Re: [time-nuts] Improving the stability of crystal oscillators

[Richard (Rick) Karlquist]

); SAEximRunCond expanded to false
Errors-To: [EMAIL PROTECTED] RETRY


Neon John wrote:

> Nah, not for this application.  A Peltier module typically has a COP of 1.  
> That is,
> it moves a watt of energy for each watt consumed.   Thus, for each watt 
> moved, two
> watts have to be dissipated to air.
> 
> I can't imagine a well-insulated quartz oscillator needing more than a watt 
> or two of
> cooling at the most.  A heat sink capable of handling 4-5 watts should do the 
> job
> just fine.
> 
> Don:  I've seen peltier-controlled "ambient" ovens before but I can't recall 
> the
> details.  I'm fairly sure one was a Fluke precision voltage transfer standard 
> in
> which the zener reference diode was controlled to a constant temperature.

The problem with Peltier coolers is that they have low thermal 
resistance to ambient.  This makes them suitable only for
non critical ovens.

Rick Karlquist N6RK

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Re: [time-nuts] Improving the stability of crystal oscillators

[Bruce Griffiths]

Brooke Clarke wrote:
> Hi Bruce:
>
> Details on your experiment please.
> Hole/pipe diameter, material?
> Depth?
> Delta T at different depths vs surface ambient?
> Soil type?
>
>
> Have Fun,
>
> Brooke Clarke
>   
Brooke

Unable as yet to find my data, it was published in some very obscure
publication if I remember correctly.
However there was extensive series of records kept in England from the
time of Lord Kelvin.

More recent data is available from the US forest service among others:
http://ncrs.fs.fed.us/pubs/rn/rn_nc032.pdf
Above is for Wisconsin, not directly applicable to California.
http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?letter=.&classic=YES&bibcode=1952AuSRA...5..303W&page=&type=SCREEN_VIEW&data_type=PDF_HIGH&send=GET&filetype=.pdf

Above paper by CSIRO is for an Australian site.
Analysis is fairly comprehensive.

http://www.mmm.ucar.edu/mm5/lsm/soil.pdf

http://www.ias.ac.in/epsci/mar2002/Esb1439.pdf

Bruce

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Re: [time-nuts] Improving the stability of crystal oscillators

[Bruce Griffiths]

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Brooke Clarke wrote:
> Hi Bruce:
>
> Details on your experiment please.
> Hole/pipe diameter, material?
> Depth?
> Delta T at different depths vs surface ambient?
> Soil type?
>
>
> Have Fun,
>
> Brooke Clarke
>   
Booke

I will try to find the data, but it was 41 years ago and may take some time.
Location was Palmerston North NZ.
Not sure of soil type, however flat region not too far (within 1km or
so) from a river.

If I remember correctly an access shaft was dug and thermometers were
inserted into the soil at the sides of the hole.
The leads were bought to the surface and the shaft/hole carefully
backfilled and left to settle for some weeks.

All measurements were logged manually.

Bruce

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Re: [time-nuts] Improving the stability of crystal oscillators

[Brooke Clarke]

Hi Bruce:

Details on your experiment please.
Hole/pipe diameter, material?
Depth?
Delta T at different depths vs surface ambient?
Soil type?


Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam


Bruce Griffiths wrote:
> ); SAEximRunCond expanded to false
> Errors-To: [EMAIL PROTECTED] RETRY
> 
> Brooke Clarke wrote:
> 
>>Hi Bruce:
>>
>>I've also looked into drilling a hole (garden hose, 90 deg fitting & pipe 
>>with 
>>end smashed flat to make a nozzle will easily drill as deep as the pipe is 
>>long).  Then using a short length of capped copper pipe at the bottom and the 
>>rest PVC.
>>
>>Like I think here in California wine country 4 feet would be plenty deep.  
>>But 
>>this method only works at your home location.  Not too good for a clock that 
>>will be sent to someone else.
>>
>>I think the long time constant method can be done in a few cubic inches.
>>
>>Have Fun,
>>
>>Brooke Clarke
>>  
> 
> Brooke
> 
> The required depth depends on the soil diffusivity and the temperature
> stability required.
> It is instructive to install thermometers at depth intervals of a foot
> or so and record the temperature fluctuations experienced by each
> thermometer.
> This was first done around 1860 by Forbes.
> I repeated the experiment in 1966.
> 
> Bruce
> 
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Re: [time-nuts] Improving the stability of crystal oscillators

[Bruce Griffiths]

); SAEximRunCond expanded to false
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Brooke Clarke wrote:
> Hi Bruce:
>
> I've also looked into drilling a hole (garden hose, 90 deg fitting & pipe 
> with 
> end smashed flat to make a nozzle will easily drill as deep as the pipe is 
> long).  Then using a short length of capped copper pipe at the bottom and the 
> rest PVC.
>
> Like I think here in California wine country 4 feet would be plenty deep.  
> But 
> this method only works at your home location.  Not too good for a clock that 
> will be sent to someone else.
>
> I think the long time constant method can be done in a few cubic inches.
>
> Have Fun,
>
> Brooke Clarke
>   
Brooke

The required depth depends on the soil diffusivity and the temperature
stability required.
It is instructive to install thermometers at depth intervals of a foot
or so and record the temperature fluctuations experienced by each
thermometer.
This was first done around 1860 by Forbes.
I repeated the experiment in 1966.

Bruce

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Re: [time-nuts] Improving the stability of crystal oscillators

[Don Collie]

Thanks for that John. I`ve always wanted to play arround with one of these 
[Peltier] modules. They can now be bought quite cheaply. I envisage a double 
oven, with the inner oven heated [to 25 degrees], by conventional means, 
while the Peltier pile cools the inner oven. This way you could use
a precision temparature regulator for the inner oven, while the outer "oven" 
would only have to cool. You wouldn`t be talking too many pumped watts, 
here.
FWIW etc.,...Don C.


- Original Message - 
From: "Neon John" <[EMAIL PROTECTED]>
To: "Discussion of precise time and frequency measurement" 

Sent: Sunday, October 14, 2007 11:19 AM
Subject: Re: [time-nuts] Improving the stability of crystal oscillators


> ); SAEximRunCond expanded to false
> Errors-To: [EMAIL PROTECTED] RETRY
>
> Nah, not for this application.  A Peltier module typically has a COP of 1. 
> That is,
> it moves a watt of energy for each watt consumed.   Thus, for each watt 
> moved, two
> watts have to be dissipated to air.
>
> I can't imagine a well-insulated quartz oscillator needing more than a 
> watt or two of
> cooling at the most.  A heat sink capable of handling 4-5 watts should do 
> the job
> just fine.
>
> Don:  I've seen peltier-controlled "ambient" ovens before but I can't 
> recall the
> details.  I'm fairly sure one was a Fluke precision voltage transfer 
> standard in
> which the zener reference diode was controlled to a constant temperature.
>
> The advantage of using room temperature, e.g., 70 deg F, is that under 
> most
> conditions, the peltier module is doing little to nothing, perhaps just 
> ridding the
> ovenized unit of the few milliwatts dissipated in the circuit itself.
>
> I've used multiple cascaded modules to cool a nuclear detector (Silicon 
> surface
> barrier diode) to reduce its noise. Not as good as LN2 but much cheaper to 
> operate.
>
> John
>
> On Sat, 13 Oct 2007 13:26:52 -0700, Brooke Clarke <[EMAIL PROTECTED]> 
> wrote:
>
>>Hi Don:
>>
>>They are very inefficient to the point that a system that's supposed to 
>>cool
>>something may heat it because of all the heat generated by the module.
>>
>>It takes a huge amount of heat sinking or liquid cooling to get them to 
>>work.
>
>>Don Collie wrote:
>>> ); SAEximRunCond expanded to false
>>> Errors-To: [EMAIL PROTECTED] RETRY
>>>
>>> Has anyone concidered using a small Peltier pile to maintain the 
>>> crystal`s
>>> temparature. I understand that these devices will heat or cool, so it 
>>> would
>>> be possible to maintain the crystal temparature at , say, 25 degrees
>>> celcius, over a range of ambient temparatures
>>> [perhaps 0 to 70  degrees]. There would be several advantages in this
>>> approach.
> --
> John De Armond
> See my website for my current email address
> http://www.neon-john.com
> http://www.johndearmond.com <-- best little blog on the net!
> Tellico Plains, Occupied TN
> I like you ... you remind me of me when I was young and stupid.
>
>
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>
>
> -- 
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.5.488 / Virus Database: 269.14.9/1069 - Release Date: 
> 10/13/2007 7:26 PM
> 


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Re: [time-nuts] Improving the stability of crystal oscillators

[Brooke Clarke]

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Hi Dick:

 From what I can Google he built maybe the first parametric amplifier but 
haven't come across details on the crystal temp stabilization.  Do you have 
info about that?

Because of the year involved I think he must have just used the long time 
constant to slow down the changes, or was he actually using the time of day as 
a function of frequency?

Have Fun,

Brooke Clarke, N6GCE
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam


Richard W. Solomon wrote:
> ); SAEximRunCond expanded to false
> Errors-To: [EMAIL PROTECTED] RETRY
> 
> Sam Harris (SK) did that over 40 years ago. It provided a stable
> reference for his pioneering work in Moonbounce Communication.
> 
> 73, Dick, W1KSZ/7
> 
> -Original Message-
> 
>>From: Bruce Griffiths <[EMAIL PROTECTED]>
>>Sent: Oct 13, 2007 2:44 PM
>>To: Discussion of precise time and frequency measurement 
>>Subject: Re: [time-nuts] Improving the stability of crystal oscillators
>>
>>Brooke Clarke wrote:
>>
>>>Hi:
>>>
>>>Something I've been thinking about is attaching a large thermal mass of 
>>>aluminum to the crystal and it's oscillator then surrounding that with 
>>>insulation.  Then surround that with a thin copper layer.  Temperature 
>>>sensors 
>>>on the copper and aluminum.  If the thermal time constant could be made very 
>>>close to 24 hours, or integer multiples of that, it might be possible to 
>>>predict the inner temperature allowing compensation.
>>>
>>>Has this already been done?
>>>
>>>  
>>
>>Brooke
>>
>>Why stop there?
>>Try burying the oscillator under 6ft or so of soil.
>>The daily temperature fluctuations at this depth are very small.
>>
>>Bruce
>>
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> 
> 
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Re: [time-nuts] Improving the stability of crystal oscillators

[Bill Powell]

I doug up and included below a time-nuts email from a couple of years 
ago on improving oscillator stability, with some thoughts from Jack 
Kusters, Tom Clark, and Brooke Clarke.

Also, there's a good history of high precision oscillators at:

http://www.ieee-uffc.org/fc_history/norton.html

Regards,
Bill Powell
af4jg



FROM: 
<[EMAIL PROTECTED]>
 | Save Address

DATE:   Wed, 26 Oct 2005 13:20:14 -0700
TO:   <[EMAIL PROTECTED]>,  SUBJECT:   
RE: [time-nuts] Xtal Oscillator Aging

Hi,

 I posed this question to Jack Kusters, now retired
from HP/Agilent.  He and Charles Adams commercialized
the SC-cut crystal for HP in the 10811A oscillator.  He
gave me permission to post his response on the reflector.

Jim Johnson
[EMAIL PROTECTED]

===

Hi Jim,

In addition to everything Tom Clark said (I agree in general with his
explanation), there is another aging mechanism associated with
stress.  When the crystal blank is manufactured, it is sawn, lapped,
ground, etched, and otherwise abused.  All of this produces stress in
the blank. In addition, there are mounting stresses that arise because
of the way the blank is mounted on its header, surface stresses that
develop because the electrode material when evaporated and then
condensed on the surface shrinks as it cools.

All of these result in long-term aging as these stresses need to
equilibrate out.  In addition, there are other stress related mechanisms
that may result in either long- or short- term aging.  The quartz
material is anisotropic, the mounts, electrode material are isotropic.

So, lets assume that we've had the crystal at an elevated, constant
temperature.  Over a period of time, all stresses, material, mount,
electrode, cracks, etc. equilibrate to their lowest energy level and it
appears that aging has stopped.

Now, take it down in temperature.  The anisotropic quartz and the
isotropic mount and electrode, have different contraction rates, so the
overall system now has a new set of stresses.

Let the unit come to full equilibrium at the new, lower
temperature.  Now take it up in temperature to where it was before.  Now
we see a whole new set of aging and stress relief.  The only virtue is
that aging due to cracks and material stress from manufacturing
processes should be mostly gone, so the unit should come to equilibrium
much faster.

One further comment, glass sealed crystals are not necessarily the best
way to seal a crystal.  It takes heat from a source sufficiently
elevated in temperature to melt the glass.  This tends to cause
contaminents to migrate from the area being sealed to a cooler spot in
the package, usually the crystal.  Contaminents come from gasses from
the torch or from junk trapped in the glass.

The cleanest mount one can do is a cold-weld seal under proper conditions.
For example, the HP crystals were put into a vacuum furnace, heated to
300+ deg-C overnite at 10E-7 torr with the can stored next to the
crystal.  After reducing the temperature to about 80-84 deg-C, the
crystal was frequency plated to within several parts in 10E7.  After
that, the mount was placed in the can, the temperature raised up to
about 150 deg-C, stabilised in temperature and vacuum, then cold-welded.

Done properly, there is essentially no contamination inside the crystal
assembly, most of the other stresses are gone, and the typical HP SC-cut
would reach an aging rate of better than 1E-7 per day, within the first
5 days.

Best regards,
Jack Kusters

==

-Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On
Behalf Of Tom Clark, W3IWI
Sent: Sunday, October 23, 2005 9:21 PM
To: time-nuts@febo.com
Subject: [time-nuts] Xtal Oscillator Aging


  Brooke (no relation) commented

Hi Richard:

>
> It's my understanding that this optimization can be done by changing
> the oscillator power level at the crystal.
>
> In the case of the 32768 Hz watch crystal, it must be run a very low
> power and it has a very low aging rate when compared to higher
> frequency crystals that are typically run at higher power levels.  I
> think this is related to the crystal throwing off atoms, so more power
> means more acceleration and more atoms thrown off.
>
> Have Fun,
>
> Brooke Clarke, N6GCE


  It has nothing to do with "throwing off atoms". A Xtal is actually a
  mechanical oscillator, with the quartz slab vibrating (in either its
  fundamental mode, or on an odd overtone); quartz is a piezo-electric
  material so the voltage across the pins of the xtal has a direct
  connection to the mechanical vibration. When an xtal oscillator starts
  up, the associated amplifier generates noise, which then starts the
  xtal vibrating, which generates signal at the right frequency and a
  feedback loop is set up. When you crank up the power to the mechanical
  resonator, the signal increases with

Re: [time-nuts] Improving the stability of crystal oscillators

[Brooke Clarke]

); SAEximRunCond expanded to false
Errors-To: [EMAIL PROTECTED] RETRY

Hi Bruce:

I've also looked into drilling a hole (garden hose, 90 deg fitting & pipe with 
end smashed flat to make a nozzle will easily drill as deep as the pipe is 
long).  Then using a short length of capped copper pipe at the bottom and the 
rest PVC.

Like I think here in California wine country 4 feet would be plenty deep.  But 
this method only works at your home location.  Not too good for a clock that 
will be sent to someone else.

I think the long time constant method can be done in a few cubic inches.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam


Bruce Griffiths wrote:
> ); SAEximRunCond expanded to false
> Errors-To: [EMAIL PROTECTED] RETRY
> 
> Brooke Clarke wrote:
> 
>>Hi:
>>
>>Something I've been thinking about is attaching a large thermal mass of 
>>aluminum to the crystal and it's oscillator then surrounding that with 
>>insulation.  Then surround that with a thin copper layer.  Temperature 
>>sensors 
>>on the copper and aluminum.  If the thermal time constant could be made very 
>>close to 24 hours, or integer multiples of that, it might be possible to 
>>predict the inner temperature allowing compensation.
>>
>>Has this already been done?
>>
>>  
> 
> Brooke
> 
> Why stop there?
> Try burying the oscillator under 6ft or so of soil.
> The daily temperature fluctuations at this depth are very small.
> 
> Bruce
> 
> ___
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Re: [time-nuts] Improving the stability of crystal oscillators

[Richard W. Solomon]

Sam Harris (SK) did that over 40 years ago. It provided a stable
reference for his pioneering work in Moonbounce Communication.

73, Dick, W1KSZ/7

-Original Message-
>From: Bruce Griffiths <[EMAIL PROTECTED]>
>Sent: Oct 13, 2007 2:44 PM
>To: Discussion of precise time and frequency measurement 
>Subject: Re: [time-nuts] Improving the stability of crystal oscillators
>
>Brooke Clarke wrote:
>> Hi:
>>
>> Something I've been thinking about is attaching a large thermal mass of 
>> aluminum to the crystal and it's oscillator then surrounding that with 
>> insulation.  Then surround that with a thin copper layer.  Temperature 
>> sensors 
>> on the copper and aluminum.  If the thermal time constant could be made very 
>> close to 24 hours, or integer multiples of that, it might be possible to 
>> predict the inner temperature allowing compensation.
>>
>> Has this already been done?
>>
>>   
>Brooke
>
>Why stop there?
>Try burying the oscillator under 6ft or so of soil.
>The daily temperature fluctuations at this depth are very small.
>
>Bruce
>
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Re: [time-nuts] Improving the stability of crystal oscillators

[Tim Shoppa]

); SAEximRunCond expanded to false
Errors-To: [EMAIL PROTECTED] RETRY

"Don Collie" <[EMAIL PROTECTED]> wrote:
> Has anyone concidered using a small Peltier pile to maintain the crystal`s 
> temparature. I understand that these devices will heat or cool, so it would 
> be possible to maintain the crystal temparature at , say, 25 degrees 
> celcius, over a range of ambient temparatures
> [perhaps 0 to 70  degrees]. There would be several advantages in this 
> approach.
> Cheers!,Don Collie jnr. 

Overall, the Petier devices don't heat or cool, just heat :-).

I don't know about crystal oven applications, but condensation on
the cold side has ruled out several applications for Peltiers
at my day job.

Tim.

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Re: [time-nuts] Improving the stability of crystal oscillators

[Bruce Griffiths]

Brooke Clarke wrote:
> Hi Don:
>
> They are very inefficient to the point that a system that's supposed to cool 
> something may heat it because of all the heat generated by the module.
>
> It takes a huge amount of heat sinking or liquid cooling to get them to work.
>
> Have Fun,
>
> Brooke Clarke
>   
Brooke

The answer depends on how much heat has to be pumped.

Peltier heat pumps are quite effective as long as the amount of heat to
be pumped is relatively small.
With a 70W module a water cooled or blown heatsink is advisable.
With a 1W module an aircooled heatsink is adequate.
Small Peltier heatpump have been used to stabilise the temperature of 
photodiodes etc (HP used them in the 8153 light wave multimeter optical
detector heads).
They have also been successfully used to stabilise the temperature of a
baseplate with devices dissipating 10W or so attached to the baseplate.
Peltier modules are also used in small aerogel insulated refrigerators
and in drink coolers.

One potential problem is that low power peltier modules are very small
so that a high thermal conductivity heat spreader is desirable to
improve the temperature uniformity.
Single stage peltier modules are quite thin so that a thick low thermal
resistance heat-spreader/spacer is required to allow a sufficiently
thick layer on insulation to be used.

Bruce


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Re: [time-nuts] Improving the stability of crystal oscillators

[Neon John]

); SAEximRunCond expanded to false
Errors-To: [EMAIL PROTECTED] RETRY

Nah, not for this application.  A Peltier module typically has a COP of 1.  
That is,
it moves a watt of energy for each watt consumed.   Thus, for each watt moved, 
two
watts have to be dissipated to air.

I can't imagine a well-insulated quartz oscillator needing more than a watt or 
two of
cooling at the most.  A heat sink capable of handling 4-5 watts should do the 
job
just fine.

Don:  I've seen peltier-controlled "ambient" ovens before but I can't recall the
details.  I'm fairly sure one was a Fluke precision voltage transfer standard in
which the zener reference diode was controlled to a constant temperature.

The advantage of using room temperature, e.g., 70 deg F, is that under most
conditions, the peltier module is doing little to nothing, perhaps just ridding 
the
ovenized unit of the few milliwatts dissipated in the circuit itself.

I've used multiple cascaded modules to cool a nuclear detector (Silicon surface
barrier diode) to reduce its noise. Not as good as LN2 but much cheaper to 
operate.

John 

On Sat, 13 Oct 2007 13:26:52 -0700, Brooke Clarke <[EMAIL PROTECTED]> wrote:

>Hi Don:
>
>They are very inefficient to the point that a system that's supposed to cool 
>something may heat it because of all the heat generated by the module.
>
>It takes a huge amount of heat sinking or liquid cooling to get them to work.

>Don Collie wrote:
>> ); SAEximRunCond expanded to false
>> Errors-To: [EMAIL PROTECTED] RETRY
>> 
>> Has anyone concidered using a small Peltier pile to maintain the crystal`s 
>> temparature. I understand that these devices will heat or cool, so it would 
>> be possible to maintain the crystal temparature at , say, 25 degrees 
>> celcius, over a range of ambient temparatures
>> [perhaps 0 to 70  degrees]. There would be several advantages in this 
>> approach.
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com <-- best little blog on the net!
Tellico Plains, Occupied TN
I like you ... you remind me of me when I was young and stupid.


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Re: [time-nuts] Improving the stability of crystal oscillators

[Bruce Griffiths]

); SAEximRunCond expanded to false
Errors-To: [EMAIL PROTECTED] RETRY

Brooke Clarke wrote:
> Hi:
>
> Something I've been thinking about is attaching a large thermal mass of 
> aluminum to the crystal and it's oscillator then surrounding that with 
> insulation.  Then surround that with a thin copper layer.  Temperature 
> sensors 
> on the copper and aluminum.  If the thermal time constant could be made very 
> close to 24 hours, or integer multiples of that, it might be possible to 
> predict the inner temperature allowing compensation.
>
> Has this already been done?
>
>   
Brooke

Why stop there?
Try burying the oscillator under 6ft or so of soil.
The daily temperature fluctuations at this depth are very small.

Bruce

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[time-nuts] Improving the stability of crystal oscillators

[Brooke Clarke]

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Hi:

Something I've been thinking about is attaching a large thermal mass of 
aluminum to the crystal and it's oscillator then surrounding that with 
insulation.  Then surround that with a thin copper layer.  Temperature sensors 
on the copper and aluminum.  If the thermal time constant could be made very 
close to 24 hours, or integer multiples of that, it might be possible to 
predict the inner temperature allowing compensation.

Has this already been done?

-- 
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam

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Re: [time-nuts] Improving the stability of crystal oscillators

[Brooke Clarke]

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Hi Don:

They are very inefficient to the point that a system that's supposed to cool 
something may heat it because of all the heat generated by the module.

It takes a huge amount of heat sinking or liquid cooling to get them to work.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Cam


Don Collie wrote:
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> 
> Has anyone concidered using a small Peltier pile to maintain the crystal`s 
> temparature. I understand that these devices will heat or cool, so it would 
> be possible to maintain the crystal temparature at , say, 25 degrees 
> celcius, over a range of ambient temparatures
> [perhaps 0 to 70  degrees]. There would be several advantages in this 
> approach.
> Cheers!,Don Collie jnr. 
> 
> 
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[time-nuts] Improving the stability of crystal oscillators

[Don Collie]

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Has anyone concidered using a small Peltier pile to maintain the crystal`s 
temparature. I understand that these devices will heat or cool, so it would 
be possible to maintain the crystal temparature at , say, 25 degrees 
celcius, over a range of ambient temparatures
[perhaps 0 to 70  degrees]. There would be several advantages in this 
approach.
Cheers!,Don Collie jnr. 


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