[time-nuts] Morion MV89A position

Dave Brown tractorb at ihug.co.nz
Wed Jan 29 16:30:58 EST 2014


Charles
It's not practicable for many people but - what about replacing your 
aluminium box with a, say 2 foot piece of  6 inch pvc pipe (ocxo suspended 
inside it clear of the wall and sealed off ends) and burying that a few feet 
in the ground?   I expect the thermal mass of the pvc is not as high as the 
aluminium but the external ground temperature will be lower than air ambient 
as well as having much less variation. Overall, I'm not sure what the result 
might be.
DaveB, NZ

 ----- Original Message ----- 
From: "Charles Steinmetz" <csteinmetz at yandex.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Thursday, January 30, 2014 9:28 AM
Subject: Re: [time-nuts] Morion MV89A position


> /tvb wrote:
>
>>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.
>>    *   *   *
>>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?
>
> That is the point of distinguishing thermal capacitance (= thermal mass) 
> from thermal resistance.  Generally, OCXOs are thermally designed to be 
> "naked" in some abient environment that is close to room temperature 
> (usually somewhat above, anticipating they will be installed in warm 
> instruments and not be sitting naked on a desktop).  They will work 
> acceptably in an ambient environment ranging some tens of degrees C above 
> and below this, as long as the ambient temperature does not change too 
> fast for the oven controller to keep up (for example, the HP 10811 is 
> rated for -55C to 71C, with degraded temperature stability below 0C).
>
> Adding additional thermal resistance (your insulation, or Paul's Dewar) 
> unbalances the heat flow to ambient.  In theory, the oscillator should 
> respond by backing down the heater current to match heater power to the 
> new thermal resistance.  However, raising the thermal resistance can 
> destabilize the thermal control loops in some OCXOs.  When that happens, 
> you get poor thermal regulation at best and you burn down the oscillator 
> at worst.  (And even if the control loop does not destabilize, it won't be 
> running at the design center "sweet spot" foreseen by the designers, so 
> thermal stability will likely be degraded.)
>
> What you want to do is keep the net thermal *resistance* similar to what 
> it is with the oscillator in its target environment, and add thermal 
> *capacitance* to slow down the changes in the oscillator's immediate 
> environment (i.e., just outside the oscillator can).  The metal box does 
> just that.  The thermally resistive path is essentially still all air, 
> just as it is with the oscillator sitting on a desktop -- from the 
> oscillator through the air surrounding it to the inside wall of the cast 
> box, then from the outside wall of the cast box through the air to ambient 
> (to a first approximation, the temperature of the inside wall of the box 
> is the same as the temperature of the outside wall).  This way, the 
> average thermal resistance from the OCXO to ambient is still similar to 
> what the thermal designers contemplated.  However, the cast box averages 
> (integrates) the outside temperature as seen by the OCXO with a time 
> constant measured in tens of minutes.
>
> The net result is that the oscillator is in (or very close to) its design 
> environment with respect to the average heat loss, so the oven controller 
> is working at (or very near) its design sweet spot -- but at the same 
> time, the rate of temperature change seen by the OCXO (i.e., its immediate 
> environment inside the cast box) is integrated over tens of minutes.  If 
> the ambient temperature did not change at all, the OCXO's immediate 
> environment inside the box would be exactly the same as if it were not in 
> the box but, rather, sitting naked on a desktop.  When the ambient 
> temperature changes fast, the OCXO's immediate environment follows it --  
> but much more slowly, giving the oven control loop time to adjust to the 
> change without introducing a transient error.
>
> As long as you end up with sufficient thermal capacitance to slow all 
> expected thermal transients down to the point that the oven control loop 
> has no trouble keeping the quartz crystal in an isothermal state, you have 
> done all you need to do.  There is nothing to calculate, and no 
> complicated design procedure.  If the oscillator still exhibits some 
> transient thermal error, just increase the mass of the box by mounting 
> aluminum plates to the outside walls of the cast box until it doesn't.
>
> The one exception to this would be if the oven controller simply has too 
> little gain to keep the crystal isothermal, in which case there would be a 
> DC (not transient) error (each ambient temperature would be associated 
> with a unique crystal temperature).  Note that this would not be the fault 
> of the method -- rather, it would be a matter of poor regulation by the 
> OCXO's oven controller.  It could be mitigated by active regulation of the 
> cast box temperature using an outer enclosure with a thermostatically 
> controlled fan.
>
> Best regards,
>
> Charles
>
>
>
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