Tim Williams wrote:
"John Larkin" wrote in
message ...
Nonlinear transmission lines are cool. You can make them with
saturating inductors, ceramic capacitors, or varicap diodes, even
ordinary power diodes. You can get the risetime to go down as a step
propagates down the line.

I've been tempted to try that before -- a necklace of ferrite beads
might run you only a couple bucks per foot, and with the velocity factor
so low, that's enough to get something snappy with jellybean transistors
(a few nanoseconds' input edge). Downside is the impedance, which must
be in the kohms range (small signal). Hard to get much current into it
like that. Supposedly, ferrite has a high e_r as well, so the impedance
probably isn't *that* bad, but it's hard to ensure a consistent airgap
(or lack thereof) with loose beads.

I expect the pulse width is limited as much by geometry (ferrite beads
being a coax structure, periodically disturbed by the imperfect faces
between beads and imperfect spacing to the conductors) as by material
dispersion (losses, etc).
* The idea is (i think the proper term is) lumped constant; transmission
line = = L-C-L-C-L etc, in this case series L and shunt C.
So use small (size) L (small beads) with small C (chip caps) to
ground plane. The caps also being the mechanical "standoff" for the wire
used.


The not-even-ludicrous speed[1] generators do it with schottky junctions
in InP (like the LeCroy 200GHz scope) IIRC. With harmonics near the THz,
there isn't any physical length to spare before the signal dissipates,
anyway.
* Dissipate? I think not. HF part attenuated tho is something to look at.


Tim

[1]
"Aah! What the hell was that?!"
"Spaceball 1!"
"They've gone to plaid!"