This takes the scope probe accessory described previously, and repeats the measurements with a higher frequency oscilloscope (>350MHz) and higher frequency probes (250MHz). The results are even more impressive at higher frequencies.
My newly acquired Tektronix 485 is a sweet little scope where the bandwidth is sigificantly greater than the specified 350MHz, and the calibration output has a clean risetime of <1ns. This makes it worthwhile repeating the original tests with the HP10074 150MHz probe, and also with 250MHz Tektronix P6106 and 500MHz P6054 probes.
The reference is the cabibration output connected directly to the scope’s input; note the 1ns/div timescale and the risetime of <1ns.
The 150MHz HP10074 probe with the standard 6″ ground lead shows pronounced ringing for >30ns, but with the bayonet spear probe tip there is a single, short well-damped ring at around 250MHz. This is a significant improvement, but also illustrates what can happen if the probe has a lower bandwidth than the scope.
The 250MHz Tektronix P6106 probe with the standard 6″ ground lead also shows pronounced ringing over ~30ns (and some ~100MHz noise picked up through the ground lead). With the bayonet spear there is virtually no ringing. That’s a very satisfying result.
The 500MHz 12.5pF Tektronix P6054 probe’s results are very similar to those P6105 results. The risetime is slightly faster at around 1.5ns, which is slightly slower than the expected risetime of the probe/scope combination.
The best probe risetime and waveform shape is, unsurprising, obtained with a 1.5GHz 0.7pF 500Ω HP10020 probe; it is indistinguishable from the direct cable connection.