Scope this out

Oct. 3, 2012
Knowledge of a few basics helps ensure oscilloscopes display signals accurately

Authored by:
Robert Repas
Associate Editor
Key points:
• Basic scopes consist of five basic functions: the display, the vertical inputs, the horizontal input, the time base, and the trigger.
• Many factors can affect the instrument’s performance and, in turn, the ability to accurately test devices.

Agilent Technologies

It’s said a picture is worth a thousand words. The truth of that statement is most apparent when viewing waveforms displayed on an oscilloscope. From the Latin oscillare, meaning to “to swing”, and the Greek skopion, “to view or examine,” the oscilloscope lets an individual look at swinging or oscillating electrical signals.

In modern oscilloscopes, an LCD or LED screen shows wave shapes captured by digital- sampling techniques. Oscilloscopes come in a variety of types, but most have the same basic features: the display, the vertical inputs, the horizontal input, the time base, and the trigger. In the past, the vertical and horizontal input stages were called amplifiers as that was the function they performed — to amplify the input signal to drive the CRT deflection plates. But that term has fallen into disfavor with the use of digital technology.

With no signal applied to either the horizontal or vertical inputs, the monitor displays a single point of light in the middle of the screen. A positive voltage applied to the vertical input moves the dot upwards on the screen, while a negative voltage moves it down. Likewise, a positive voltage applied to the horizontal input moves the dot to the right, while a negative voltage moves it to the left. The distance the dot moves depends on the amplitude of the voltage.

For the majority of measurements made by an oscilloscope, the signal to be measured goes to the vertical input. The horizontal input receives its signal from the time-base section of the oscilloscope. The time-base signal moves the dot on the screen from left to right at a fixed rate of speed. The vertical position of the dot shows the instantaneous voltage of the vertical input signal at that point in time.

Digital oscilloscopes use slightly different methods to create the same style of display. The value of the input signal is digitized using an analog-to-digital converter and the value logged along with a time reference as to when the signal was digitized. The computer then draws a graph of the logged data on the monitor.

Measuring the horizontal distance traveled by the dot of light determines the time or period of the input signal, while measuring the vertical distance determines its amplitude.

Signal integrity
The main purpose of an oscilloscope is to give an accurate visual representation of electrical signals. For this reason, signal integrity is important. Signal integrity refers to the oscilloscope’s ability to reconstruct the waveform so that it accurately represents the shape of the original signal.

In early analog oscilloscopes signal integrity was not a critical issue. Signals that exceeded the bandwidth response of the scope displayed well-defined characteristics known to come from exceeding the oscilloscopes rating. However, digital oscilloscopes do not display those characteristics. One such problem is associated with the Nyquist Criteria.

Nyquist simply states that to determine the frequency of a waveform, one must sample at a rate at least twice the waveform frequency. To measure a 100-MHz signal, the scope must sample at a minimum rate of 200 million samples every second. To accurately reconstruct a waveform requires many more than two samples. Industry accepted practices lean toward at least 10 samples. Current oscilloscopes have sampling rates up to 5 Gsamples, permitting frequency responses up to 500 MHz. Measurement of signals exceeding that number requires special techniques beyond the scope of this article.

An oscilloscope with low signal integrity is useless. It is pointless to perform a test when the waveform on the oscilloscope does not have the same shape or qualities as the true signal. However, it is important to remember that the waveform on an oscilloscope will never be an exact representation of the true signal, no matter how good the oscilloscope is. When you connect an oscilloscope to a circuit, the oscilloscope becomes part of the circuit creating loading effects that change signal parameters. Instrument makers strive to minimize loading effects, but they always exist to some degree.

Many high-end oscilloscopes now have operating systems that let them behave like computers. The oscilloscope contains connectors for a mouse and keyboard that lets you adjust the controls through drop-down menus and buttons on the display. In addition, some oscilloscopes have touchscreens to permit stylus or fingertip access to the menus.

In a turnabout, several companies now make add-ons for computer systems that turn them into oscilloscopes. The extra hardware performs the digital measurements sending the data to the computer for logging and display through a USB connector.

The probes that connects the oscilloscope to the circuit under test come in many varieties depending on the type of measurement being taken. The most common type of probe is all a 10× probe. It gets its name from the fact that it reduces the amplitude of the signal to the oscilloscope to a tenth of its original value. This lets the probe maker build-in compensating circuitry to reduce any distortion the probe creates. Other probes include unity (1×), 100×, and current probes.

The purpose of the probe is to accurately convey the shape and amplitude of the signal to the oscilloscope for measurement. Unfortunately, no probe is able to perfectly reproduce a signal. The reason is that when connected, the probe becomes part of the circuit, changing its electrical characteristics. This effect is called loading.

There are three types of loading: resistive, capacitive, and inductive. Resistive loading will change the amplitude of the displayed signal. A good rule of thumb is to make sure the resistance of the probe is at least 10× the resistance of the circuit under test.

Capacitive loading reduces displayed rise times and lowers overall bandwidth. Choose a probe with at least 5× the bandwidth of your signal to keep capacitive loading effects low.

Inductive loading produces ringing effects on the signal, making true amplitude measurements difficult. Keep ground leads as short as possible to minimize this effect. Many probes have ground leads as a short extension wire from the probe handle to keep lengths minimum.

Probes are classified as active or passive. Passive probes are typically inexpensive, easy to use, and rugged. However, while their resistive loading is low, they can exhibit high capacitive loading. In addition, most only work at bandwidths up to 600 MHz. An active probe is needed to go beyond that frequency.

Active probes are considerably more expensive than passive probes and tend to be less rugged and heavier. But they provide the best combination of resistance and capacitance loading to test much higher-frequency signals.

The probe connects to the vertical input jack. Most oscilloscopes today contain more than one vertical input, with pairs and quads being the most common. A special type of vertical input may accept only digital signals. It’s not uncommon for an oscilloscope to have both types of inputs. For example, the Agilent MSOX2024A oscilloscope contains four standard input or analog channels and eight digital channels.

Multiple input channels permit comparing one waveform against another to check integrity, symmetry, and phase relationship between the waveforms. Each channel is individually controlled. The three most-common controls for the vertical inputs are the scaling, positioning, and input-selection controls.

The scaling knob sets the range for the level of voltage to be measured on a grid of 1-cm squares that appears on the oscilloscope screen. This grid is called a graticule, with the scaling factor called out as the number of volts per centimeter, V/cm. A scaling factor of 10 V/cm means the dot will move 1 cm for every 10-V change in the input signal.

The position control sets the zero reference of the signal. Unlike many metered test instruments, the zero-voltage point on an oscilloscope screen is an arbitrary setting. A mostly positive signal would have zero set near the bottom of the screen. Conversely, a mostly negative signal would see zero near the top. A signal that goes positive and negative would have its zero in the middle of the screen.

Finally, the input selector determines the type of signal to be displayed. Typically, there are three settings: off, ac, and dc. Off blocks the input, producing a zero reference value. The dc setting lets the entire signal through, while the ac setting only lets the changing signal through. The latter is used when a small ac signal rides atop a high dc value, as seen in many types of amplifiers. For example, a 1-Vac rms signal has a 100-Vdc bias voltage. If the scaling factor is set for the 100-Vdc signal, the 1-Vac signal is imperceptible. By using the ac input, the 100-Vdc signal is blocked, and the scaling factor can now be set for the 1-Vac signal.

Besides controlling the left-to-right position of the screen display, the horizontal input also accepts an input signal, which is scaled in V/cm. XY mode, named after the X (horizontal input) and Y (vertical input) axes of a graph, displays a function of the two signals. It is useful when graphing I-V plots or Lissajous patterns where the shape of the pattern measures the phase difference and frequency ratio between the two signals.

However, most of time the horizontal input gets its signal from another part of the oscilloscope: the time base. The purpose of the time base is to generate a signal that has a consistent rate of voltage change. When applied to the horizontal input, it creates a motion from left to right that can be used to measure a period of time rather than voltage. Scaling in the time base is specified as a unit of time per centimeter such as 0.2 msec/cm or 10 sec/cm. By measuring the width of a cycle displayed on the screen in centimeters, it’s period may be determined. The waveform’s frequency can then be calculated by taking the reciprocal of the period. Today’s digital oscilloscopes perform that calculation internally and can display the frequency of the waveform on the screen.

Once the time base moves the horizontal input from the left to far right, it resets the display back to the left and the timing cycle repeats. However, chances are good that the starting point on the input wave does not match the prior timing cycle. The result is a smear of signals on the screen as the waveforms from different timing cycles overlap. Ideally, the horizontal motion should start at the same point in the waveform for each timing cycle. The display signals would then overlay the prior displayed waves, displaying only one waveform on the screen.

The trigger stage is the part of the oscilloscope that controls when the time base begins its timing cycle. It monitors two basic criteria: whether the signal is rising or falling, and the specific instantaneous voltage that triggers the timebase timing cycle.

Edge triggering is the most widely used triggering mode. First, a selection is made to trigger on a rising or falling edge. Then a specific threshold trigger voltage is set. When the input signal surpasses that threshold, the time base starts its timing cycle.

Glitch triggering looks for an event or pulse whose width exceeds some specified length of time. This capability is useful for finding random glitches or errors. If these glitches happen infrequently, it may be difficult to see them. However, glitch triggering lets you catch many of these errors.

Pulse-width triggering resembles glitch triggering in that it looks for specific pulse widths. In general it can trigger on pulses of any specified width and polarity.

Digital scopes offer a capability not found in earlier models. Older oscilloscopes can only capture what happens from the trigger point on. Now, digital scopes can capture events before the trigger, letting the observer see pretrigger as well as posttrigger signals. The ability may greatly improve the possibility of discovering why an action happens that would otherwise not be visible.

Oscilloscope properties
Many oscilloscope properties dramatically affect the instrument’s performance and, in turn, the ability to accurately test devices. One of the most important of these properties is the scope bandwidth.

The bandwidth of an oscilloscope indicates the range in frequency to which the scope responds. In other words, it dictates the frequency range of signals the scope can accurately display and test.

Bandwidth is measured in Hertz. Without sufficient bandwidth, the oscilloscope will not display an accurate representation of the actual signal. For example, the amplitude of a signal outside the scope bandwidth may be incorrect, its edges may not be clean, and its waveform details may be lost.

The bandwidth of an oscilloscope is the lowest frequency at which an input signal is attenuated by 3 dB. Another way to look at bandwidth: The bandwidth will be the minimum frequency where the displayed amplitude of a pure sinewave input is 70.7% of the actual signal amplitude.

The sample rate of an oscilloscope is the number of samples it can acquire per second. An oscilloscope should have a sample rate that is at a least 2.5× greater than its bandwidth. However, ideally the sample rate should be 3× the bandwidth or greater.

It’s critical to carefully evaluate an oscilloscope’s sample-rate banner specifications. Manufacturers typically specify the maximum sample rate an oscilloscope can attain. Sometimes this maximum rate is possible only when one or two channels are in use. If more channels are used simultaneously, the sample rate may drop. Therefore, it is wise to check the number of channels that are usable while still maintaining the specified maximum sample rate.

If the sample rate of an oscilloscope is too low, the signal on the scope display may not be very accurate. As an example, assume you are trying to view a waveform, but the sample rate only produces two points per period. Obviously, two points would not permit a complete view of the waveform shape.

The greater the number of samples per second, the more clearly and accurately the waveform displays. The sampled points would eventually look almost continuous. In fact, oscilloscopes usually use sin(x)/x interpolation to fill in between the sampled points.

A final specification to watch is the memory depth of the oscilloscope. Memory depth refers to exactly how many samples or points the memory of the scope can store and, thus, the length of time the scope can store.

In an ideal world, sampling rate would remain constant regardless of the scope setting. However, a high time/division setting would need huge amounts of memory to store all of the samples. So as the time per division rises, most oscilloscopes reduce the sampling rate to prevent running out of memory. The greater the memory depth of the scope, the more time you can spend capturing waveforms at full sampling speed.

© 2012 Penton Media, Inc.

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