[time-nuts] Thunderbolt Supply
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Tue Feb 15 03:18:39 UTC 2011
Bob wrote:
>If all you are doing is running a Thunderbolt, you don't need a
>supply that's more quiet than most batteries.
Most batteries are *very* quiet -- it takes heroic measures to get
*any* actively regulated supply into that ballpark. Indeed, one
might be tempted to run a Tbolt off of three batteries, each one
charged by a low-noise, high-impedance current source that puts out
about .05 CV more than the Tbolt draws. One could even turn the
charging off for short periods of "minimal noise" operation, if the
batteries were suitably sized. However, in either case I would be
concerned that the drift of one or more of the battery voltages (poor
absolute regulation) might introduce another source of XO drift --
but I have not tried it.
>The idea is to stop spending money when you have reached the "good
>enough" point.
I quite agree with you on this point (maybe I'd say "when you are
clearly past the point where other errors dominate the performance
envelope"). However:
>People get reasonable performance off of straight switcher outputs.
>Adding simple linear + filtering gets you well into the overkill
>region on this application.
I'm not sure we know for certain how quiet is "good enough" for a
Tbolt, or where the "overkill region" is -- particularly when the
residual noise contains impulse hash from a switching regulator. I
presume most time nuts would consider a 5 dB improvement in phase
noise worthwhile for the relatively low cost/effort of using a good
linear supply, if that level of improvement can be attained. The
only systematic study I've seen is what tvb has on his web site, and
it does not include data from a switching supply with external
post-regulation and/or post-filtering.
Best regards,
Charles
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