In addition to the sampling gate and pulse generator circuits discussed in Part 2, the Tektronix SD-24 sampling head also contains a flat pulse generator circuit used for Time-Domain Reflectometry (TDR) measurements.
Unlike the main sampling pulse generator, this TDR pulse generator is designed to produce a flat-topped square wave (instead of a very narrow pulse) suitable for TDR applications. The idea is to generate a ugly looking square wave with a high amplitude and very fast rising edge (There's no clipped line, so the negedge is still slow), and then use two pairs of back-to-back Schottky diodes to clip the top and bottom of the waveform. This results in a waveform with a flat top and bottom.
Detailed operation is descibed in a patent [FPG Patent].
TDR Flat Pulse Generator Circuit of Tektronix SD24
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...TBD
