Oscilloscope

In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. One of the controls, the timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division. If the input voltage departs from zero, the trace is deflected either upwards or downwards. Another control, the vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time (the present plotted at a varying position, the most recent past to the left, the less recent past to the right).

If the input signal is periodic, then a nearly stable trace can be obtained just by setting the timebase to match the frequency of the input signal. For example, if the input signal is a 50 Hz sine wave, then its period is 20 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 20 ms. This mode is called continual sweep. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.

To provide a more stable trace, an oscilloscope has a function called the trigger. This causes the scope to pause after reaching the right hand side of the screen, and wait for a specified event before returning to the left hand side of the screen and drawing the next trace.

Oscilloscope

The effect is to resynchronise the timebase to the input signal, preventing horizontal drift of the trace. Trigger circuits allow the display of nonperiodic signals such as single pulses, as well as periodic signals such as sine waves and square waves.

Types of trigger include:

• external trigger, a pulse from an external source connected to a dedicated input on the scope.
• edge trigger, an edge-detector that generates a pulse when the input signal crosses a specified threshold voltage in a specified direction.
• video trigger, a circuit that extracts synchronising pulses from video formats such as PAL and NTSC and triggers the timebase on every line, a specified line, every field, or every frame.
• delayed trigger, which waits a specified time after an edge trigger before starting the sweep. No trigger circuit acts instantaneously, so there is always a certain delay, but a trigger delay circuit extends this delay to a known and adjustable interval.

Some oscilloscopes have cursors, which are lines that can be moved about the screen to measure the time interval between two points, or the difference between two voltages.

Most oscilloscopes have two or more input channels, allowing them to display more than one input signal on the screen. Usually the oscilloscope has a separate set of vertical controls for each channel, but only one triggering system and timebase.

A dual-timebase oscilloscope has two triggering systems so that two signals can be viewed on different time axes. This is also known as a "magnification" mode. The user traps the desired, complex signal using a suitable trigger setting. Then he enables the "magnification", "zoom" or "dual timebase" feature, and can move a window to look at details of the complex signal.

Sometimes the event that the user wants to see may only happen occasionally. To catch these events, some oscilloscopes are "storage scopes" that preserve the most recent sweep on the screen.

Some digital oscilloscopes can sweep at speeds as slow as once per hour, emulating a strip chart recorder. That is, the signal scrolls 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 the dot cannot be seen.

Many newer scopes have a "reset options" or "auto set up" button. Use it when you get confused, or when you first approach an unfamiliar scope. Some scopes have a "beamfinder" button. It limits the size of the scan so the trace will appear on the screen.

Make sure that at first you set the options of a channel to "DC" coupling, with automatic triggering. Decrease the channel's volts per division until a line appears. Set the sweep time per division near the speed of the desired event, and then adjust the volts per division 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 are working. When approaching an unfamiliar oscilloscope, it's wise to measure this signal first.

In general, the ground connection of the oscilloscope should 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.

"AC" coupling blocks any DC in the signal. This is useful when measuring a small signal riding on a DC offset.

"DC" coupling must be used when measuring a DC voltage.

Make sure you are triggering from the correct channel. Set the trigger delay to zero. Adjust the trigger level until the desired event triggers. Last of all, adjust the trigger delay until the desired signal feature appears.

The capacitance of the wire in the test probe can cause an oscilloscope to inaccurately display high speed signals. If the signal looks distorted, that is, if it shows unusual spikes ("ringing") or weird humps, try adjusting the scope probe's capacitance. Many scope probes have a small adjustment screw on the probe. Most oscilloscopes provide a test output that produces a square wave for adjusting the probe. Adjust the probe so that the corners of the square wave appear square.

Scope probes are both expensive and fragile. To reduce capacitance, the conductor in a scope probe's wire is sometimes narrower than a human hair. The plastic "pen" part of the probe is often easy to break. Never leave a probe on the floor where one can walk on it. If you must share a scope, consider having and protecting your own set of probes.

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

Digital storage scopes (almost the only kind now available at the higher end of the market) used to display misleading signals at low sample rates, but this "aliasing" problem is now much rarer due to increased memory length. It's worth asking about in the used market, though.

As of 2002, a 150 MHz dual-channel storage scope costs about $1200 new, and is good enough for general use. Oscilloscopes are commercially available with signal bandwidths up to 70 GHz, but faster scopes become much more expensive.

Cathode-Ray Oscilloscope (CRO)

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

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. 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 uses electrostatic deflection.

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 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 at constant speed from left to right across the screen, then quickly return the beam to the left in time to begin the next sweep. The timebase can be adjusted to match the sweep time to the period of the signal.

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 draws only a tiny current from the signal source. 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, so that the vertical deflection of the dot shows the value of the input. The response of this system is much faster than that of mechanical measuring devices such as the multimeter, where the inertia of the pointer slows down its response to the input.

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. Instead, they display only one dot at a time, but switch the dot between one channel and the other either on alternate sweeps (ALT mode) or many times per sweep (CHOP mode).

The vertical amplifier and timebase controls are calibrated to show 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.

Later analogue 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 analogue waveforms. The extra features that this system provides include:

• on-screen display of amplifier and timebase settings;
• voltage cursors - adjustable horizontal lines with voltage display;
• time cursors - adjustable vertical lines with time display;
• on-screen menus for trigger settings and other functions.

Analogue Storage Oscilloscope

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. DSOs with colour LCD displays are common. The data set can be sent over a LAN or a WAN for processing or archiving. The scope'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.

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