On 07/12/2012 05:54 AM, Chris Albertson wrote:
On Wed, Jul 11, 2012 at 6:55 PM, Mark Spencer<mspencer12...@yahoo.ca>wrote:
I tried putting my thunderbolt in a styrofoam box. It got hotter than I
liked and didn't seem to perform any better than when I left it in a
cardboard box.
I worry about the long term implications for component life as the
temperature goes up.
The advantage of the insulation comes in when you build the fan controller.
Mine uses a temperature sensor and an IC comparator that drives a
transistor that drives a 12V fan. The fan does on when the set point is
reached then goes off. If you set the operating point a little higher then
the inside of yur house then the fan cycles and keeps the inside of the box
and roughly, more or less constant. The parts to build a fan controller
are about $5
You *really* don't want an on/off regulation scheme. The OCXO gets
dynamic thermal stress from it which causes it to go off in frequency as
the heat wave (or cooling wave) comes in and has a rate faster than the
oven can steer. The oven gain will be much less than for slow
semi-static temperature changes. Old ovens used on/off schemes, but it
was dropped as electronics allowed for continuous feedback loops.
You will see a bump in your ADEV at about the cycling period of your
fan. If you have quick cycling period, it will affect your close in
phase noise and ADEV, but you might suppress your room/house AC/heating
loop. If you run continuous, there will be a much quieter response.
Another side-effect of cycling power for an OCXO is that it will for
each cycle shifts its phase. The OCXO control will return it to
temperature as fast as it can, which should return it to frequency. What
happens is that the curve around the balancing point isn't completely
symmetric, but the heat-up/cool-off temperature profile certainly isn't.
The end result becomes that the frequency error under the curve isn't 0
over such a cycle, and that integrated over time will become phase. So,
this unstable phase creep will keep the control loop active to fight it
back, until you reach holdover, when the error will be exposed completely.
I've learned this the hard way, from observing frequency and phase
fluctuations and fighting them.
If you don't have a temperature controlled fan then the next best thing is
a well vented cardboard box whose only purpose is to keep air currents off
the unit. Insulation without an active temperature controller is only going
to make it hotter, not more stable
Using a cardboard box or other "wall" around the oscillator will as a
passive oven work well, if the unit is still allowed to dissipate it's
excess heat to the surrounding. The effect is really great and helps the
control loop as temperature shifts occurs more gradually such that the
control loop can track it within it's bandwidth. Long term temperature
shifts like the bang-bang regulator of the building AC/heating will
still eat into the box.
Cheers,
Magnus
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