Re: Fast-assed signal switch

On Wed, 15 Aug 2007 22:57:32 -0500, "Tim Williams"
<tmoranwms@xxxxxxxxxxx> wrote:

"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:j0v6c3leusdr5eouh0t1uhdm64flij864a@xxxxxxxxxx
Get the Tek book. It explains all of this stuff.

Way to dodge the question ...NOT ;-)

Unless you're offering, of course.

Tim

Oh all right.

Imagine a 25 ohm source (50 in, terminated), a series switch, and some
small hold capacitance. Say that the switch has another 10 ohms of
series resistance, and the hold cap is 3 pF. If the switch is closed,
the hold cap follows the input with about a 100 ps time constant. So
your off-switch will be speed limited by that tau.

If we blip the switch closed only for, say, 25 ps, the cap samples the
signal for 25 ps but it only charges up to about 25% of the difference
between the pre-sample cap voltage and the new voltage. That's a 25%
sampling efficiency, but we're 4x as fast. All it takes to fix the
efficiency problem is a cheap opamp, to give low-frequency gain. Most
sampling scopes only run a few percent efficient.

And fast samplers usually use only 2 diodes, to reduce the switch
resistance.

This was all worked out in about 1961, when HP discovered the SRD
(known then as the Boff Diode) and invented the dual-diode feedback
sampler, first used in the HP185 scope.

The only two serious advances since then have been the shockline
sampling pulse generator and the 6-diode traveling-wave sampler. The
fastest electrical samplers are now upwards of 250 GHz, using shock
lines and dual diodes. PSPL sells a commercial 100 GHz sampling head,
but not the whole oscilloscope.

John



.

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