajous pattern, the two signals can be connected to Channel One and to Channel Two via ordinary probes or AFG-supplied signals connected via BNC cables. With the internal AFG, there is a problem because only one of the thirteen available waveforms can be accessed at any given time.
However, there is another route to go. Press the button labeled R (for Reference) directly under the Math button. This brings up the Reference Menu, so if signals have been previously saved in memory, they can be toggled on, and they become available as material to create Lissajous Patterns.
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