In this video: https://www.youtube.com/watch?v=9cP6w2odGUc A simple cheap circuit is described to get 2nS rise times for use in ringing cables for a TDR test.

I started thinking that there might be an inexpensive way less than $100 to get a much faster rise time that could be used to trace pcb faults down to a narrow area say less than a few sq cm. But even assuming FR4 has velocity factor around .66 this is still about a thousand times faster than the impulse generator in the video.

One possibility that occurred to me is building an oscillator using a hex transistor array and intentionally running progressively longer trace widths to base/gate connections. Another possibility is using a tunnel diode or SRD to sharpen an edge from an oscillator. The Tek 485 had a cal out based on avalanche osc using 2n6304. Or perhaps a driver IC for SERDES.

I have been reading Linears app note 47 https://www.analog.com/media/en/technical-documentation/application-notes/an47fa.pdf and have renewed respect for circuit designers and pcb layout folks who do work at high frequencies.

Anyone already made such a fixture that can deliver a circuit and PCB art?


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