ABCs of Probes Primer

Precision Measurements Start at the Probe Tip

Probes are vital to oscilloscope measurements. To understand how vital, disconnect the probes from an oscilloscope and try to make a measurement. It can’t be done. There has to be some kind of electrical connection, a probe of some sort between the signal to be measured and the oscilloscope’s input channel.

In addition to being vital to oscilloscope measurements, probes are also critical to measurement quality. Connecting a probe to a circuit can affect the operation of the circuit, and an oscilloscope can only display and measure the signal that the probe delivers to the oscilloscope input.

Thus, it is imperative that the probe have minimum impact on the probed circuit and that it maintain adequate signal fidelity for the desired measurements.

If the probe doesn’t maintain signal fidelity, if it changes the signal in any way or changes the way a circuit operates, the oscilloscope sees a distorted version of the actual signal. The result can be wrong or misleading measurements.

In essence, the probe is the first link in the oscilloscope measurement chain. And the strength of this measurement chain relies as much on the probe as the oscilloscope. Weaken that first link with an inadequate probe or poor probing methods, and the entire chain is weakened.

In this and following sections, you’ll learn what contributes to the strengths and weaknesses of probes and how to select the right probe for your application. You’ll also learn some important tips for using probes properly.

What Is a Probe?

As a first step, let’s establish what an oscilloscope probe is.

Basically, a probe makes a physical and electrical connection between a test point or signal source and an oscilloscope. Depending on your measurement needs, this connection can be made with something as simple as a length of wire or with something as sophisticated as an active differential probe.

At this point, it’s enough to say that an oscilloscope probe is some sort of device or network that connects the signal source to the input of the oscilloscope. This is illustrated in Figure 1.1, where the probe is indicated as an undefined box in the measurement diagram.

Whatever the probe is in reality, it must provide a connection of adequate convenience and quality between the signal source and the oscilloscope input (Figure 1.2). The adequacy of connection has three key defining issues – physical attachment, impact on circuit operation, and signal transmission.

To make an oscilloscope measurement, you must first be able to physically get the probe to the test point. To make this possible, most probes have at least a meter or two of cable associated with them, as indicated in Figure 1.2. This probe cable allows the oscilloscope to be left in a stationary position on a cart or bench top while the probe is moved from test point to test point in the circuit being tested. There is a tradeoff for this convenience, though. The probe cable reduces the probe’s bandwidth; the longer the cable, the greater the reduction.

In addition to the length of cable, most probes also have a probe head, or handle, with a probe tip. The probe head allows you to hold the probe while you maneuver the tip to make contact with the test point. Often, this probe tip is in the form of a spring-loaded hook that allows you to actually attach the probe to the test point.

Physically attaching the probe to the test point also establishes an electrical connection between the probe tip and the oscilloscope input. For useable measurement results, attaching the probe to a circuit must have minimum affect on the way the circuit operates, and the signal at the probe tip must be transmitted with adequate fidelity through the probe head and cable to the oscilloscope’s input.

These three issues – physical attachment, minimum impact on circuit operation, and adequate signal fidelity – encompass most of what goes into proper selection of a probe. Because probing effects and signal fidelity are the more complex topics, much of this primer is devoted to those issues. However, the issue of physical connection should never be ignored. Difficulty in connecting a probe to a test point often leads to probing practices that reduce fidelity.

The Ideal Probe

In an ideal world, the ideal probe would offer the following key attributes:

• Connection ease and convenience
• Absolute signal fidelity
• Zero signal source loading
• Complete noise immunity

Connection Ease and Convenience

Making a physical connection to the test point has already been mentioned as one of the key requirements of probing. With the ideal probe, you should also be able to make the physical connection with both ease and convenience.

For miniaturized circuitry, such as high-density surface mount technology (SMT), connection ease and convenience are promoted through subminiature probe heads and various probe-tip adapters designed for SMT devices.

Such a probing system is shown in Figure 1.3a. These probes, however, are too small for practical use in applications such as industrial power circuitry where high voltages and larger gauge wires are common. For power applications, physically larger probes with greater margins of safety are required. Figures 1.3b and 1.3c show examples of such probes, where Figure 1.3b is a high voltage probe and Figure 1.3c is a clamp-on current probe

From these few examples of physical connection, it’s clear that there’s no single ideal probe size or configuration for all applications. Because of this, various probe sizes and configurations have been designed to meet the physical connection requirements of various applications.

Absolute Signal Fidelity

The ideal probe should transmit any signal from probe tip to oscilloscope input with absolute signal fidelity. In other words, the signal as it occurs at the probe tip should be faithfully duplicated at the oscilloscope input.

For absolute fidelity, the probe circuitry from tip to oscilloscope input must have zero attenuation, infinite bandwidth, and linear phase across all frequencies. Not only are these ideal requirements impossible to achieve in reality, but they are impractical. For example, there’s no need for an infinite bandwidth probe, or oscilloscope for that matter, when you’re dealing with audio frequency signals. Nor is there a need for infinite bandwidth when 500 MHz will do for covering most high-speed digital, TV, and other typical oscilloscope applications.

Still, within a given bandwidth of operation, absolute signal fidelity is an ideal to be sought after.

Zero Signal Source Loading

The circuitry behind a test point can be thought of as or modeled as a signal source. Any external device, such as a probe, that’s attached to the test point can appear as an additional load on the signal source behind the test point.

The external device acts as a load when it draws signal current from the circuit (the signal source). This loading, or signal current draw, changes the operation of the circuitry behind the test point, and thus changes the signal seen at the test point.

An ideal probe causes zero signal source loading. In other words, it doesn’t draw any signal current from the signal source. This means that, for zero current draw, the probe must have infinite impedance, essentially presenting an open circuit to the test point.

In practice, a probe with zero signal source loading cannot be achieved. This is because a probe must draw some small amount of signal current in order to develop a signal voltage at the oscilloscope input. Consequently, some signal source loading is to be expected when using a probe. The goal, however, should always be to minimize the amount of loading through appropriate probe selection.

Complete Noise Immunity

Fluorescent lights and fan motors are just two of the many electrical noise sources in our environment. These sources can induce their noise onto nearby electrical cables and circuitry, causing the noise to be added to signals. Because of susceptibility to induced noise, a simple piece of wire is a less than ideal choice for an oscilloscope probe.

The ideal oscilloscope probe is completely immune to all noise sources. As a result, the signal delivered to the oscilloscope has no more noise on it than what appeared on the signal at the test point.

In practice, use of shielding allows probes to achieve a high level of noise immunity for most common signal levels. Noise, however, can still be a problem for certain low-level signals. In particular, common mode noise can present a problem for differential measurements, as will be discussed later

Realities of Probes

The preceding discussion of The Ideal Probe mentioned several realities that keep practical probes from reaching the ideal. To understand how this can affect your oscilloscope measurements, we need to explore the realities of probes further.

First, it’s important to realize that a probe, even if it’s just a simple piece of wire, is potentially a very complex circuit.

For DC signals (0 Hz frequency), a probe appears as a simple conductor pair with some series resistance and a terminating resistance (Figure 1.4a). However, for AC signals, the picture changes dramatically as signal frequencies increase (Figure 1.4b).

The picture changes for AC signals because any piece of wire has distributed inductance (L), and any wire pair has distributed capacitance (C). The distributed inductance reacts to AC signals by increasingly impeding AC current flow as signal frequency increases. The distributed capacitance reacts to AC signals with decreasing impedance to AC current flow as signal frequency increases. The interaction of these reactive elements (L and C), along with the resistive elements (R), produces a total probe impedance that varies with signal frequency. Through good probe design, the R, L, and C elements of a probe can be controlled to provide desired degrees of signal fidelity, attenuation, and source loading over specified frequency ranges. Even with good design, probes are limited by the nature of their circuitry. It’s important to be aware of these limitations and their effects when selecting and using probes.

Bandwidth and Rise Time Limitations

Bandwidth is the range of frequencies that an oscilloscope or probe is designed for. For example, a 100 MHz probe or oscilloscope is designed to make measurements within specification on all frequencies up to 100 MHz. Unwanted or unpredictable measurement results can occur at signal frequencies above the specified bandwidth (Figure 1.5).

As a general rule, for accurate amplitude measurements, the bandwidth of the oscilloscope should be five times greater than the frequency of the waveform being measured. This “five-times rule” ensures adequate bandwidth for the higherfrequency components of non-sinusoidal waveforms, such as square waves.

Similarly, the oscilloscope must have an adequate rise time for the waveforms being measured. The rise time of an oscilloscope or probe is defined as the rise time that would be measured if an ideal, instantaneous-rise pulse were applied. For reasonable accuracy in measuring pulse rise or fall times, the rise time of the probe and oscilloscope together should be three to five times faster than that of the pulse being measured (Figure 1.6).

In cases where rise time isn’t specified, you can derive rise time (Tr) from the bandwidth (BW) specification with the following relationship:

Tr = 0.35/BW

Every oscilloscope has defined bandwidth and rise time limits. Similarly, every probe also has its own set of bandwidth and rise time limits. And, when a probe is attached to an oscilloscope, you get a new set of system bandwidth and rise time limits.

Unfortunately, the relationship between system bandwidth and the individual oscilloscope and probe bandwidths is not a simple one. The same is true for rise times. To cope with this, manufacturers of quality oscilloscopes specify bandwidth or rise time to the probe tip when the oscilloscope is used with specific probe models. This is important because the oscilloscope and probe together form a measurement system, and it’s the bandwidth and rise time of the system that determine its measurement capabilities. If you use a probe that is not on the oscilloscope’s recommended list of probes, you run the risk of unpredictable measurement results.

Dynamic Range Limitations

All probes have a high-voltage safety limit that should not be exceeded. For passive probes, this limit can range from hundreds of volts to thousands of volts. However, for active probes, the maximum safe voltage limit is often in the range of tens of volts. To avoid personal safety hazards, as well as potential damage to the probe, it’s wise to be aware of the voltages being measured and the voltage limits of the probes being used.

In addition to safety considerations, there’s also the practical consideration of measurement dynamic range. Oscilloscopes have amplitude sensitivity ranges. For example, 1 mV to 10 V/division is a typical sensitivity range. On an eight-division display, this means that you can typically make reasonably accurate measurements on signals ranging from 4 mV peak-to-peak to 40 V peak-to-peak.

This assumes, at minimum, a four-division amplitude display of the signal to obtain reasonable measurement resolution.

With a 1X probe (1-times probe), the dynamic measurement range is the same as that of the oscilloscope. For the example above, this would be a signal measurement range of 4 mV to 40 V.

But, what if you need to measure a signal beyond the 40 V range?

You can shift the oscilloscope’s dynamic range to higher voltages by using an attenuating probe. A 10X probe, for example, shifts the dynamic range to 40 mV to 400 V. It does this by attenuating the input signal by a factor of 10, which effectively multiplies the oscilloscope’s scaling by 10. For most general-purpose use, 10X probes are preferred, both because of their high-end voltage range and because they cause less signal source loading. However, if you plan to measure a very wide range of voltage levels, you may want to consider a switchable 1X/10X probe. This gives you a dynamic range of 4 mV to 400 V. However, in the 1X mode, more care must be taken with regard to signal source loading.

Source Loading

As previously mentioned, a probe must draw some signal current in order to develop a signal voltage at the oscilloscope input. This places a load at the test point that can change the signal that the circuit, or signal source, delivers to the test point.

The simplest example of source loading effects is to consider measurement of a battery-driven resistive network. This is shown in Figure 1.7. In Figure 1.7a, before a probe is attached, the battery’s DC voltage is divided across the battery’s internal resistance (Ri) and the load resistance (Ri) that the battery is driving. For the values given in the diagram, this results in an output voltage of:

Eo = Eb * R_I/( R_i + R_I)

= 100 V * 100,000/(100 + 100,000)

= 10,000,000 V/100,100

= 99.9 V

In Figure 1.7b, a probe has been attached to the circuit, placing the probe resistance (R_p) in parallel with R_I. If R_p is 100 kΩ, the effective load resistance in Figure 1.7b is cut in half to 50 kΩ.

The loading effect of this on E_o is:

E_o = 100 V * 50,000/(100 + 50,000)

= 5,000,000 V/50,100

= 99.8 V

This loading effect of 99.9 V versus 99.8 V is only 0.1% and is negligible for most purposes. However, if R_p were smaller, say 10 kΩ, the effect would no longer be negligible.

To minimize such resistive loading, 1X probes typically have a resistance of 1 MΩ, and 10X probes typically have a resistance of 10 MΩ. For most cases, these values result in virtually no resistive loading. Some loading should be expected, though, when measuring high-resistance sources.

Usually, the loading of greatest concern is that caused by the capacitance at the probe tip (see Figure 1.8). For low frequencies, this capacitance has a reactance that is very high, and there’s little or no effect. But, as frequency increases, the capacitive reactance decreases. The result is increased loading at high frequencies.