[time-nuts] SMPS or conventional?

[Dana Whitlow]

I prefer the use of non-switching power supplies, in part because I am
forced to use
indoor receiving antennas not very far from my equipment setup.  However,
one of my
Rb standards, an used L-Pro which I bought from another ham, came with a
SMPS
and I've never noted any particular problem.  So, my advice is: if you
have  SMPS
that you trust not to fail and blow up your Rb, first give it a try to see
what happens.

Regarding cooling of Rb standards:
Truly, the addition of a fixed-speed fan will reduce the thermal resistance
between
"ambient" temperature and the device being cooled.  However, said reduction
of
the thermal resistance shortens the thermal time constant, increasing the
responsivity
to ambient temperature variations.

I see two likely approaches to improving the situation:

a)  As has been mentioned several times, substantially increase the thermal
mass of
     the device being cooled, combined with fan cooling of the heatsink.  A
downside
     of this is that system warmup time may be increased enough to be
bothersome.

b)  Use a variable-speed fan controlled by a loop which strives to maintain
a constant
     temperature of the heatsink to which the device is attached once a
reasonable
     temperature (moderately above ambient yet still comfortable for the
device) is
     reached.

I believe that (b) is probably the more effective way to go.  If done well,
it could
provide superior frequency stability as well as the shortest warmup time.
However,
choice (b) adds to the system complexity hence risk of an expensive failure.

Dana




On Wed, Oct 21, 2020 at 7:46 AM Wannes Sels <wannes.sels at gmail.com> wrote:

> There are conflicting requirements regarding temperature in Rb's:
>
> - For best performance, the rubidium and quartz oscillators must be kept at
> a high and stable temperature.
> - For reliability, the supporting electronics must be kept at a lower
> temperature.
>
> The heater takes care of the high temperature. The stability of the
> temperature can be improved by increasing the thermal mass, i.e. adding a
> block of aluminium or copper.
> The electronics can be kept cooler with a heatsink, and forced ventilation
> if needed.
>
> When you stick both the electronics and physics in a small package, this
> becomes more difficult. For the PRS-10 some form of heatsink is pretty much
> required to keep the electronics cool enough. Although I seem to remember
> reading somewhere that the "benchtop" heatsink they offer now is lighter
> than older units.
>
> If the room temperature is not stable enough, you might want to increase
> the size of the heatsink and add a fan. This reduces the temperature swing
> inside the unit, while still cooling the electronics, at the cost of
> increased power draw for the heater.
>
>
>
>
> On Wed, Oct 21, 2020 at 5:38 AM Hal Murray <hmurray at megapathdsl.net>
> wrote:
>
> >
> > > I spent a lot of years buying Rb’s and putting them on small heatsinks.
> > I
> > > always was disappointed in their reliability. That continued to be the
> > case
> > > up to the point that the baseplate temp’s got into the 40C region.  In
> my
> > > case, that took a fan ….
> >
> > How well did it work if the heat sink wasn't small?  What is your version
> > of
> > small?
> >
> > Do you have any data (or vague memories) of how much it helps to orient
> > the
> > heat sink so the fins are vertical so they encourage warm air to flow up
> > past
> > the fins?
> >
> > --
> > These are my opinions.  I hate spam.
> >
> >
> >
> >
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