[time-nuts] Power supply for OCXO using "USB power blocks"

[ed breya]

Jim said:

The question is really more one of "how much filtering do I need to
design into the downstream power supply circuits"


The answer of course depends on the source and receiver of the noise. 
Common USB type systems and accessories are built to maximize cheapness, 
pass EMC testing well enough to get by, be small, and provide the 
required power.

The normal-mode ripple and noise can be made as good as necessary with 
enough filtering and active regulation, at one point of use. The bigger 
problem is the common-mode signals that permeate everything.

If you're using a compact, battery-powered SMPS to run your OCXO, then 
you can take advantage of the situation by "canning it up," as I always 
say. Don't have a bunch of cords and cables and wallwarts going all over 
the place - everything will carry some common-mode current and make a 
mess. If you package up the battery/power supply unit along with the 
OCXO inside of a metal case that more or less completely encloses it, 
then it will be very much easier to get control of the C-M currents 
within the box, and make them nearly invisible outside the box.

Picture a metal box with the proper guts inside, and say, the outside 
sports a BNC 10 MHz out, a power switch, maybe some indicator lights, 
and a charging port. All the action inside can be well hidden from the 
outside, and the desired output signal will have one ground reference 
point. During charging, there will be another C-M current issue due to 
connection to the port, so the EMC performance will be degraded 
somewhat. But, that can be improved internally too, because the system 
is mostly canned up.

Ed