[time-nuts] Re: Rubidium oscillator : pack it in styrofoam or attach it to a heath-sink?

[Magnus Danielson]

Hi Paul,

So, sure, most of the electronics is actually better served away from 
the heat, but that is the compromise of the cheap telco rubidiums that 
need things fit into a small space. Another aspect is that isolating 
them as you suggest can help reduce the amount of heat we need to 
produce, and with that the current through transistors to burn energy 
for heating, which is another source of failure, a strain on MTBF in 
itself. At the same time, as you isolate, you need to leak more heat 
from the colder filter/cell so that it can dump heat from the lamp side 
and still have a regulating heating to maintain the temperature you 
want. So you need to understand the balances and keep everything there. 
Chopping up an LPRO like this is possible for sure. The LPRO also uses 
the temperature of the filter/Cell block to stabilize the crystal, and 
the oscillator part needs to be more or less straight there or you end 
up in trouble. If you have a couple, you can let most of the electronics 
be dead but at high temperature and another bord cold to do what can be 
done at a bit of distance, if that is what makes you go. I have enough 
LPROs not to care, I have spares.

Hope it helps.

Cheers,
Magnus

On 2021-10-05 22:17, paul swed wrote:
> This is a good discussion and has my brain cell working.
> Totally agree on te temperatures in the filter and such.
> But I have always disliked the temperature everything is running at in 
> the telco RB's.
> So would it make sense to actually seperate the boards and get them 
> away from the heat while leaving the hot items as is? The leads can be 
> lengthened, even the RF.
> Regards
> Paul
> WB8TSL
>
> On Tue, Oct 5, 2021 at 4:09 PM Magnus Danielson via time-nuts 
> <time-nuts at lists.febo.com <mailto:time-nuts at lists.febo.com>> wrote:
>
>     Hi,
>
>     It's a complex picture as depending on temperature of the components,
>     and other aspects such as RF intensity, sensitivity to this or that
>     changes, line with changes etc. It also depends on the actual buffert
>     gas mix, which by the way changes over time due to leakage.
>
>     There is really three parts, the lamp, the filter and the detector
>     cell.
>     Turns out that  the filter and cell temperatures end up in about 65
>     degrees C. For some I've seen 73 degrees C. For the lamp, you end up
>     with something in 110-120 degrees C. Physically these two
>     temperatures
>     is just next to each other, as the lamp needs to shine straight
>     into the
>     microwave cavity of the cell but the filter cell needs the same
>     temperature as the cell.
>
>     One can optimize the temperatures for strongest signal, which sounds
>     like a good thing for S/N, one can optimize them for minimal
>     sensitivity
>     for lamp or RF intensity or you can optimize it for low line width.
>     Depending on the conditions, you end up with a bit different
>     settings.
>     If it is easy to stabilize RF intensity for instance, then one can
>     relax
>     that optimization, similarly for lamp intensity. Then you can push it
>     for a balance between line-width (Q) and S/N. For others, this is not
>     feasible, for instance for simplicity/cost and/or size.
>
>     Regardless, temperatures of rubidiums is quite a different mess to
>     that
>     of cesiums or hydrogen masers, and let me tell you that
>     temperature of
>     the later is a mess I look at quite a bit at the moment.
>
>     Cheers,
>     Magnus
>
>     On 2021-10-05 17:45, Bob kb8tq wrote:
>     > Hi
>     >
>     > Rubidiums are somewhat unusual beasts. They typically have two
>     heated zones ( = two ovens) in
>     > them. One is a bit hotter than the other. Because of the basic
>     physics, those ovens are right next
>     > to each other / in contact with each other.
>     >
>     > If you go to crazy with the insulation, the “colder” oven will
>     heat up due to heat leakage from the
>     > “hotter” oven.  You need a certain amount of heat coming off the
>     package to allow this to happen.
>     >
>     > The bigger issue is that there is a pretty big batch of
>     electronics near the ovens in the typical telecom
>     > Rb. Unless you heatsink things pretty well these parts heat up.
>     When they do their MTBF drops
>     > quite a bit. You save a couple of watts of heat (maybe) and
>     loose the Rb after a year or two. Not
>     > a great tradeoff.
>     >
>     > Yes, there are a lot of different designs for lab grade Rb’s.
>     There are also some really tiny little
>     > guys running around. Neither category is all that easy to get on
>     the surplus market. If you want
>     > to dive into either of those categories, there are issues, they
>     just may not be quite the same.
>     >
>     > Bob
>     >
>     >> On Oct 4, 2021, at 1:39 PM, Wim Peeters <peeters_w at scarlet.be
>     <mailto:peeters_w at scarlet.be>> wrote:
>     >>
>     >> Insulation decreases the power consumption.  But it will also
>     increase the temperature of the electronics.
>     >>
>     >> A heath-sink will cool the electronics but will increase the
>     power consumption.
>     >>
>     >> Or maybe insulate the  part of the case that gets hot, and put
>     a heat-sink on the other parts?
>     >>
>     >> Wim Peeters
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