[time-nuts] Morion MV89A position

[Tom Van Baak]

Charles,

I tried something like that and ended up frying the oscillator! The problem was that I used too much insulation. That was good to keep ambient temperature transients out but it also kept internal heat generation in. This is not a problem for some low power circuit board (like a GPS receiver or RF distribution amp), but OCXO's generate significant heat and if the clever enclosure(s) surrounding it prevent that heat from flowing out, then over hours or days, you gradually melt what's inside...

My question is, how does one design an enclosure to prevent this mistake? Or is it trial and error. Perhaps put a thermistor on the OCXO and if the case temperature rises beyond what is normal case temperature in free air, then the enclosure has too much insulation?

With all this talk about thermal capacitance and resistance, perhaps what we need is a thermal diode; it lets heat out but prevents thermal fluctuations from getting in. A thermal Gore-Tex layer.

/tvb

----- Original Message ----- 
From: "Charles Steinmetz" <csteinmetz at yandex.com>
To: "TimeNuts" <time-nuts at febo.com>
Sent: Wednesday, January 29, 2014 9:27 AM
Subject: Re: [time-nuts] Morion MV89A position


> Poul-Henning Kamp wrote:
> 
>>Sorry, I had read it as you had the OCXO in close contact with the box.
> 
> No, I agree that wouldn't work well.  Isolating the oscillator from 
> the cast box is what the teflon or nylon standoffs are for.  As I 
> said in response to Paul, I recommend at least 3-4 cm of air space on 
> all 6 sides of the oscillator.  If the cast box is mounted in another 
> enclosure (whether the second enclosure has a thermostatically 
> controlled fan or not), it should be insulated from that box, too 
> (so, no direct thermally conducting path from the oscillator to the 
> cast box, and no direct thermally conducting path from the cast box 
> to any additional enclosure or rack).
> 
> Typically, the cast box can just sit in an out-of-the-way place on 
> some rubber feet (preferably, not right in front of an HVAC 
> vent).  If the environment is brutal with respect to large, fast 
> temperature changes, then an outer enclosure and perhaps even a 
> thermostatically controlled fan are indicated.
> 
> This system has worked extremely well for me, and does not attract 
> the same attention as requisitioning 4U rackspace for a cinderblock.
> 
> Best regards,
> 
> Charles