PRObE SElECTION?

eTech - ISSUE 6 21

NO PROblEm.by Trevor J Smith, market Development manager, Tektronix

In addition to imposing bandwidth and rise-time constraints, the real-life probe also acts as a load at the test point, which can change the signal that the circuit or signal source sees. Especially at higher frequencies, capacitive loading reduces bandwidth and increases rise times for the overall measurement system. This effect can be minimized by selecting probes with low tip capacitance values.

A further consideration is the probe’s inductance, which interacts with capacitance to cause ringing at a certain frequency. The impact is reduced by designing probe grounding so that the ringing frequency occurs beyond the bandwidth limit of the probe+oscilloscope system.

a complex decision? Not any more.Probe selection doesn’t stop there. It should also factor-in the type of signal being measured: voltages, logic, currents, optical

signal or vibrations, for example. By selecting a probe appropriate to the type of signal, direct measurement results will be attained more quickly.

Moreover, designers need to consider the amplitudes of the signals being measured. Are they within the dynamic range of the ‘scope? If not, a probe is required that can adjust dynamic range. Generally, this will be through attenuation with a 10X or higher probe.

If all this sounds over-complex, there is plenty of help available to guide users through the selection process. RS and Tektronix have recently announced a new on-line, interactive tool that enables users to select by series, model number, or standards/application, then fi ne tune the search with specifi c testing requirements. The list of matching products updates with each step.

RS also carries a constantly expanding library of manufacturer’s technical briefs, application notes and other resources will help get the most out of probes and other equipment.

Specialised probes provide a convenient physical connection to high density SmT boards.

Test and measurement with oscilloscopes is key

aspect of electronics design. Of course, the fi rst

priority is choosing the correct scope for the task, but probe selection should

not be overlooked either. Using the wrong probe can

potentially compromise measurements and

ultimately derail the fi nal design.

Get more online...RS and Tektronix have announced an online interactive probe selection tool, available at:rswww.com/tektronix

Fundamentally, the probe needs to provide a connection of adequate quality between the signal source and oscilloscope input. The ideal probe would offer convenient and easy physical connection and absolute signal fi delity with zero signal source loading and total immunity from noise.

For miniaturized circuitry, such as high-density surface mount technology (SMT), connection ease and convenience are promoted through subminiature probe heads; and most suppliers offer a range of probe-tip adapters specifi cally designed for SMT devices. At the other end of the spectrum, applications such as industrial power circuitry demand physically larger probes with greater margins of safety, in order to handle high voltages and heavier-gauge wires.

Oscilloscopes can also have different input connector types and input impedances. For example, most scopes use a simple BNC-type input connector; others may use SMA connectors; and still others broaden the options to include support for readout, trace ID, probe power or other special features.

Delivering signals faithfullyBeyond these requirements, the ideal probe needs to take the signal at the probe tip and faithfully deliver it unchanged to the oscilloscope input. This is impossible to achieve in practice, but designers should strive to minimise attenuation, maximise bandwidth, and deliver as close as possible to linear phase at the voltages and frequencies of interest.

For accurate amplitude measurements the bandwidth of the measurement system should as a general rule be fi ve times greater than the frequency of the waveform being measured; and in measuring pulse rise or fall times, the rise time of the probe and oscilloscope together should be three to fi ve times faster than that of the pulse being measured.

Analysing the system bandwidth or rise times from the individual oscilloscope and probe bandwidths is far from simple, so manufacturers of quality oscilloscopes specify these values based on specifi c combinations of ‘scope and recommended probe models. Using a probe that is not on the oscilloscope’s recommended list runs the risk of unpredictable measurement results.

Standard BNC Probes. Probes with a plain BNC connector will connect with virtually all Tektronix oscilloscopes. Low cost passive probes generally have a plain BNC connector.

TekProbeTM Level 1 BNC Probes. TekProbe Level 1 BNC connector equipped probes communicates scale information to the oscilloscope so that the oscilloscope correctly conveys accurate amplitude information.

TekProbeTM Level 2 BNC Probes. The TekProbe Level 2 BNC shares the scale information of the Level 1 but also provides power for a whole host of active electonic probe designs.

TekVPI® Probes. TekVPI equipped probes offer advances in power management and remote control. TekVPI probes are an ideal choice for applications where computer control is important.

TekConnect® Probes. Probes with TekConnect interface support the highest bandwidth active probes offered by Tektronix. The TekConnect interface is designed to support probe requirements >20 GHz.

bNC and Sma type connectors and special features.

20 eTech - ISSUE 6