Oscilloscope probe hints

An oscilloscope is useless without the probes necessary to get signals from a device under test (DUT) to the instrument. And probes themselves are far more complicated than simple, ideal conductors that transmit those signals. For effective probing, you'll need to know when to deploy passive or active probes, how to check for probe loading, how to compensate probes, how to deal with resonance, and how to make low-current and floating measurements.

In the Webcast “Six hints for better scope probing,” Agilent Technologies product manager Jae-yong Chang covers all these topics. He notes, for example, that passive probes suffice up to 600 MHz, while active probes operate to 13 GHz. He demonstrates a two-probe technique that indicates probe loading. He recommends that probe impedance be at least 10 times source impedance and that probe bandwidth be at least five times signal bandwidth.

If you need to add a wire to a probe tip to make a measurement in a crowded environment, you can use a resistor measuring 100 Ω or so to damp resonance. Courtesy of Agilent Technologies.  (View larger image)

In addition, Chang provides sample waveforms that suggest when you need to compensate your probes, and he describes how to adjust a probe's RC network divider to maintain an appropriate attenuation ratio over the entire probe bandwidth. He provides details on using AC/DC current clamp-on probes to make the low-current measurements necessary to characterize and troubleshoot the signals common to ubiquitous battery-operated consumer products, noting that it's best to degauss and perform offset adjustments on these probes before each measurement.

He also covers measurements on high-power DUTs, describing how to use differential probes and isolation transformers. Finally, he describes a nifty method of using a probe-tip resistor (figure) to damp resonance.

View the archived Webcast (presented live December 12, 2007, and sponsored by Agilent Technologies and Test & Measurement World).

If you need to add a wire to a probe tip to make a measurement in a crowded environment, you can use a resistor measuring 100 Ω or so to damp resonance. Courtesy of Agilent Technologies.