Oscilloscopes

The oscilloscope is undoubtedly the most versatile electronic instrument in use today. Unlike meters, which only allow the user to measure amplitude information, the oscilloscope (or “scope”) allows the user to view the instantaneous voltage vs. time. This reveals the shape of the waveform, and parameters such as frequency and phase can also be measured.

The Oscilloscope Concept

The primary function of an oscilloscope is to display an exact replica of a voltage waveform as a function of time. This picture of the waveform can be used to determine quantitative information such as the amplitude and frequency of the waveform, as well as qualitative information such as the waveform’s shape. The oscilloscope can also be used to compare two different waveforms and measure their time and phase relationships.

The initial discussion of oscilloscopes will be centered around the typical two-channel analog scope (Figure 4-1). Single- channel scopes are available, but they have largely been eclipsed by the two-channel variety. Four-channel scopes are also quite prevalent in the marketplace and are particularly useful for measuring signals in digital systems. (Digital systems usually have multiple data and address lines which may need to be viewed simultaneously.) Four-channel scopes operate similar to two- channel scopes, with the exception of having twice as many inputs and vertical amplifiers. For general purpose use, analog scopes are still more economical than digital scopes (for equivalent frequency range). As analog-to-digital conversion technology improves, this will undoubtedly change. In concept, analog oscilloscopes and digital oscilloscopes per form the same types of measurements, but with different techniques internal to the instrument. These differences will be addressed later in the section.

Figure 4-1. A typical two-channel 40-MHz analog oscilloscope.

A sine wave would be displayed by an oscilloscope as shown in Figure 4-2. The vertical axis represents voltage and the horizontal axis represents time. This results in the same voltage vs. time plot for a sine wave that was introduced in Section 1. The oscilloscope display has a set of horizontal and vertical lines called the graticule which aid the user in estimating the value of the waveform at any particular point. (The graticule is shown in Figure 4-2, but is omitted in the other figures.) Fundamentally, the concept of an oscilloscope is simple: accurately pro duce the voltage vs. time plot of a waveform. Actually accomplishing this may be more complicated.

Figure 4-2. The oscilloscope display with a sine wave connected to the oscilloscope input.

Time-Base Operation

Figure 4-3. Simplified block diagram of an oscilloscope showing the time-base operation.

A simplified block diagram of an oscilloscope is shown in Figure 4-3. The display of the oscilloscope (usually a cathode ray tube or CRT) requires two pieces of information: the vertical position to be plotted and the horizontal position to be plotted. The vertical axis represents the waveform to be plotted, so the input of the oscilloscope is connected to an amplifier which drives the vertical position of the display. Time is plotted on the horizontal axis. Driving the horizontal position of the display is an internally generated waveform that represents time. The section of circuitry that produces the time waveform is called the time base. We know what the vertical signal looks like—it’s just whatever the input waveform is, perhaps a sine wave or square wave. What we want the time base to do is constantly increase the horizontal position as the input voltage goes through a cycle. This results in the input waveform being swept or painted across the display at some rate, depending on the time base. The time-base waveform required to do this is a constantly increasing voltage called a ramp. Since the display is usually updated repetitively, the ramp starts over when it reaches its maximum value, resulting in a sawtooth waveform (Figure 4-4). Each cycle of the sawtooth waveform corresponds to one sweep across the display. A small delay (called the retrace time) occurs between each ramp of the sawtooth as the horizontal position is reset to the left side of the display.

Figure 4-4. The time-base generator produces a ramp voltage. (A) Horizontal axis signal. (B) Repeating ramp produces a sawtooth waveform.

Triggering

A time base alone is not sufficient to produce a usable display. The time base needs to know when to start a sweep. Without this information, the display will attempt to show the correct waveform, but with a random start time. At best, this results in an unstable waveform wandering across the display. At worst, the display is a jumbled collection of waveforms smeared together filling the entire display area (Figure 4-5A).

A properly triggered waveform, having a predictable and repeatable starting point, will be stationary from sweep to sweep (Figure 4-5B). Consider the sine wave shown in Figure 4-6. If the trigger circuit is set up to trigger at the start of the sine wave, then triggers will occur at the points identified in the figure. Each piece of the sine wave is then displayed, with the trigger point at the leftmost edge of the display. Notice that each trigger point occurs at the same place on each cycle. This results in a stable display, as shown in Figure 4-6B. The figure implies that none of the cycles of the waveform are missed by the oscilloscope. In reality, since the oscilloscope may not be able to prepare for a new sweep instantaneously (due to retrace time), some cycles may be lost. With waveforms that are relatively consistent, this is not a problem.

Figure 4-5. Examples of oscilloscope triggering with a sine wave input. (A) Improperly triggered waveform results in an unstable display. (B) Properly triggered waveform has a stable display.

The trigger point on a waveform is usually defined using both a voltage level and a slope (positive or negative). The trigger level control determines the voltage level at which the trigger will occur. The trigger slope control determines whether the trigger will occur on rising (positive slope) or falling (negative slope) portions of a waveform. Figure 4-7A shows a waveform and some possible trigger times. All of the trigger points shown are at a trigger level of 0.5 volt, but some are for a positive slope and the others are for a negative slope. The resulting scope displays for the two triggering conditions are shown in Figures 4-7B and 4-7C.

Figure 4-6. Explanation of triggering. (A) The sine wave with multiple cycles and multiple trigger points. (B) The resulting properly triggered oscilloscope display.

Figure 4-7. Oscilloscope trigger control Includes level and slope. (A) Example showing both positive slope and negative slope trigger points. (B) The scope display if positive slope triggering is used. (C) The scope display if negative slope triggering is used.

When the waveform being measured is used as the trigger source, the oscilloscope is using internal trigger. Scopes that have more than one channel usually allow the user to select which of the channels is to be used as the trigger source. Internal trigger is used the most, but it's possible to trigger with other waveforms. An external trigger input is provided to allow the user to connect an external signal to the oscilloscope to be used for triggering purposes. This signal usually can't be viewed on the scope display, but some scopes have this capability to help the user in setting up the triggering. The line trigger selection uses the AC power line voltage as the trigger signal (usually 60 Hz). Line trigger is useful for observing waveforms that are either directly related to the power line frequency (including its harmonics) or have power line related voltages superimposed on the original waveform.

The trigger may be AC coupled or DC coupled. DC coupling presents the trigger circuit with a waveform containing both AC and DC voltages, while AC coupling removes any DC that may be present. AC coupling is useful when the desired trigger signal is riding on top of a DC voltage which needs to be ignored. Many oscilloscopes include additional filters that can be switched into the trigger circuit to condition the trigger signal. The low-frequency reject filter will remove low frequencies such as the 60-Hz line frequency (and its harmonics) which may be present in the trigger signal, causing triggering problems. The high- frequency reject filter will similarly remove high- frequency noise which also may cause triggering problems.

The trigger-holdoff control disables the triggering circuit for a period of time after the end of a sweep. This is useful when the waveform has several places in its cycle that are the same as the trigger condition. The oscilloscope would normally trigger on all of them, but with the trigger holdoff, the scope ignores all but the first one. Suppose we want to display the first cycle of the pulsed sine wave shown in Figure 4-8. The trigger could be set to zero volts and positive slope, which will trigger off of the beginning of the sine wave. Unfortunately, this trigger condition exists at the start of every one of the sine wave periods so the oscilloscope will display every cycle. However, if the trigger holdoff is adjusted properly, the subsequent triggers can be ignored and only the first cycle of each sine pulse will be displayed. The trigger holdoff disables the oscilloscope’s trigger circuit for an adjustable length of time.

Triggering is probably the most troublesome part of using an oscilloscope. Most scopes provide a variety of ways to trigger on a signal so that the user can customize the triggering to a particular measurement problem. Of course, this also means that the user must understand and make decisions about what type of triggering to use.

Figure 4-8. Example of trigger holdoff operation.

Sweep Control

In the single-sweep mode, the oscilloscope displays only one sweep and then waits until the user resets the scope for another sweep. This allows the user to capture a particular one-shot, or transient event, rather than continuously updating the display with unwanted information. Although this feature is supplied on almost all oscilloscopes, it's most useful when the scope also provides a method of storing the waveform, at least temporarily, on the screen. Otherwise, the single-sweep mode tends to be a brilliant, but brief, flash across the display.

Automatic vs. Normal Sweep

Most oscilloscopes supply two types of continuous sweeps: automatic and normal. The difference between the two is rather subtle, but important. With normal sweep, the oscilloscope is swept when a valid trigger occurs. This is fine for most AC waveforms, but is inconvenient if the input is a DC voltage. In this case, no trigger will occur since the DC input is constant and will not be passing through whatever trigger level happens to be selected. The result is a blank display.

Automatic sweep operates just like normal sweep except that if no trigger occurs for a period of time (like in the case of a DC input or an incorrectly adjusted trigger level), the scope generates a sweep anyway. This is convenient because it gives the user a look at the waveform, even if it isn’t triggered properly. The user can then determine whether the waveform is on screen and what action is necessary for correct triggering. In the case of a pure DC voltage, triggering is rather meaningless— the user just wants to view the constant voltage. If a trigger starts 0 at a fast rate (typically anything greater than 40 Hz) then the scope triggers just like the normal sweep mode. Automatic sweep should be the default choice since it will give good results on most waveforms. The exception is when the desired trigger condition occurs so infrequently (a very low frequency sine wave, for example) that the automatic mode will begin triggering before the appropriate time. In that case, normal sweep should be used.

Chop and Alternate Modes

Most oscilloscopes have at least two channels, both of which can be displayed at the same time. One way to do this is to have a display that can plot two waveforms simultaneously. Analog oscilloscopes that have true simultaneous dual-trace capability are called dual-beam oscilloscopes. This is because the usual method of displaying two simultaneous channels is to use a CRT display that has two electron beams active at the same time. Another, more cost effective, method is to use a single display but electronically switch between two possible input signals (Figure 4-9). II this is done quickly enough, it will not be noticeable to the user nor will it affect most measurements. This consideration does not apply to digital scopes which are not limited by the type of display, but by the analog-to-digital conversion circuitry.

Figure 4-9. Oscilloscope block diagram for two-channel chop and alternate modes. The electronic switch is used to display both channels (almost) simultaneously.

There are two different methods of controlling the electronic switch. Chop mode switches between the two inputs as fast as possible while the waveforms are being plotted on the display. Figure 4-10 shows how chop mode can be used to simultaneously display two different waveforms. For clarity, channel A is shown as a sine wave and channel B is shown as a square wave. The display trace switches back and forth (chops) between the two waveforms during the sweep, resulting in both of them appearing on the screen. Figure 4-10 exaggerates the chop effect to illustrate the point. The two channels can be switched at such a high rate that the chopping can't be seen. This works well for sweeps that are much slower than the rate at which the two channels chop, so chop mode is best for low-frequency signals. If chop mode is used on high- frequency waveforms (and with very fast sweeps) the chopping effect is noticeable on the display (as illustrated in Figure 4-10).

Figure 4-10. In chop mode, the display switches between the two channels as fast as possible, to display both waveforms simultaneously without any noticeable chopping.

On the other hand, alternate mode takes a complete sweep of one waveform, then switches to the other input and takes another sweep. As shown in Figure 4-11, the first sweep displays channel A, the second sweep displays channel B, the third sweep displays channel A again, and so on. This works well during fast sweeps, because if the update rate to the display is fast, the user perceives that both inputs are being plotted simultaneously. Therefore, alternate mode works best with high- frequency signals. If alternate mode is used on low-frequency waveforms (and with slow sweeps), the individual sweeps of each channel become apparent and the effect of simultaneously displayed channels is lost. One potentially misleading problem with alternate mode is that even though the two waveforms appear to be displayed simultaneously, they really were measured at two distinct points in time. For most measurements, this is not a problem since the waveform repeats the same cycle every time. There are cases, however, where the user needs to measure both channels simultaneously. Oscilloscopes that don't have true dual-trace capability will almost always supply both chop and alternating modes (for maximum flexibility).

Figure 4-11. In alternate mode, a complete sweep from the first channel is plotted and then a complete sweep from the second channel is plotted.

Vertical Amplifier

The gain of the vertical amplifier will determine how big the waveform appears on the display. In an analog scope, the vertical sensitivity will determine the gain of the vertical amplifier and will be calibrated in volts/division; the user can determine the amplitude of the signal by counting the number of vertical divisions on the display. For example, the waveform in Figure 4-2 shows a sine wave that's 4 vertical divisions 1 to peak. If the vertical sensitivity control was set to 0.2.volt / division then the sine wave would be 4 x 0.2 = 0.8 volt peak to peak.

Oscilloscopes have a vertical amplifier for each of their input channels. Two-channel scopes are quite common and four-channel scopes are no longer unusual. The two channels may be labeled in a variety of ways—channels A and B, channels 1 and 2, etc.—but here they will be called A and B. In the usual time-base mode of the oscilloscope both A and B are displayed as a function of time. As stated previously, either one may be the trigger source and they may be displayed using the chop or alternate feature on some scopes.

In addition to displaying channel A and /or channel B, many scopes provide the capability of displaying A + B or A - B. Also, one or both of the channels may be capable of being displayed inverted (with its polarity reversed). (A - B might not be provided, but A + B with B inverted can achieve the same result.)

AC and DC Coupling

Each input can be selectively AC or DC coupled. DC coupling allows both DC and AC signals through, while AC coupling accepts only AC signals. Figure 4-12A shows a waveform containing both AC and DC. If the oscilloscope is DC coupled then the waveform is displayed as drawn in Figure 4-12A. if the oscilloscope is AC coupled, then the DC portion of the waveform is blocked and only the AC portion is displayed, as shown in Figure 4-12B.

The previous example seems very straightforward, but the issue of AC coupling may show up in other unexpected ways. Consider the pulse waveform in Figure 4-13A, shown as a DC coupled scope would display it. This waveform appears to be a typical AC waveform so one might think that it would be unaffected by coupling. However, when the scope is AC coupled, the display does change. The waveform shifts down by about one third of its original zero-to-peak value (Figure 4-13B). The original waveform did have some DC present in it (remember DC is just the average value of the waveform). The AC coupling removed the DC, leaving a waveform whose average value is zero. Notice that the wave form is not centered exactly around zero volts, since its duty cycle is 1/3. AC coupling may also cause voltage “droop” or “sag” in the waveform (Figure 4-13C), due to the loss of low frequencies.

Most oscilloscopes have a convenient means of grounding the input (usually a switch near the connector). This is symptomatic of one of the most confusing things in using a scope—where is zero volts on the display? The ground switch allows the user to quickly ground the input and observe the flat trace on the display which is now at zero volts. The line may then be set anywhere on the display that's convenient, using the display’s position controls. Knowing where zero is defined along with the volts/division selection determines the scale on the display. Fortunately, the more recent digital scopes have eliminated the confusion by always displaying the data in a known calibrated manner.

Many scopes provide a bandwidth-limit control which activates a fixed frequency low-pass filter in the vertical amplifier. This has the effect of limiting the bandwidth of the scope (typically to about 20 MHz). Since bandwidth is such a desirable thing it may seem backwards to intentionally limit it. Figure 4-1 4A shows an oscilloscope display of a sine wave with a noticeable amount of high-frequency noise riding on it. When the bandwidth-limit control is switched on (Figure 4-14B) the high-frequency noise is eliminated, but the original sine wave remains uncorrupted. Of course, this works only when the interfering noise is mostly outside the bandwidth of the filter and the desired signal is inside the filter bandwidth.

Figure 4-12. The effect of DC and AC coupling. (A) DC coupling causes the entire waveform to be displayed, including the DC portion. (B) AC coupling removes the DC portion of the signal.

X-Y Display Mode

Most two-channel oscilloscopes have the ability to plot the voltage of one channel on the vertical (Y) axis, and the voltage of the other channel on the horizontal (X) axis (Figure 4-15). This results in a voltage vs. voltage display, usually called A vs. B, Y vs. X, or simply X-Y. The time-base and triggering circuits are not used when operating in this manner.

Figure 4-13. The effect of AC coupling on a pulse train. (A) The original waveform displayed with DC coupling. (B) The pulse train with DC component removed due to AC coupling. (C) AC coupling may cause voltage droop due to the loss of low frequencies.

Figure 4-14. The effect of using the bandwidth-limit control. (A) A noisy sine wave. (B) The same sine wave with the noise reduced by limiting the bandwidth.

This feature greatly enhances the usefulness of the oscilloscope. The horizontal axis is no longer limited to only time. Any other parameter that can be represented as a voltage can now be used as the X axis. More precisely, both the vertical and horizontal axes can be used to display any two parameters represented by voltages. For instance, if a current-sensing resistor were used to convert a current into a voltage, a current could be plotted on the vertical axis while another voltage is plotted along the horizontal axis. This, and other X-Y applications, are discussed in Section 5.

Z-Axis Input

The Z-axis input (also known as the intensity modulation input) provides a means for controlling the intensity of the display while in X-Y operation. The name “Z axis” comes from the fact that in addition to the X axis and Y axis information, the intensity of the display can be varied to provide an additional “axis” of information—the Z axis. If a positive voltage (typically several volts) is applied to the Z-axis input, the trace is blanked (no intensity). If a negative voltage is applied to the Z-axis input, then the trace has full intensity. Voltages in between the two extremes produce less than full intensity (but not blanked). The actual voltages and even the polarity of the Z-axis input vary, depending on the model of instrument. Given the proper Z-axis control signals, different sections of the trace can have different intensities. This is useful for jgh1ight a particular point on a display or for turning the trace off to start the plot over at a particular point (without having a trace drawn to that point).

Figure 4-15. A simplified block diagram showing the X-Y display capability of the oscilloscope. The Z-axis input can be used to control the intensity of the plot at any given point.

Oscilloscope Inputs

High Inputs

The typical oscilloscope has a high-impedance input so that the circuit under test is not loaded significantly. The input can be modeled by a 1-megohm resistor in parallel with a capacitance (Figure 4-16). The value of the capacitance depends on the particular model of oscilloscope, but is generally in the range of 10 to 30 pF. The magnitude of a typical input impedance is plotted in Figure 4-17. At low frequencies the capacitance acts like an open circuit and the impedance consists only of the 1- megohm resistor. At about 8 kHz, the capacitor’s impedance becomes significant as it just equals the 1-megohrn resistor impedance. The impedance of the parallel combination continues to gradually decrease for higher frequencies. Although the input impedance is very high at low frequencies, it will tend to load the circuit being measured as the frequency increases. Remember, the 1-megohm input is not 1 megohm at high frequencies.

Figure 4-16. Circuit model for the input of an oscilloscope.

Figure 4-17. The magnitude of the input impedance for the high-impedance and 50-ohm scope inputs.

50-Ohm Inputs

Figure 4-18. The circuit model for the 50-ohm scope input. The capacitance is often neglected in a 50-ohm system.

Some oscilloscopes offer a second type of input having a 50-ohm input impedance (Figure 4-18). It is often the same connector as the high- impedance input, with a switch selecting between the two, and is often implemented by placing a 50-ohm resistor in parallel with the 1-megohm input. Since I megohm is much larger than 50 ohms, the effective parallel impedance is approximately 50 ohms. If a scope does not have the 50-ohm input built in, an appropriate load can be placed in parallel with the high-impedance input to produce the same result. The input impedance is modeled as a single 50-ohm resistor, with the input capacitance in parallel. In a 50-ohm system, the capacitive effect is less critical, resulting in a wider bandwidth system. Figure 4-17 shows that even though the 50-ohm input impedance starts out much smaller than the high-impedance input, it remains constant out to a higher frequency. The major disadvantage of the 50-ohm input is that it's too low of a load resistance for many circuits. (For these cases, very low capacitance active probes which are designed to drive the 50-ohm input are used to provide minimal circuit loading and greater overall bandwidth.) Of course, the 50-ohm input is especially convenient for systems that have an inherent 50-ohm impedance.

Grounding

Most oscilloscopes have inputs with one side directly connected to ground. This is the most practical and economical way to build the instrument. This is usually not a problem, since most voltage measurements are made with respect to ground. For some measurement situations, however, it's desirable to connect the scope input between arbitrary points in the circuit, including ones that are not grounded. Some scopes have floating or differential inputs that allow both leads of the input to be connected away from ground. In that case, grounding is not a problem.

A two-channel scope with the ability to display A — B (the difference between the two channels) can be used as a one channel floating input scope. The oscilloscope is set up to display A — B. The A channel is connected to the point in the circuit taken to be the positive voltage. Channel B is connected to the other voltage point and the oscilloscope ground is connected to the circuit ground. Thus, the scope displays the difference between the two voltage points with neither one required to be at ground.

Oscilloscope Specifications

A set of typical scope specifications is shown in Chart 4-1. Bandwidth and rise time have already been covered in the general discussion in Section 1. The deflection factor tells what vertical volts per division settings are available. In this example, settings between 1 mV and 5 volts per division are available in a 1-2-5 sequence (1 mV, 2 mV, 5 mV, 10 mV, etc.). Included with the deflection factor is a percent error (± 3%) which defines the fundamental accuracy of the instrument. Similarly, the time per division settings on the horizontal axis are from 0.1 p to 0.5 sec per division. Again, an error specification which defines the accuracy of the time scale is included (± 3%).

Chart 4-1. Specifications of a Typical Oscilloscope

Oscilloscope Block Diagram

The block diagrams shown previously highlighted the configuration for a particular type of oscilloscope operation. A block diagram incorporating all of the features previously discussed is shown in Figure 4-19. The various switches allow the scope to be configured such that the wide variety of measurement functions can be performed. Both input channels can be configured for DC or AC coupling, the electronic switch allows both alternate and chop modes for two-channel operation and the display can be setup for either time-base or X—Y operation.

Figure 4-19. Block diagram of a typical two-channel oscilloscope.

Scopesmanship

An oscilloscope is a fairly complex instrument, especially when com pared with a voltmeter, for example. The large number of switches and knobs on the front panel can be intimidating to the novice user. A few comments are probably in order to help the first time user get started.

After carefully reading the manufacturer’s operating manual, the best way to get started with a measurement is to put the oscilloscope into a known state which will at least get something on the display. Some of the digital scopes have included a key called “Autoscale” which automatically evaluates the waveform, chooses an appropriate trigger condition and selects reasonable horizontal and vertical scales. Pushing this one button can prevent a lot of user frustration. Assuming that this feature is not available, a suggested starting point for an oscilloscope measurement is listed in Chart 4-2. This can only be a start, as each measurement is somewhat different.

Chart 4-2. Suggested Starting Point for Oscilloscope Measurements

Vertical Amplifier

Input coupling = DC

Volts/div = 1 volt or (expected Vo-p) / 4

Time Base

Time-base operation

Time/div = 1 msec or 1 /(4 X expected frequency)

Auto Sweep

Trigger

Internal Trigger

Trigger level = 0 volts

Trigger slope = positive

Trigger coupling = DC

Hopefully, a trace will appear on the display after the oscilloscope is set up and the scope settings can be optimized for the particular measurement. If there is no trace at all, then things like the power switch (yes, the power switch) and intensity control should be checked. Perhaps the waveform is just off screen because it's much larger than the volts/ division setting will allow on screen. Try grounding the input—a horizontal line corresponding to zero volts should appear. Some scopes have a beam finder button which, when pushed, gives the user some idea where an off screen trace is hiding.

If the trace is on screen, but is not stable, then the triggering controls should be adjusted. Try tweaking the trigger level to make the waveform stable. The slope and trigger coupling may also be helpful. If the display is stable but is scaled improperly, adjust the time/division and /or the volts/division knobs.

Probably the best advice for operating an oscilloscope is carefully try something. The two approaches that don't work are: (1) just sitting in front of the instrument, staring at it; and (2) twisting every knob until all of the controls are guaranteed to be in the wrong position. Instead, the user should make an educated guess as to what control might fix the problem and try it. If it does not improve the situation, the control should probably be returned to the original setting. Try not to get the oscilloscope so fouled up that only a seasoned technician can straighten it out if in doubt, revert back to the suggested starting point.

Digital Oscilloscopes

Conceptually, analog and digital oscilloscopes do the same thing—they display voltage waveforms. The analog scope uses traditional circuit techniques to display the voltage waveform on a CRT. A digital scope, in contrast, converts the original analog signal to a series of binary numbers which can then be displayed or stored in memory. This means that a digital scope is inherently a storage scope, because the waveform is stored in digital form. Contrast this with the analog scope, where the waveform is a short lived voltage waveform. (When the input disappears, the displayed waveform disappears.) There are techniques for storing waveforms on an analog display, but they generally are expensive, temperamental and have a finite storage time. As mentioned earlier, the ability to store waveforms is especially important when capturing one time events. Without waveform storage, the waveform is plotted across the display and then abruptly disappears. With storage, the waveform remains on the screen so that the user can carefully analyze the data. Typical digital oscilloscopes are shown in Figure 4-20 and Figure 4-21.

Figure 4-20. This two-channel digital oscilloscope also operates in a traditional analog mode for high frequency measurements.

Figure 4.21. High performance analog-to-digital converter technology is used to digitize and display analog waveforms with this digital scope. Attenuating probes are connected to the two inputs as well as the external trigger.

Another advantage of digital scopes is the ability to display the wave form that occurred before the trigger. This may seem slightly impossible at first glance, but a digital scope can accomplish this by constantly storing the input signal into memory waiting for the trigger to occur. When the trigger occurs, the portion of the waveform that occurred just before the trigger is already stored.

Like other digital instruments, the digital scope is well suited for applications that require transferring waveform data to a computer. Since it stores the waveform internally as digital numbers, the data is already in a form compatible with computers. Also, since the data is in digital form and most digital scopes are microprocessor controlled, the scope can simplify many measurements. For instance, the microprocessor can automatically compute and display the rise time, fall time, frequency, etc. of a waveform, rather than having the user manually do it. A good digital scope can even find the RMS voltage of the waveform by performing a root-mean-square calculation on the data.

The performance of a digital scope is generally limited by the rate at which the analog signal can be converted into digits. The analog wave form must be sampled often enough so that when the digital version of the waveform is reconstructed on the display, it gives a good representation of the actual waveform. The effects of sampling are discussed further in Section 8.

Oscilloscope Probes

The high-impedance input of the oscilloscope can be connected directly to the circuit under test using a simple cable. It is highly recommended that any cable used be shielded in order to minimize noise pickup. However, for maximum performance an oscilloscope probe matched to the input of the scope should be used.

1 x Probes

1 x probes, also known as 1:1 (one-to-one) probes, simply connect the high-impedance input of the oscilloscope to the circuit being measured. They are designed for low capacitance, minimum loss, and easy connection but otherwise they are equivalent to using a cable to connect the scope. Figure 4-22 shows the circuit diagram for a high-impedance scope input connected to a circuit under test. The circuit under test is modeled as a voltage source with a series resistor.

The impedance of the circuit and the input impedance of the oscilloscope together produce a low-pass filter. For very low frequencies, the capacitor acts as an open circuit and has little or no effect on the measurement. For high frequencies, the capacitor’s impedance becomes significant and loads down the voltage seen by the oscilloscope. Figure 4-23 shows this effect in the frequency domain. If the input is a sine wave, the amplitude will tend to decrease (due to the loading effect) and the phase will be shifted. As shown in Section 1, the bandwidth and rise time of a system are inversely related. Since the bandwidth of the instrument is effectively being decreased, the rise and fall times of pulse inputs will be increased.

10 x Probes

10 x probes (also called 10:1 probes, divider probes or attenuating probes) have a resistor and capacitor (in parallel) inserted into the probe. Figure 4-24 shows the circuit for the 10 X probe connected to a high- impedance input of an oscilloscope. If R1C1 = R2C2 this circuit has the amazing result that the effect of both capacitors is cancelled. In practice, this condition may not be met exactly but can be approximated. Capacitor C is usually made adjustable and can be tweaked for a near perfect match. Under these conditions, the relationship of Vs to Vin is:

Vin = Vs R2 (R1 + R2)

which should be reminiscent of the voltage divider equation. R is the input resistance of the scope’s high-input impedance (1 megohm) and R1 = 9 R2. From the previous equation, this results in

Vin = (1/10) Vs.

Figure 4-22. The high-impedance input is connected to a circuit using a 1x probe.

So the net result is a probe and scope input combination that has a much wider bandwidth than the 1 x probe, due to the effective cancellation of the two capacitors. The penalty that's incurred is the loss of voltage. The oscilloscope now sees only one tenth of the original voltage (hence the name 10X probe). Also notice that the circuit being measured sees a load impedance of R1 + R2 = 10 megohm, which is much higher than with the 1 X probe. Some probes are designed to be conveniently switched between 1 X and 10 x operation.

Figure 4-23. In the frequency domain, the response of the 1 x probe rolls off at the higher frequencies.

Figure 4-24. Circuit diagram showing 10 x probe used with the oscilloscope high-impedance input.

This factor of ten loss in voltage is not a problem as long as the voltage that's being measured is not so small that dividing it by ten makes it unreadable by the scope. This means that the scope’s sensitivity may be a factor in deciding whether to use a 10 x probe. On most oscilloscopes, the user must remember that a 10x probe is being used and must multiply the resulting measurements by a factor of ten. This is sort of a nuisance and fortunately some scopes include two scale markings: one valid for a 1x probe and the other valid for a 10x probe. Some of the new digital scopes have gone one step further and automatically adjust the readings by the correct amount when an attenuating probe is used.

Probe Compensation

In order to maximize the bandwidth of the 10x probe it's necessary to precisely adjust the probe capacitor to cancel the input capacitance of the scope. This is accomplished by a procedure known as compensation.

Figure 4-25. Examples of 10x probe compensation. (A) Undercompensated. (B) Overcompensated. (C) Properly compensated.

The scope probe is connected to a square wave source called the calibrator, which is built into the scope. The probe is then adjusted to make the square wave as square and flat-topped as possible. Figures 4-25A and 4-25B show the oscilloscope display during compensation with an overcompensated and undercompensated probe. Figure 4-25C shows the display when the probe is properly compensated.

As discussed in Section 1, the square wave is a wide bandwidth signal rich in harmonics. If the probe is adjusted so that it measures a square wave with a minimum of waveform distortion, the probe will be correctly compensated for wide bandwidth signals in general. This concept is also used for square wave testing of amplifiers, as discussed in Section 5.

Other Attenuating Probes

Other types of attenuating or divider probes are available, including 50:1 and 100:1 probes. The general principles of these probes are the same as the 10X divider probe: voltage level and bandwidth are traded off. To obtain wider bandwidth, more loss is incurred in the probe and less voltage is supplied to the input of the scope. This may require a more sensitive scope for low-level measurements. Some divider probes use the scope’s 50-ohm input instead of the 1-megohm input.

Active Probes

So far, all of the probes discussed have been simple passive circuits with no active components such as transistors or integrated circuits. In cases where extremely low capacitance is required for high-frequency measurements, an active probe may be used. An active probe has a small amplifier built into it that's designed to have very little capacitance at its input. The output of the amplifier is usually matched to drive the 50- ohm input of the oscilloscope. This allows a length of 50-ohm cable to be used between probe and scope without any additional capacitive loading effects.

Table 4-1 summarizes the typical specifications of the various types of scope probes that have been discussed. Actual characteristics will vary according to manufacturer and model.

Table 4-1. Typical Specifications of Oscilloscope Probes

Current Probes

Oscilloscopes are designed for a voltage input, but can be used to mea sure current using a current probe. A current probe has a set of jaws that encircle the wire carrying the current to be measured. No electrical connection is needed. The circuit does not have to be broken or altered in anyway, as the current probe measures whatever current is passing through its closed jaws.

Current probes generally use one of two technologies. The simplest uses the principle of a transformer, with one winding of the transformer being the current-carrying wire. Since transformers work only with AC voltages and currents, current probes of this type don't measure direct current.

The other type of current probe works on the hall-effect principle. This technique requires the use of an external power supply, but does measure both alternating and direct current (AC and DC).

Since current probes measure the current through their jaws, several techniques can be used that are unique to the current probe. If the sensitivity of the probe and oscilloscope combination is too low for a particular measurement to be made, several turns of the current-carrying wire can be inserted into the jaws. The probe will effectively have a larger current to measure (the original current times the number of turns). In a similar manner, the difference between two currents can be measured if the two wires in question are inserted, but with the currents flowing in opposite direction (the sum will be measured if the currents are flowing in the same direction). Of course, the physical size of the wires and the current probe will be a factor in determining how many wires can be inserted. Although the current probe does not require a direct electrical connection, it still removes energy from the circuit under test. Normally, this small amount of energy will not disturb the circuit, but can be a factor in some cases.

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