Forums

Crystal oven theory

Started by bitrex May 21, 2022
Here's a paper on the theory of crystal ovens:

<http://www.karlquist.com/oven.pdf>

There's a lot more to it than just slapping a crystal & heater in a 
metal box and calling it a day! There are probably some pathologically 
bad geometries even a really fast control loop can never stabilize very 
well.

The part about the outer can acting like a Faraday shield is 
interesting, shunting ambient gradients around the core. Does anyone 
know how they make the negative space look like a linear tehrmal 
resistance over a wide range? There's some kind of insulating foam in 
there, is main heat thermal transfer radiative or conduction?

I picked up an otherwise really nice HP 5334b frequency counter with 
both the 1.3 GHz input and OCXO options for a song. Unfortunately 
there's a fault somewhere inside the OCXO module and it outputs about 
1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the 
Colpitts section and the crystal and heater sections are OK.

The heater transistors screws to the internal mass have to be re-torqued 
to spec if you mess with them, I didn't mess with 'em.

bitrex wrote:
> Here's a paper on the theory of crystal ovens: > > <http://www.karlquist.com/oven.pdf> > > There's a lot more to it than just slapping a crystal & heater in a > metal box and calling it a day! There are probably some pathologically > bad geometries even a really fast control loop can never stabilize very > well. > > The part about the outer can acting like a Faraday shield is > interesting, shunting ambient gradients around the core. Does anyone > know how they make the negative space look like a linear tehrmal > resistance over a wide range? There's some kind of insulating foam in > there, is main heat thermal transfer radiative or conduction? > > I picked up an otherwise really nice HP 5334b frequency counter with > both the 1.3 GHz input and OCXO options for a song. Unfortunately > there's a fault somewhere inside the OCXO module and it outputs about > 1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the > Colpitts section and the crystal and heater sections are OK. > > The heater transistors screws to the internal mass have to be re-torqued > to spec if you mess with them, I didn't mess with 'em. >
Rick Karlquist used to post here from time to time--a very smart guy. (I know him slightly from a side gig some years back, before he retired.) He also did some interesting ham radio work on direct frequency synthesizers based on cheap 455 kHz ceramic IF filters, and got pretty impressive results. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Am 22.05.22 um 03:46 schrieb Phil Hobbs:

> Rick Karlquist used to post here from time to time--a very smart guy. (I > know him slightly from a side gig some years back, before he retired.) > > He also did some interesting ham radio work on direct frequency > synthesizers based on cheap 455 kHz ceramic IF filters, and got pretty > impressive results.
10.7 MHz filters IIRC. He is a regular in the Time Nuts email group on febo.com.
> Cheers
Gerhard
On Sat, 21 May 2022 20:50:12 -0400, bitrex <user@example.net> wrote:

>Here's a paper on the theory of crystal ovens: > ><http://www.karlquist.com/oven.pdf> > >There's a lot more to it than just slapping a crystal & heater in a >metal box and calling it a day! There are probably some pathologically >bad geometries even a really fast control loop can never stabilize very >well. > >The part about the outer can acting like a Faraday shield is >interesting, shunting ambient gradients around the core. Does anyone >know how they make the negative space look like a linear tehrmal >resistance over a wide range? There's some kind of insulating foam in >there, is main heat thermal transfer radiative or conduction? > >I picked up an otherwise really nice HP 5334b frequency counter with >both the 1.3 GHz input and OCXO options for a song. Unfortunately >there's a fault somewhere inside the OCXO module and it outputs about >1.8 MHz instead of 10...seems to be maybe a bad cap somewhere around the >Colpitts section and the crystal and heater sections are OK. > >The heater transistors screws to the internal mass have to be re-torqued >to spec if you mess with them, I didn't mess with 'em.
Here's my EO modulator oven, with the big top cover removed. https://www.dropbox.com/s/f6h8tfyq0xkqx1q/Oven_Cables_pub.jpg?raw=1 There are six mosfets on the big blue board on the bottom of the box as heaters. The temp sensors are thermistor wheatstone bridges on the small blue board on the bottom of the modulator, with a 24-bit ADC. The modulator is mounted on spacers inside the main oven block, not very thermally conductive, so thermally it's a second-order system. It's stable to way better than a millikelvin. Note the SMA feedthrus and long squiggly cables, which reduce cold sneaking into the EOM over the coaxes. -- Anybody can count to one. - Robert Widlar
On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote:
> On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote: > > >Here's a paper on the theory of crystal ovens: > > > ><http://www.karlquist.com/oven.pdf>
He didn't survey the literature all that well.
> >There's a lot more to it than just slapping a crystal & heater in a > >metal box and calling it a day! There are probably some pathologically > >bad geometries even a really fast control loop can never stabilize very > >well. > > > >The part about the outer can acting like a Faraday shield is > >interesting, shunting ambient gradients around the core. Does anyone > >know how they make the negative space look like a linear tehrmal > >resistance over a wide range? There's some kind of insulating foam in > >there, is main heat thermal transfer radiative or conduction?
Conduction. Foam pretty much stops convection.
> >The heater transistors screws to the internal mass have to be re-torqued > >to spec if you mess with them, I didn't mess with 'em. > > Here's my EO modulator oven, with the big top cover removed. > > https://www.dropbox.com/s/f6h8tfyq0xkqx1q/Oven_Cables_pub.jpg?raw=1 > > There are six mosfets on the big blue board on the bottom of the box > as heaters. > > The temp sensors are thermistor wheatstone bridges on the small blue > board on the bottom of the modulator, with a 24-bit ADC. The modulator > is mounted on spacers inside the main oven block, not very thermally > conductive, so thermally it's a second-order system. It's stable to > way better than a millikelvin.
People have been managing that since the 1960's. It's not easy, though good inter-changeable thermistors and 20-bit A/D converter chips have made it easier.
> Note the SMA feedthrus and long squiggly cables, which reduce cold sneaking into the EOM over the coaxes.
Probably more helpful to say that they increase the thermal time constant of modulator whose temperature is being controlled. -- Bill Sloman, Sydney
Gerhard Hoffmann wrote:
> Am 22.05.22 um 03:46 schrieb Phil Hobbs: > >> Rick Karlquist used to post here from time to time--a very smart guy. >> (I know him slightly from a side gig some years back, before he retired.) >> >> He also did some interesting ham radio work on direct frequency >> synthesizers based on cheap 455 kHz ceramic IF filters, and got pretty >> impressive results. > > 10.7 MHz filters IIRC.
Right, thanks. Cool gizmo, anyway.
> He is a regular in the Time Nuts email group on febo.com.
Cheers Phil Hobbsn -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On 5/22/2022 4:31 AM, Anthony William Sloman wrote:
> On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote: >> On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote: >> >>> Here's a paper on the theory of crystal ovens: >>> >>> <http://www.karlquist.com/oven.pdf> > > He didn't survey the literature all that well. > >>> There's a lot more to it than just slapping a crystal & heater in a >>> metal box and calling it a day! There are probably some pathologically >>> bad geometries even a really fast control loop can never stabilize very >>> well. >>> >>> The part about the outer can acting like a Faraday shield is >>> interesting, shunting ambient gradients around the core. Does anyone >>> know how they make the negative space look like a linear tehrmal >>> resistance over a wide range? There's some kind of insulating foam in >>> there, is main heat thermal transfer radiative or conduction? > > Conduction. Foam pretty much stops convection.
It was mentioned on another thread that the thermal resistance of radiation between two concentric blackbodies at 300K is about 6mm, interestingly that's very close to the same thickness of foam-filled negative space between the outer and inner shells of the OCXO module on my 5334B. Don't know if that's a coincidence or not, obviously the two aren't at exactly 300K. The interior I think runs about 355K, while the exterior (I haven't measured it) seems cool enough to keep your hand on a while.
On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote:

>On 5/22/2022 4:31 AM, Anthony William Sloman wrote: >> On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote: >>> On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote: >>> >>>> Here's a paper on the theory of crystal ovens: >>>> >>>> <http://www.karlquist.com/oven.pdf> >> >> He didn't survey the literature all that well. >> >>>> There's a lot more to it than just slapping a crystal & heater in a >>>> metal box and calling it a day! There are probably some pathologically >>>> bad geometries even a really fast control loop can never stabilize very >>>> well. >>>> >>>> The part about the outer can acting like a Faraday shield is >>>> interesting, shunting ambient gradients around the core. Does anyone >>>> know how they make the negative space look like a linear tehrmal >>>> resistance over a wide range? There's some kind of insulating foam in >>>> there, is main heat thermal transfer radiative or conduction? >> >> Conduction. Foam pretty much stops convection. > >It was mentioned on another thread that the thermal resistance of >radiation between two concentric blackbodies at 300K is about 6mm, >interestingly that's very close to the same thickness of foam-filled >negative space between the outer and inner shells of the OCXO module on >my 5334B. > >Don't know if that's a coincidence or not, obviously the two aren't at >exactly 300K. The interior I think runs about 355K, while the exterior >(I haven't measured it) seems cool enough to keep your hand on a while. >
In a reasonably close-fitting box, without a lot of space for convection, air is a better thermal insulator than foam or fiberglass. -- Anybody can count to one. - Robert Widlar
jlarkin@highlandsniptechnology.com wrote:
> On Sun, 22 May 2022 13:24:02 -0400, bitrex <user@example.net> wrote: > >> On 5/22/2022 4:31 AM, Anthony William Sloman wrote: >>> On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote: >>>> On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote: >>>> >>>>> Here's a paper on the theory of crystal ovens: >>>>> >>>>> <http://www.karlquist.com/oven.pdf> >>> >>> He didn't survey the literature all that well. >>> >>>>> There's a lot more to it than just slapping a crystal & heater in a >>>>> metal box and calling it a day! There are probably some pathologically >>>>> bad geometries even a really fast control loop can never stabilize very >>>>> well. >>>>> >>>>> The part about the outer can acting like a Faraday shield is >>>>> interesting, shunting ambient gradients around the core. Does anyone >>>>> know how they make the negative space look like a linear tehrmal >>>>> resistance over a wide range? There's some kind of insulating foam in >>>>> there, is main heat thermal transfer radiative or conduction? >>> >>> Conduction. Foam pretty much stops convection. >> >> It was mentioned on another thread that the thermal resistance of >> radiation between two concentric blackbodies at 300K is about 6mm, >> interestingly that's very close to the same thickness of foam-filled >> negative space between the outer and inner shells of the OCXO module on >> my 5334B. >> >> Don't know if that's a coincidence or not, obviously the two aren't at >> exactly 300K. The interior I think runs about 355K, while the exterior >> (I haven't measured it) seems cool enough to keep your hand on a while. >> > > In a reasonably close-fitting box, without a lot of space for > convection, air is a better thermal insulator than foam or fiberglass.
That depends very much on the thermal emissivity of the surfaces and on the thickness of the gap. The JWST doesn't have to deal with a lot of convection, but its sun shield has many layers on account of the radiation issue. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On Monday, May 23, 2022 at 5:14:27 AM UTC+10, Phil Hobbs wrote:
> jla...@highlandsniptechnology.com wrote: > > On Sun, 22 May 2022 13:24:02 -0400, bitrex <us...@example.net> wrote: > > > >> On 5/22/2022 4:31 AM, Anthony William Sloman wrote: > >>> On Sunday, May 22, 2022 at 1:15:05 PM UTC+10, jla...@highlandsniptechnology.com wrote: > >>>> On Sat, 21 May 2022 20:50:12 -0400, bitrex <us...@example.net> wrote: > >>>> > >>>>> Here's a paper on the theory of crystal ovens: > >>>>> > >>>>> <http://www.karlquist.com/oven.pdf> > >>> > >>> He didn't survey the literature all that well. > >>> > >>>>> There's a lot more to it than just slapping a crystal & heater in a > >>>>> metal box and calling it a day! There are probably some pathologically > >>>>> bad geometries even a really fast control loop can never stabilize very > >>>>> well. > >>>>> > >>>>> The part about the outer can acting like a Faraday shield is > >>>>> interesting, shunting ambient gradients around the core. Does anyone > >>>>> know how they make the negative space look like a linear tehrmal > >>>>> resistance over a wide range? There's some kind of insulating foam in > >>>>> there, is main heat thermal transfer radiative or conduction? > >>> > >>> Conduction. Foam pretty much stops convection. > >> > >> It was mentioned on another thread that the thermal resistance of > >> radiation between two concentric blackbodies at 300K is about 6mm, > >> interestingly that's very close to the same thickness of foam-filled > >> negative space between the outer and inner shells of the OCXO module on > >> my 5334B. > >> > >> Don't know if that's a coincidence or not, obviously the two aren't at > >> exactly 300K. The interior I think runs about 355K, while the exterior > >> (I haven't measured it) seems cool enough to keep your hand on a while. > > > > In a reasonably close-fitting box, without a lot of space for > > convection, air is a better thermal insulator than foam or fiberglass.
You add foam or fibre-glass to stop air convecting.Both the walls of the foam cells and the glass in the glass-fibre have much higher thermal conductivities than air, but you don't put in enough to short-circuit the air. Convection does depend on the space available. 6mm isn't much. https://en.wikipedia.org/wiki/Rayleigh_number If it is less than 500 you don't get significant heat transfer by convection. If it's more than 100,000 you get turbulent convection
> That depends very much on the thermal emissivity of the surfaces and on > the thickness of the gap. The JWST doesn't have to deal with a lot of > convection, but its sun shield has many layers on account of the > radiation issue.
Radiation goes as the fourth power of temperature, and the sun is at 6000K. It's not really a relevant example. -- Bill Sloman. Sydney