Differential scope probe delivers 6-GHz bandwidth to the screen

With digital scopes that offer 6-GHz bandwidth suddenly almost commonplace and with industry participants now recognizing differential signaling as all but essential for sending multigigahertz signals across pc boards and backplanes, you shouldn't be surprised at the proliferation of ultrawideband, active, differential scope probes. LeCroy Corp now joins Agilent Technologies (www.agilent.com) in offering a differential active probe that maintains its top-of-the-line scopes' 6-GHz bandwidth from the probe tip to the screen. To accomplish this feat, LeCroy's WaveLink series gets an undistorted signal to the input of the probe's wideband silicon-germanium differential amplifier via a passive resistor and an attenuator near the signal source followed by a transmission line terminated at both ends in Z₀. Like Agilent's probes, LeCroy's new probes can acquire single-ended signals without your having to terminate the unused input.

The LeCroy probes' mechanical configuration emphasizes ease of browsing (probing various points on a board for interesting signals)—especially browsing for differential signals. A handy thumb wheel enables you to set the spacing between the nickel-titanium-alloy probe tips to any distance from 0 in. with the probes touching to 0.3 in. The spacing holds its setting, and you can twist and bend the tips without undue concern about damaging or permanently deforming them. LeCroy says that the probe's mechanical architecture is conducive to additional designs for different styles of probing and that the company will probably introduce such versions during 2003.

Despite their similarities, the LeCroy and Agilent probes' electrical architectures differ in significant ways. LeCroy offers a lower input resistance of approximately 4 kΩ and a lower attenuation ahead of the probe amplifier. LeCroy says that the lower attenuation improves the probe's SNR, thereby reducing the jitter the probe amplifier adds to signals, particularly the low-amplitude signals common in high-speed differential-signaling systems. Although 4 kΩ may seem to be a low input resistance, applying voltages as high as 25V causes no permanent damage. Moreover, the probes' input impedance varies less across the probe bandwidth than does the input impedance of probes that have higher resistance. At frequencies higher than approximately 2 GHz, distributed capacitance and series inductance make the specified shunt resistance of any wideband probe a matter of little practical interest.

Because it recognizes that the details of the probes' characteristics are important to signal-integrity engineers, LeCroy provides a free characterization fixture with each probe, whereas competitors charge for such fixtures. The WaveLink D600 differential active probe with an adjustable twin-tip module has a probe-tip-to-screen bandwidth of 6 GHz and costs $7490 (Picture). (The bandwidth of the probe alone is greater.) The D300, a 3.5-GHz-probe-tip-to-screen-bandwidth version, costs $4490.

LeCroy Corp, 1-800-453-2769, www.lecroy.com.