[time-nuts] Morion MV89A position

Paul A. Cianciolo paulc at snet.net
Wed Jan 29 03:53:38 UTC 2014


Charles,

Thank you for the information and I apologize for responding late.
I understand the concept of the "sweet spot". I made the changes you
mentioned isolate the OCXO thermally from the chasiss.
It is out of the Dewar and sitting on a piece Styrofoam.  It is then mounted
in a chassis that 19" wide 19" deep and 5" or so high.
Sure enough after I made the changes the,  ADev on time lab shows at 2 sec.
all the way up to 3600 sec.
With a sample rate of Hz dictated by the 1 PPS out of the thunder bolt the
reading are below

 2 sec.  1.79 -10,   
20 sec  1.95 -11
200 sec 6.83 - 12
1000 sec 1.70 --12

Can you mention the size of the aluminum box you are using to add thermal
capacitance?
I am very interested to try and improve my results

Thank you


Paul A. Cianciolo
W1VLF
http://www.rescueelectronics.com/
Our business computer network is  powered exclusively by solar and wind
power.
Converting Photons to Electrons for over 20 years










-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Charles Steinmetz
Sent: Friday, January 24, 2014 6:00 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Morion MV89A position

Paul wrote:

>1) Is the fact that the mv89a is inside a Dewar causing any short term 
>or long term adverse effects?

Dewars are appropriate for OCXOs that are designed to work in Dewars.  They
do not generally improve the temperature regulation of OCXOs that were not
designed to work in Dewars, and can significantly compromise the temperature
regulation of some OCXOs.

An oven works by heating the crystal (and often other circuitry) with a
heating element (this is the "pull up"), which is balanced by heat loss to
the ambient environment (this is the "pull down").  By putting the MV89 in a
Dewar, you have reduced the "pull down" that balances the heater.  This
means that the heater must stably deliver much less heat than it was
designed to, and the heater control circuitry is operating far from its
target design point.  It may have enough range to work this way, but it is
not operating at the design-center "sweet spot" that the designers chose.

This applies to any OCXO, and any form of insulation you use.  What you want
to achieve is a "pull-down" rate (rate of heat loss to
ambient) similar to the rate anticipated by the thermal designers, but with
some integration to slow down the change in cooling rate that occurs when
the ambient temperature changes quickly.  This gives the heater control
servo more time to adjust, thereby improving regulation as the ambient
temperature changes.

To accomplish this, you want to add thermal capacitance, NOT thermal
resistance.  The Dewar adds thermal resistance.

My solution is to seal the OCXO up in a fairly heavy cast aluminum box (I
put the OCXO on teflon or nylon standoffs so there is no direct metal
heat-conducting path from the OCXO to the box).  You don't really need to do
anything more than this for most OCXOs.  If you want, you can mount the cast
box in another enclosure (again, on thermally-insulating standoffs) with a
thermostatically-controlled fan.  When I do this, I bond a thermal sensor to
the cast aluminum box and use that to drive the fan, so the cast box remains
at a constant temperature regardless of changes in the ambient temperature.

Best regards,

Charles



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