Re: Isolating Oscilloscope Channels

Tim Williams wrote:

"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:9zOVh.11836$Kd3.1172@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

You can get fast transformers at places like Mini-Circuits. I don't know
how high LAN transformers go and where they saturate but a data*** of
one should tell. Reason I don't know is because I always keep a pound of
#43 ferrites in my lab so I can quickly wind one up if in need. Faster
than filling out a Digikey order ;-)


Heh yep. I've had excellent response from even really crappy transformer
designs. 100 turns of 30AWG on a high-permeability core (toroid or
etcetera) will get you there. Primary then secondary, don't really even
need to interleave them. Use *two* layers of masking tape between windings
if insulation spec is really high. ;-) My experience gives bandwidth into
the 10s of MHz (risetime comparable to the driving circuit: 100-200ns).

I would use a series capacitor to block DC and a (parallel) resistor to
dampen oscillations. With small C, you can get away with pretty wimpy
windings, since the pulse ends up really narrow. With large C (and L!), you
can see the whole square wave with maybe just a little droop along the way.


Looks like Ed is only interested in the transition times. My staple for that looks like this: About 6" PTFE wire, 6" of a CAT-5 strand or whatever, twist the two together in 1/2" turns or so, wrap four times through a 1/2" OD toroid of #43 material. I get sub-10nsec transitions through those. The PTFE insulation is only needed where there is a lot of bzzzt between primary and secondary.

--
Regards, Joerg

http://www.analogconsultants.com
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