On 5/22/2022 2:21 PM, 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. >Here's a glamour shot of the internals: <https://imgur.com/a/W4hmKt4> Don't know what the foam is. It doesn't seem to conduct heat too good.
Crystal oven theory
Started by ●May 21, 2022
Reply by ●May 22, 20222022-05-22
Reply by ●May 22, 20222022-05-22
In article <bt9j8h5plkg05fmbgr8ra4fj9rqktuup2v@4ax.com>, jlarkin@highlandsniptechnology.com says...> > 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.My design for a foyer quartz clock back in the early 1960s included a crystal in a thermostatic oven made from Perspex. I have a sketch and isometric of the oven and a note that there were five thermistors in a bridge, but no schematic. I do remember that on first testing the control circuit initially ran at full power heating, then cut off, then gradually settled on steady warmth!
Reply by ●May 22, 20222022-05-22
On Sun, 22 May 2022 15:14:12 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>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.JSTW is dealing with a huge delta-t, and vacuum is a poor thermal conductor. I've experimented with heated gadgets inside a deep-drawn aluminum box, which has a low emissivity. My testing usually showed that I got less heater power dissipation with air, as opposed to styrafoam or fiberglas. It's not a difficult experiment. I think most people just assume that insulation always helps. -- Anybody can count to one. - Robert Widlar
Reply by ●May 22, 20222022-05-22
On Sun, 22 May 2022 16:46:44 -0400, bitrex <user@example.net> wrote:>On 5/22/2022 2:21 PM, 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. >> > >Here's a glamour shot of the internals: > ><https://imgur.com/a/W4hmKt4> > >Don't know what the foam is. It doesn't seem to conduct heat too good. > >The interesting test would be to measure heater power with and without the foam. -- Anybody can count to one. - Robert Widlar
Reply by ●May 22, 20222022-05-22
jlarkin@highlandsniptechnology.com wrote:> On Sun, 22 May 2022 15:14:12 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> 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. > > JSTW is dealing with a huge delta-t, and vacuum is a poor thermal > conductor.The delta-T is only huge in the first gap. After that it's much much smaller. The temperature of the first shield is near the subsolar temperature, but the amount it radiates into the first gap is much lower than a black body's, on account of the very low emissivity (i. e. high reflectivity) of its surfaces. The subsolar temperature of a flat plate at a radial distance r from a star of radius a is T_ss = T_star * sqrt(a/( r sqrt(2) ) ), i.e. the temperature at which the thermal emission of the star (sigma T_star**4 * a**2) balances that of a notional sphere of unit emissivity (sigma T_ss**4 * r**2). (The factor of sqrt(2) is there because the plate radiates from both sides--for a spherical planet, whose area is 4 pi R**2, it's a full factor of 2.) The temperature of the solar photosphere is around 6000K, and its radius is 7e5 km. The mean radius of the Earth's orbit is 1.5e8 km. Thus the subsolar temperature for a flat plate is T_ss = 6000 K * sqrt(7e5 km / 1.5e8 km / sqrt(2)) = 334 K. (For a spherical object such as the Earth and Moon, it would be sqrt(sqrt(2)) less than that, i.e. 281 K.) There are two main contributors to the effective thermal resistance of a multilayer stack of spaced reflectors. One is just breaking up the gap into N subgaps, which does what you'd expect--M layers gets you about M times the thermal resistance. (The effect is bigger at larger deltaT, where the emission from inner layers is much less.) The other contributor is that, since the layers of JWST's heat shield are spaced wide apart, the great majority of the thermal emission of each surface bounces around until it escapes into space. Of course the ambient radiation comes in as well, but since its temperature is very low, that's not a big worry.> I've experimented with heated gadgets inside a deep-drawn > aluminum box, which has a low emissivity. My testing usually showed > that I got less heater power dissipation with air, as opposed to > styrafoam or fiberglas. > > It's not a difficult experiment. I think most people just assume that > insulation always helps.Plain air is better than insulation for narrow spaces and small delta-T, and insulation is better otherwise. Just where the break-even point happens depends a lot on the situation. 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
Reply by ●May 23, 20222022-05-23
On Sunday, May 22, 2022 at 4:57:25 PM UTC-7, jla...@highlandsniptechnology.com wrote:> ... I've experimented with heated gadgets inside a deep-drawn > aluminum box, which has a low emissivity. My testing usually showed > that I got less heater power dissipation with air, as opposed to > styrafoam or fiberglas. > > It's not a difficult experiment. I think most people just assume that > insulation always helps.Insulation in the sense of low-conductivity material is NOT the intended function of fiberglass; it is used to reduce air convection, which (thermal convection is delta-T squared heat transfer) is a tiny effect in a low-temperature-gradient system. In a house-on-fire, fiberglass in the walls is not tiny, but a big help in getting out alive. In a very real sense, fiberglass filled cavities are air-insulated.
Reply by ●May 23, 20222022-05-23
On a sunny day (Sun, 22 May 2022 22:51:09 +0100) it happened Mike Coon <gravity@mjcoon.plus.com> wrote in <MPG.3cf4c0956a87c90d9896be@usenet.plus.net>:>My design for a foyer quartz clock back in the early 1960s included a >crystal in a thermostatic oven made from Perspex. I have a sketch and >isometric of the oven and a note that there were five thermistors in a >bridge, but no schematic. I do remember that on first testing the >control circuit initially ran at full power heating, then cut off, then >gradually settled on steady warmth!Yes that is overshot, maybe over-compensated This was pretty good too: http://panteltje.com/panteltje/tri_pic/ Little box inside big box... temperature within a few ADC steps over many years.. Took about half an hour to get the PID working correctly in PIC asm .. For the rest the 1 ppm TCXOs as in my RTL-SDR sticks are very good and very small.
Reply by ●May 23, 20222022-05-23
jlarkin@highlandsniptechnology.com wrote:> On Sun, 22 May 2022 16:46:44 -0400, bitrex <user@example.net> wrote: > >> On 5/22/2022 2:21 PM, 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. >>> >> >> Here's a glamour shot of the internals: >> >> <https://imgur.com/a/W4hmKt4> >> >> Don't know what the foam is. It doesn't seem to conduct heat too good. >> >> > > The interesting test would be to measure heater power with and without > the foam.Air at room temperature has an alpha of about 0.026 W/m/K. High-density styrofoam (the kind that crunches when you poke it with your finger) is about 0.040, and low-density styrofoam (the kind that squeaks) is around 0.030. 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
Reply by ●May 23, 20222022-05-23
Jan Panteltje wrote:> On a sunny day (Sun, 22 May 2022 22:51:09 +0100) it happened Mike Coon > <gravity@mjcoon.plus.com> wrote in > <MPG.3cf4c0956a87c90d9896be@usenet.plus.net>: > >> My design for a foyer quartz clock back in the early 1960s included a >> crystal in a thermostatic oven made from Perspex. I have a sketch and >> isometric of the oven and a note that there were five thermistors in a >> bridge, but no schematic. I do remember that on first testing the >> control circuit initially ran at full power heating, then cut off, then >> gradually settled on steady warmth! > > Yes that is overshot, maybe over-compensatedOr else windup, which is pretty likely if the sensors and heaters aren't well coupled.> > This was pretty good too: > http://panteltje.com/panteltje/tri_pic/ > > Little box inside big box... > > temperature within a few ADC steps over many years.. > Took about half an hour to get the PID working correctly in PIC asm .. > > For the rest the 1 ppm TCXOs as in my RTL-SDR sticks are very good and very small. >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
Reply by ●May 23, 20222022-05-23
On Mon, 23 May 2022 01:18:34 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Sunday, May 22, 2022 at 4:57:25 PM UTC-7, jla...@highlandsniptechnology.com wrote: > > >> ... I've experimented with heated gadgets inside a deep-drawn >> aluminum box, which has a low emissivity. My testing usually showed >> that I got less heater power dissipation with air, as opposed to >> styrafoam or fiberglas. >> >> It's not a difficult experiment. I think most people just assume that >> insulation always helps. > >Insulation in the sense of low-conductivity material is NOT the intended function of >fiberglass; it is used to reduce air convection, which (thermal convection is >delta-T squared heat transfer) is a tiny effect in a low-temperature-gradient system. >In a house-on-fire, fiberglass in the walls is not tiny, but a big help in getting out alive. > >In a very real sense, fiberglass filled cavities are air-insulated.Try it. -- Anybody can count to one. - Robert Widlar
