A steady stream of advances has elevated test and measurement instruments to the point where they can reveal minute details of signals with lightning-quick rise and fall times. So, then, what about test probes? The last thing test engineers want or need is a probe that’s going to influence their measurements or fail to deliver the full bandwidth that’s available to them on the scope.
Fortunately, today’s high-end probes are constructed to sidestep these issues. In this article, you’ll learn about what makes modern active test probes tick and how they can help you pin down (literally as well as figuratively) the signals you need and want to see. There also are considerations related to squeezing the maximum lifespan and performance out of active probes (see “Pick The Right Probe And Get The Most Out Of It”).
THE PERFECT PROBE
An active probe has active components such as transistors, amplifiers or preamplifiers, and sometimes FETs. The basic distinction between an active and passive probe is that the active probe requires a power source applied to it.
In a perfect world, the perfect test probe would serve as a completely electrically transparent interface between the device under test (DUT) and the oscilloscope’s input. It would impart no effect whatsoever on the signal being acquired, accurately reproducing the signal under test with more than adequate fidelity.
Unfortunately, here in the real world, many physical factors come into play to make the perfect test probe practically unattainable. The probe that’s incapable of loading the circuit under test has not been and cannot be built. “All probes will load the circuit,” says Jae-Yong Chang, Agilent Technologies’ product manager for test probes. “The question is how much loading the probe imposes and how much you can tolerate.”
The challenge for test-instrument makers, then, is to come as close to perfect as possible. A number of considerations must be balanced in the creation of high-bandwidth active probes. Many of these considerations are borne of the market drivers behind the probes’ development.
Today’s serial high-speed data standards, such as PCI Express, are among the top challenges for test engineers. “Signals for all of these serial data standards are differential signals,” says Andy Heltborg, product marketing engineer at Tektronix. “Many of them are moving to multiple lanes, faster speeds, and lower voltages. So this is where the market is going, and it creates issues we need to address with probes.”
HOW PROBES AFFECT TESTING
Traditional active test probes have always had the facility to add accessories to the tip of the probe to enhance the actual connection to the DUT. These accessories allow users to more easily make measurements, but they also affect measurement performance in terms of bandwidth, loading, linearity, and flatness of bandwidth (Fig. 1). But once signals get into the section of the probe where its amplifier resides, there’s little or no further effect on these signal parameters.
A closer look at the nature of the connection of an active probe reveals more about its electrical characteristics (Fig. 2). The interconnect itself dominates these characteristics.
“We view it as an LC-resonant tank circuit,” says Chang. “Depending on the length of the cables, input leads, and connectors, you have inductance and capacitance on the front end that make up a resonant circuit. Because of the resonance, you get certain characteristics over the frequency range.”
Input impedance falls to nearly zero at some frequency, which is the LC-resonant frequency. “At that point, the response of the circuit rises at the resonant frequency. This is an inevitable characteristic of the probe,” says Chang. A common approach to correcting this peaky frequency response, which Agilent employs in its high-end InfiniiMax active probes, is to use a damping resistor of about 100 to 150 O at the probe tip.
“This resistor prevents the input impedance from going below the damping resistance,” says Chang. The added resistance reduces the loading and peaking of the probe caused by input connection conductors. So while the damping resistance helps to flatten out the probe’s frequency response, it does not influence any increase in the probe’s bandwidth.
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