Have you ever wondered why all those accessories were included with your oscilloscope probe or what some of them were for? Maybe you inherited your oscilloscope probes from someone else and it didn't come with any accessories and you would like to find some but you don't know what makes a good accessory good or where to get them.
Basic questions—like why are there so many accessories, what are they used for, what makes a good accessory good, and where can they be found—can arise among casual and frequent users of oscilloscopes. This article focuses on accessories for passive oscilloscope probes (figure 1), though the topics covered apply to active probes as well.
Click on image to enlarge.
Why so many different probe accessories?
The number of different probe accessories is driven by multiple factors. First, you'll find several different physical probe sizes, and the accessories are designed to function optimally with a specific probe size. The most common sizes are 5 mm diameter, 3.5 mm diameter, and 2.5 mm diameter (which refers to the diameter of the probe body near the tip). Different-sized probes offer differences in cost, maximum input voltage, or compatibility with legacy equipment.
The second factor is whether the accessory is to be used with the probe signal input or the probe ground. The third factor influencing the number of accessories is use model—either the probe is going to be used hands-free or in a browsing fashion (that is, it's handheld while making the measurement). The forth factor is the specific measurement and the application environment. For example, high-voltage accessories tend to be large for safety and breakdown voltage reasons, while high-bandwidth accessories tend to be small to minimize loading.
What are they used for?
Probe accessories are designed for optimum performance when used for specific intended purposes. The hands-free model is just that, hands-free. It makes it easier to use multiple probes at one time or to operate the oscilloscope while using the probe. A probe intended to be used in a hands-free manner will probably have accessories that tend to be highly specialized. For example, they might accommodate a connector designed into the target, you might be able to clip them onto a component, or you might use them with a positioner (which is itself an accessory) or in some other self-retaining manner.
When used in a browsing fashion, the accessories are designed to be more generic in their usage, enabling you to perform probing of signals and grounds on components, traces, pads, and vias with frequent, easy movement from point to point.
What makes a "good" accessory good?
A "good" probe accessory enables faithful signal reproduction, provides a reliable connection, and is durable, easy to use, and affordable. To enable faithful signal reproduction, the accessory should minimize signal loading by minimizing capacitive loading on the target system while at the same time providing low inductance in the signal and ground paths.
A reliable connection is achieved by an accessory that has no shorting hazards (zero to minimal exposed metal), is immune to environmental conditions such as temperature or humidity, and is not affected by sources of vibration like shaky hands or slight movements of the target system. In addition, it should have a sharp point that pierces through surface contaminants or employ a self-wiping action to displace minor contamination. An easy-to-use accessory's function should be relatively intuitive. The final qualifier for a good accessory is price; a good accessory should be affordable relative to the cost of other accessories or probes.
When considering which accessories to use, a primary consideration should be the performance or bandwidth needed for the measurement to be made. As a general guideline, for accurate amplitude measurements, the system bandwidth of the probe and oscilloscope together should be three to five times greater than the frequency of the waveform being measured. This rule of thumb ensures adequate bandwidth for signals such as square waves with high-frequency content.
For reasonable accuracy when measuring rise times, the rise time of the oscilloscope and probe system should be three to five times faster than the pulse being measured. Usually, oscilloscope manufactures specify the system bandwidth of their probe and oscilloscope combinations. Many times a designer may not know the bandwidth of the target to be measured but knows the rise time of the fastest signals to be measured. In these cases, there is a handy relationship that proves useful for determining bandwidth:
Babdwidth × Rise Time = 0.35.
The convenience offered by some accessories may be attractive but may come at a cost. Any accessory that is placed between the probe and the target system has the potential to adversely affect the target and the measurement because of the extra capacitance and inductance of the accessory. Extra capacitance will load the target system, while the extra capacitance and inductance can affect the probe's response.
Generally speaking, the larger or longer the accessory, the more impact it will have on the measurement being made. Therefore, it may be prudent to reserve the use of larger or longer accessories for slower signal measurements. Consider the example shown in figure 2.
Fig 2: an 80-MHz sine wave (the yellow trace) being measured by a 500-MHz
passive probe with its standard short ground spring (the green trace).
Applying the "three to five times the waveform frequency" guideline, a 500-MHz probe is adequate for this measurement, as you can see by noting the same peak-to-peak voltages of the green and yellow traces. Then compare the original signal's yellow trace with the blue trace, measured using the same 500-MHz probe but with a 15-cm ground lead instead of the short ground spring. The measurement error is obvious. (Note that the phase difference between the waveforms was intentionally set to let you more clearly visualize the different waveforms; it is not a result of the probe or its accessories.)