Oscilloscope

An oscilloscope is an electronic measuring instrument that creates a visible two-dimensional graph of an electrical potential difference. The signal is normally displayed with one axis representing time, making it useful for displaying periodic signals.

Description

Prior to the introduction of the oscilloscope in its current form, the cathode ray tube had already been in use as a measuring device. Turned on normally a CRT displays a single bright dot in the center of the screen, but this can be moved about electically or magnetically.

By placing a set of plates on either side of the beam path and attaching them to an input electrical signal, events that take very short times will still be visible. Amplifying them is This is not the case for mechanical measuring devices like multimeters, where the needle may take longer to move than the signal lasts.

By attaching a second set of plates at right angles to the first, the tube can be used to display a two dimensional image given two inputs. Now the original "offset" created in the early tube can be moved along the screen. Typically this is set up such that the original input signal deflects the dot upward with increasing voltage, and the "new input" deflects the beam further to the right with increasing voltage.

Typically this new set of plates is attached to a "function generator" that produces a periodic increasing voltage. This makes the dot autmatically "sweep" across the screen from left to right, and then start over at the left again. By increasing the frequency of this signal, the dot can be made to most so fast that it blurs into a continuous line. Meanwhile the dot is still moving up and down in response to the "input", tracing out the signal on the display. For instance, if you plug the input into a wall socket and make the display sweep at 60 times a second (50 in Europe), the increasing and decreasing voltage of the AC signal will trace out a sine wave on the screen.

Oscilloscopes typically also include a "trigger" system for measuring events that are not periodic, or where the event time is much shorter than the time between them (lots of little pulses for instance). In this case the input signal is also fed into the function generator, telling it to start the sweeping motion only when the input voltage reaches some value. By selecting a fast sweep and triggering on the input, single events of even very short duration can be displayed.

Most oscilloscopes also allow you to bypass the function generator and supply both signals directly. This is useful in cases where there are two signals that are related, which is commonly used in radio and television. It's also possible to use these signals with external function generators to produce lissous patterns.

Other common features include a grid pattern of known size laid over the top of the tube, calibrated to the input signal. The controls on the input signal's amplifier

allowing for measurements.

In this case the system is only really useful if the inputs have some sort of relationship to each other,

The screen has a graticule of vertical and horizontal lines to help the user to measure times or voltages associated with a signal. The speeds and voltages on the controls usually refer to divisions, which are the time and voltage intervals marked on the graticule. Some oscilloscopes also have cursors, or lines that be moved about the screen to measure the time interval between two points, or the difference between two voltages.

Now, of course the dot runs into the right side of the screen all too quickly. There are tricks to cope with this:

1. "Triggered sweep" means that electronic circuits pick a time to start the sweep. The most common trigger starts the sweep when the voltage goes up or down to some level. Triggering circuits can be very complex. Another popular triggering option on many scopes is to trigger on the start of a video frame.
2. A trigger delay just waits some time after the trigger before starting the sweep. If there's a trigger, there should be trigger delay.
3. Continual sweep, with no triggering.

Another problem is that often, several signals have to happen in some relation to each other. To solve this problem, some oscilloscopes have several dots (the right term is "channels"). This lets the user watch several signals at once. Usually each channel has its own controls to display the channel, but since the dots have to move at the same rate in order to compare the channels, there is only one triggering system and sweep speed.

Another problem is that sometimes the event that the user wants to see may only happen occasionally. To catch these events, some oscilloscopes are "storage scopes" that continuously display the most recent sweep.

Some premium "deep" digital oscilloscopes have a "magnification" mode. Basically, one traps the desired, complex signal. Then one enables the "magnification" or "zoom" feature, and one can move a window to look at details of a complex signal.

Some digital oscilloscopes can sweep at speeds as slow as once per hour. They emulate a strip chart recorder at low speeds. That is, the signal begins to scroll across the screen from right to left. Most fancy oscilloscopes switch from a sweep to a strip-chart mode right around one sweep per ten seconds. This is because otherwise, the scope looks broken: it's collecting data, but one would not see the dot.

How It Works

Cathode-Ray Oscilloscope (CRO)

The earliest and simplest type of oscilloscope ('scope' for short) consisted of a cathode ray tube, a vertical amplifier, a timebase, a horizontal amplifier and a power supply. These are now called 'analogue' scopes to distinguish them from the 'digital' scopes that are mostly sold today (early 21st century).

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 phosphorescent material (the phosphor). Because the instrument is viewed at arm's length, the screen is much smaller than that in a television set, typically about 20cm in diameter.

In the neck of the tube is an electron gun, which is a heated metal plate with a wire mesh (the grid) in front of it. A potential difference of several hundred volts is applied to make the heated plate (the cathode) negatively charged and the grid (or anode) positively charged. The electric field tears electrons from the cathode and propels them like bullets past the anode and towards the screen. Where the electron beam hits the phosphor it causes it to glow, generating a bright spot on the screen.

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 must pass. When the field is zero, the beam is unaffected. When the field is positive, the beam is deflected upwards, and when the field is negative, 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. Electrostatic deflection is cheaper and lighter, but is suitable only for small tubes.

The timebase is an electronic circuit that generates a ramp voltage. This is a voltage that repeatedly changes from one value to another, linearly with respect to time. When it reaches the second value it jumps quickly back to the first value and begins increasing again. The timebase voltage drives the horizontal amplifier. Its effect is to sweep the electron beam smoothly from left to right across the screen, then quickly return the beam to the left in time to begin the next sweep.

The speed of the timebase can be adjusted to match the period of the signal being measured to the width of the screen.

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, of the order of megohms or gigohms, so that it hardly disturbs the input signal. The amplifier drives with vertical deflection plates with a voltage that is proportional to the vertical input. 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.

When all these components work together, the result is a bright trace on the screen that represents a graph of voltage against time. Voltage is on the vertical axis, and time on the horizontal.

Multichannel scopes do not actually have multiple electron beams; they display only one dot at a time. What they do is display one channel and then the other either on alternate sweeps or alternately faster than the sweep. This is controlled by a button labeled ALT/CHOP.

The vertical amplifier and timebase controls are calibrated so that the user knows the vertical distance on the screen that corresponds to a given voltage difference, and the horizontal distance that corresponds to a given time interval.

The power supply is an important component of the scope. It provides low voltages to power the cathode heater in the tube, and the vertical and horizontal amplifiers. High voltages are needed to drive the electrostatic deflection plates. These voltages must be very stable. Any variations will cause errors in the position and brightness of the trace.

Storage Oscilloscope

An extra feature available on some analogue scopes is called 'storage'. This feature allows the trace pattern, that normally decays in a fraction of a second, to be stored for several minutes or longer. An electrical circuit can be activated to store and erase the trace on the screen.

Digital Storage Oscilloscope

The digital storage oscilloscope, or DSO for short, began as an expensive up-market alternative to the analogue scope, but is now the most commonly sold type.

The vertical input, instead of driving the vertical amplifier, is digitised 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.

Tips for use

The most typical problem when approaching an unfamiliar scope is that it just sits there, and doesn't display.

Many newer scopes have a "reset options" button. Use it when you get confused, or when you first approach an unfamiliar scope. Some older scopes have a "beamfinder" button, which is a similar sort of feature.

Make sure that at first you set the options of a channel to "DC" coupling, with continuous sweep, not triggered sweep. Decrease the channel's volts per CM until a line appears. Dial the sweep frequency near the speed of the desired event, and then adjust the sweep and the CM per volt until the event appears at a useful size.

Oscilloscopes almost always have a test output that one can measure to assure that a channel and probe is working. When approaching an unfamiliar oscilloscope, it's wise to measure this signal first.

Sometimes it's not convenient for the oscilloscope to share a ground with an electronic circuit. For these situations, almost all oscilloscopes provide "AC" coupling. When the scope can be attached to a circuit's ground, one can use "DC" coupling. When using "DC" coupling, the ground connection of the oscilloscope MUST be attached to the ground of the circuit under test, or results may be very odd. Most test leads for oscilloscopes have the ground clip built into their end.

Make sure that the triggering circuit and channel use the same coupling (AC or DC), and make sure you are triggering from the correct channel. Set the trigger delay to zero. Dial the trigger to a level at which it has to trigger, and then back it off until the desired event triggers. Last of all, adjust the trigger delay until the desired signal feature appears.

The capacitance of the wire to the test probe can cause an oscilloscope to inaccurately display high speed signals. Capacitance meaasures how much the wire pushes back as more charge is pushed into it. To reduce this capacity for charge, and make measurements more accurate, the wires to most oscilloscope probes are thinner than a human hair. The wire is wrapped in a rugged, thick plastic cable, but the probes are very fragile, and can cost several hundred dollars. Users can be very protective of their scope probes. Respect them.

If the signal looks distorted, that is, if it shows unusual ringing or weird humps, try adjusting the scope probe's capacitance. Many scope probes have a small adjustment screw on the probe.

Selection

Oscilloscopes generally have a checklist of some set of the above features. The basic measure of virtue is the maximum frequency of sweep that an oscilloscope can reliably display. A minimally-useful 'scope will sweep from one second to 10MHz, with triggering and delayed sweep. For work on digital signals, dual channels are also necessary, and a storage scope with a sweep speed of at least 1/5 your system's maximum frequency is recommended.

The chief intangible benefit of a quality oscilloscope is the quality of the trigger circuit. If the trigger is unstable, the display will always be hashed. The quality improves roughly as the frequency response and voltage stability of the trigger increase.

Digital storage scopes (almost the only kind now available) used to display nonexistent signals at high frequencies, but this "aliasing" problem is now much rarer. It's worth asking about in the used market, though.

As of 2002, a 150MHz dual-channel storage scope costs about $1200 new, and is good enough for most things. Oscilloscopes are commercially available with signal bandwidths up to 1THz, but faster scopes become much more expensive.

There is an affordable alternative to an oscilloscope that is useful for many tasks. One can listen to the signals. The basic plan is to mix an intermediate frequency with the signal, and then amplify and listen to the result through a speaker. With modern solid-state circuits, such equipment only costs a few dollars and can run from a small battery. This diagnostic system was widely used for almost all early radio development, and is still used in Asia, and by impoverished amateur radio operators.