Re: CTI OSC5A2B02 OXCO module high precision frequency reference project
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You might need to consider some kind of chopper-type amplifier for stabilizing the voltage at those very low input offsets and microvolt-ranges over long time scales.? Otherwise drift is going to be problematic over minutes-to-hours time scale as even small input offsets vary.? Even a LM399 has to have its own aging effects, given it's constructed from diffusion, ion implantation, and other deposition processes that experience relaxation over time.? These are likely to be significant over a year. ?
Also, trying to fit the various exponential-type aging processes, which could vary over orders of magnitude of time, to a sum of exponentials is going to be a poorly conditioned problem.? This is the kind of thing that NIST standards were created for.? I suppose it's worth a try, but I think it would need to be compared to something like a rubidium clock.
On Mon, Aug 7, 2023 at 8:42?PM Reginald Beardsley via <pulaskite= [email protected]> wrote: I've?discussed?these?in?threads?on?several?lists?and?concluded?that?it?would?be?best?to?stick?to?a?single?thread?here.
These?are?available?for?$3?each?on?ebay.??They?are?salvaged?from?old?cell?tower?GPSDOs.??The?new?specs?are?very?good?and?they?are?well?aged?so?long?term?stability?should?be?excellent?and?an?order?of?magnitude?or?more?better?than?a?new?OXCO.??10?year?aging?rate?is?specified?at?0.4?ppb/yr.
The?tuning?rate?is?1?ppb/mV?or?0.001?ppb/uV.??On?the??datasheet?the?short?term?stability?is?stated?as?0.02?ppb/s.??That?happens?to?correspond?to?the?20?uVrms?spec?of?an?LM399?which?are?also?available?as?salvage?parts?and?which?I?plan?to?use?for?the?Vref.??That?gives?me?some?modest?hope?that?if?I?clean?up?the?noise?and?other?factors?I?may?be?able?to?achieve?0.01?ppb.
There?are?a?couple?of?Chinese?PCBs?that?have?one?of?these?on?them.??I?have?two?of?the?version?with?the?trimmer?next?to?the?OXCO?module?that?are?0.1?ppb?after?adjustment?using?a?GPSDO?and?DSO.??Two?others?have?drifted?out?of?range?of?the?tuning?voltage?divider?and?can't?be?set?to?10?MHz?without?changing?the?fixed?resistors?in?the?divider.
After?some?study,?I've?concluded?that?the?main?limitation?on?the?stability?of?these?is?power?supply?noise?and?temperature?stability.??I'd?like?to?get?to?0.01?ppb/yr?just?to?see?if?I?can.??The?10x?rule?of?thumb?dictates?that?I?need?noise?and?voltage?drift?in?the?single?uV?range?which?will?not?be?easy?or?quick.
Preliminary?error?budget?analysis?reveals?that?mains?line?voltage?varies?+/-?5%?between?114?and?126?V.??After?reviewing?the?datasheet?for??the?LM317?it?appears?that?a?mains?supply?which?uses?two?LM317s?in?series?with?an?AD?584JH?to?set?the?10?V?to?feed?the?LM399?will?prevent?mains?fluctuations?from?affecting?the?LM399?output.
There?are?also?temperature,?device?noise?and?EMI?to?contend?with?which?I?shall?get?to?in?due?course.??To?save?time?and?effort?I?am?going?to?attempt?to?use?Chinese?LM317?and?AD584?PCBs?which?I?have?on?hand?to?supply?the?LM399?and?an?AD8429?ultra?low?noise?instrumentation?amp?to?buffer?the?LM399?from?the?OXCO?Vctl?line.?The?20?uVrms?noise?of?the?LM399?will?need?to?be?filtered?out?and?I'll?have?to?find?a?truce?among?the?various?tempcos.
In?general?I?intend?to?follow?Ken?Thompson's?aphorism,?"When?in?doubt,?use?brute?force."?and?rely?on?lots?of?filtering?from?the?wall?to?the?OXCO?and?several?levels?of?thermal?isolation?and?regulation.
As?I?am?certain?that?any?knowledgeable?reader?will?grasp?that?0.01?ppb/yr?for?an?OXCO?is?wildly?ambitious,?I?should?like?to?explain?why?I?think?it?might?be?possible.
The??viscothermoelastoplastic?continuum?mechanics?of?crystals?and?voltage?references?produces?an?aging?profile?which?asymptotically?approaches?a?constant?value.??There?can?be?several?different?aging?rates?which?combine,?but?after?enough?time?one?will?come?to?dominate?the?others.??In?general?the?form?of?the?functions?is?very?similar?to?the?1D?heat?equation?with?its?infinite?sum?of?exponentials.
Some?years?ago?I?spent?quite?a?bit?of?time?studying?sparse,?underdetermined?solutions?of?arbitrary?sums?of?such?functions.??In?my?numerical?experiments?I?found?that?I?could?predict?future?behavior?to?within?1-2%?for?a?period?in?the?future?equal?to?the?duration?of?the?prior?observations.That?result?is?due?to?the?work?by?Emannuel?Candes?and?David?Donoho?in?the?2004?-2008?period.??In?particular,?Donoho?proved?in?2004?that?if?and?only?if?a?sparse?L1?solution?to?an?underdetermined?system?of?linear?equations?existed?it?was?the?L0?optimal?solution?which?is?NP-Hard.
I?was?already?solving?massively?underdetermined?systems?with?50,000?unknowns?and?100?or?fewer?equations?following?the?discussion?of?bais?pursuit?in?the?3rd?edition?of?Mallat's?"A?Wavelet?Tour?of?Signal?Processing"?when?it?sank?it?I?had?always?been?taught?that?was?not?possible?because?there?were?an?infinite?number?of?solutions.??As?a?result?I?spent?the?next?3?years?reading?3000?pages?of?the?most?difficult?math?I've?ever?seen.??A?single?proof?in?one?of?Donoho's?papers?ran?14?pages.
Ever?since?I?have?been?searching?for?data?with?which?to?demonstrate?the?power?of?sparse?L1?decompositions?for?the?solution?of?inverse?problems.??A?bunch?of?cheap,?high?spec?voltage?references?and?OXCOs?led?me?to?speculate?on?whether?I?could?combine?solutions?for?the?aging?rate?of?individual?OXCOs?to?eliminate?the?systematic?errors?caused?by?aging?by?continuously?adjusting?the?control?voltages?using?an?L1??solution?for?the?proper?control?voltages?to?hold?each?OXCO?to?within?0.01?ppb/yr.??The?nominal?set?point?for?10?MHz?output?is?2?V?so?I?need?something?on?the?order?of?2-5?ppm/yr?voltage?stability?which?is?extremely?difficult?to?achieve?*if*?it?can?be?done?at?all.??A?figure?I?was?recently?sent?showing?the?100?day?variation?of?3?high?quality?OXCOs?is?dominated?by?correlated?noise.??For?each?OXCO?the?magnitude?of?the?excursions?and?the?slopes?of?the?curves?is?different.
Having?spent?my?career?in?reflection?seismology,?I?am?quite?accustomed?to?suppressing?both?correlated?and?uncorrelated?noise?in?multi-channel?time?series?by?a?wide?range?of?methods?chosen?to?fit?the?nuances?of?the?noise?in?question.??Recovering?good?signals?which?are?well?below?the?noise?is?daily?fare?in?seismic?exploration.??There?is?not?much?energy?that?makes?it?6?miles?into?the?earth?and?back.??But?that's?all?that?one?has?to?work?with.??At?$100+?million?per?well,?a?lot?of?time?and?money?gets?spent?trying?to?make?marginal?improvements?in?the?success?rate?be?means?of?better?seismic?imaging.
This?is?a?low?financial?cost,?hobby?project.??Failure?is?a?perfectly?acceptable?outcome,?but?only?after?I?have?made?a?best?effort?attempt..?You?don't?know?until?you?try.??I?think?I'll?learn?enough?to?enjoy?the?attempt?however?it?turns?out.
My?next?post?will?be?on?the?topic?of?the?results?I??achieve?with?the?LM317s.??I?have?Variacs,?so?I?can?measure?the?effect?of??mains?voltage?variations?quite?easily.??It?will?become?a?lot?more?difficult?when?I?start?looking?at?uV?level?noise.??I?expect?amplifying?uV?signals?so?I?can?see?them?on?a?1?mV/div?DSO?will?be?"interesting".??Fortunately?I?won't?need?that?until?I?start?work?with?the?LM399.
Have?Fun!
Reg
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