Few questions on Rubidium standards operation - Page 1
Forum section: electronics / general

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: December 28, 2019, 10:37:13 am
content: I am repairing an old Efratom FRK from 1974 - one of the original ones, before they moved to FRK-L, FRK-H, M-100 versions.What started as repair ended up in redesigning its electronics, trying to keep physics package intact. I have a few questions.How wide is the interrogation resonance dip? In the context of how wide the FM excitation of the 6.8GHz carrier can be? Assuming Q factor 10-7it should be in the order of 1kHz...Is there a significance in using 127Hz for RF modulation freqeuncy? Specification says it should be adjusted to +-1Hz. Is this to maintain fundamental and 2nd harmonic filter alignments? Can it be significantly lower?127Hz modulation is asynchronous to everything else. Would it be bad to derive it from OCXO 10MHz?Can one modulate C-field instead of the RF excitation signal by mixing in a 127Hz sinewave? It would simplify a lot of things.Does C-field affects optical pumping, RF interrogation or both?Does Rb filter cell need heating or is this a byproduct of compact design?Can RF cavity SRD be excited with a higher order harmonic? At the moment it is excited with 60MHz, using its 114th harmonic with 5.3125MHz mixed in to produce 6,834,687,500Hz RF excitation.CheersLeo

author: syau
author_rank: Frequent Contributor
timestamp: December 28, 2019, 11:24:18 am
content: This manual may be useful:http://www.wriley.com/Rubidium%20Frequency%20Standard%20Primer%20102211.pdf

author: jpb
author_rank: Super Contributor
timestamp: December 28, 2019, 11:30:13 am
content: I was going to suggest that manual too but I think I was put onto it by you Leo anyway!Another description of the physics package is in :https://www.orolia.com/sites/default/files/document-files/iSource_LPFRS_Manual.pdfwhich may help a bit but it is probably not going to answer your questions directly.There is also the repair guide:http://blemings.org/hugh/time-nuts/EFRATOM-LPRO-repair-guide-F-de-Vries-rev7-jan2011.pdf

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: December 28, 2019, 04:57:02 pm
content: Cheers for good links.Bill Riley, as usual, has myriad of details availablehttp://www.wriley.com/Rb%20Freq%20Std%20Links.htmI have missed linewidth details - it's all there.I did not know that Efratom uses specific resonance cell buffer gas formula to shift the resonance line from natural87Rb line frequency 6,834,682,610Hz by almost 5kHz to 6,834,687,500Hz to make RF synthesiser simpler.This might explain why emission bulbs and filter cells might be interchangeable between various RFS models but resonance cells are not necessarily so.LeoQuote from: jpb on December 28, 2019, 11:30:13 am

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: December 28, 2019, 04:58:50 pm
content: Leo,Here are answers to some of the questions.The 127 Hz (137Hz in the 5065A) is chosen so that it is offset from any 60Hz harmonic.I have seen some designs using 64Hz so lower will work as long as you change the filters in the electronics from the original value.I'm not sure modulating the C-field coil will work as it changes the output frequency but does not change the amplitude of the resonance dip. For instance you can change the C-field across its range and the P-P amplitude of the resonance signal does not change. Might work for really large P-P excursions? If you try it let me know if it works!You could use a divided synchronous signal to interrogate but I'm not sure if there is an advantage.The FRK and its daughters don't have a filter cell. A classic design has a lamp a filter cell and a resonance cell.Efratom and most modern telecom era units have an integrated cell that combines the function of the filter and resonance cells. They do by running the lamp and cell much hotter than a classic design.This answers the heated cell question, its temperature is critical for proper operation.Depending on the design you can indeed drive the SRD with a higher frequency. The 5065A has an integral filter in the SRD assy and cannot be driven by other than 60Mhz. As long as your SRD does not have such a filter higher frequencies should work.Sound like a fun project!Cheers,Corby

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: December 31, 2019, 02:33:56 pm
content: Random probing of FE-5680A shows that modulation frequency is 416.67Hz, sounds unusually high.Leo

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: January 01, 2020, 12:51:45 am
content: Leo,The mod rate is indeed 416.67 HZTake a look a the info herehttp://www.ko4bb.com/doku2015/doku.php?id=precision_timing:fe5680a_faqCheers,Corby

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 03, 2020, 04:43:54 pm
content: Interesting, I'd like to see if I can play with some settings in firmware.The vintage FRK project will probably end up mostly in digital domain. It makes so much more sense to sample photodetector after the transimpedance amplifier and do DSP fundamental and 2nd harmonic filtering.Efratom frontend analogue filters use Hall network bandpass filters, so have large film caps and are quite twitchy. Sync detector integrator uses 5MOhm resistors and guard rings. All very scary analogue stuff. Amazing that they have pulled it off with single-sided hand-drawn PCBs. If I remember right, FRK model went into space in 1974 on GPS proof-of-concept sats.Leo

author: testpoint1
author_rank: Frequent Contributor
timestamp: January 07, 2020, 03:46:06 pm
content: Quote from: Leo Bodnar on January 03, 2020, 04:43:54 pmforget the vintage Rubidium Oscillator, some of the components already aged, I tested more than 100 units of Datum, Spectratime and SRS and do the research for them, these component can not be found any more, such as SRD, if you want to the research, just do the following 2 types, one is Spectratime SRO (swiss made), one is SRS PRS10 (US made).

author: thinkfat
author_rank: Unknown
timestamp: January 07, 2020, 04:27:19 pm
content: The 127Hz modulation basically paints the RF tone you're interested in. This tone must have a strict link to the signal coming from the synthesizer / VCXO. Modulating the C field would equally affect all tones, no? So it'd be useless.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 07, 2020, 06:49:07 pm
content: I have redesigned the front end and the sweep controller. I wanted to stay fully analogue but it's hard work - next revision will have a DSP on it. Analogue bandpass filters need either very accurate components or manual trimming. 127Hz calls for hefty caps or large resistances. Integrator needs a guard ring to work well... Took me awhile to get the sweep controller to work reliably.I don't like digital hipstery but this is one of the cases when it's a no-brainer.Leo

author: thinkfat
author_rank: Unknown
timestamp: January 07, 2020, 07:10:36 pm
content: That's beautiful! I love how you put the text on the copper layer instead of silk screen. Also, seems you don't skimp on ground viasWhat I also see: lots of free space and lots of small components. I'm happy I don't have to assemble it manuallyPS: what's the buck converter you're using to generate the voltage supplies? Looks like an AD part but I cannot read the markings. Are these LDOs?? From 20V down to 6.8V, that's brave!

author: james_s
author_rank: Super Contributor
timestamp: January 07, 2020, 07:15:30 pm
content: That is a very nice looking PCB, very clean and elegant.I'm curious though why the redesign? Is there some shortcomings in the original design that this is improving on? It seems like a lot more work than repairing the original hardware so I assume there must be some significant improvement here.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 07, 2020, 07:49:47 pm
content: Are you familiar with Corby's (5065AGuru) work on using optical filters to improve the performance of Rb standards? AFAIK, the only work he's released is on the 5065A, but I think I read that there's work being done on the FRK. If you're going to the effort of redesigning the FRK, you really should talk to him to coordinate your efforts.Ed

author: Grandchuck
author_rank: Super Contributor
timestamp: January 07, 2020, 08:06:38 pm
content: Beautiful work Leo!

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 07, 2020, 08:38:50 pm
content: We have three greens down and locked!Quote from: james_s on January 07, 2020, 07:15:30 pmI am just fooling around at the moment.My long term goal is to build the whole Rubidium oscillator from scratch using just a donor resonance cell and and an excitation lamp (maybe, eventually, try a locked 780nm LASER.)I took the easy way out, replacing existing modules until all I have left is the stuff I cannot make.CheersLeo

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 07, 2020, 08:49:53 pm
content: Quote from: thinkfat on January 07, 2020, 07:10:36 pmThey are just ADP7142 LDOs. There are no high current devices on this board so SMPS are not necessary.Quote from: edpalmer42 on January 07, 2020, 07:49:47 pmUnfortunately, I don't have or ever had HP 5065A. Sadly, they have become a bit of a cult item - I think most of them are driving Nixie clocks.As far as I have seen, IR bandpass filters have been part of typical Rb physics package since the beginning. My 1974 FRK sample has a slot in the optical path that seem to have been provisioned just for that but it is blanked with some foam insert that turned to crumble over the course of 45 years. I got a few 780nm bandpass filters off eBay to play with when it comes to it. Are you saying 5065A does not have one?Leo

author: edpalmer42
author_rank: Super Contributor
timestamp: January 07, 2020, 10:07:43 pm
content: Quote from: Leo Bodnar on January 07, 2020, 08:49:53 pmI'm not aware of any Rb standards that have an IR filter. Some have a sheet of glass to block some of the heat from the lamp so it doesn't overheat the cell, but that's a lot different from a bandpass filter. Here's Corby's document on his "Super 5065A".http://leapsecond.com/corby/Super-5065A-Project.pdfEd

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 07, 2020, 10:56:09 pm
content: Well, I thought most were like this but, apparently, not...Quotehttp://www.wriley.com/136_3093.pdfQuoteLeoQuote from: edpalmer42 on January 07, 2020, 10:07:43 pm

author: edpalmer42
author_rank: Super Contributor
timestamp: January 07, 2020, 11:37:43 pm
content: In hindsight, it does seem like a rather obvious idea, doesn't it? What surprises me is that HP didn't include it in the 5065A. Maybe at that time, it hadn't occurred to anyone.Ed

author: james_s
author_rank: Super Contributor
timestamp: January 08, 2020, 04:14:48 am
content: Maybe they didn't have suitable filters? I know the IR reflective coating was a crucial part of the development of low pressure sodium lamps.

author: Theboel
author_rank: Frequent Contributor
timestamp: January 08, 2020, 03:13:20 pm
content: Hi Leo,If You looking for a good read about laser rubidium clock may I suggest You to read T.Bandi thesis.https://core.ac.uk/download/pdf/20660469.pdfIn My Opinion replace Rb Lamp to Laser is not interesting I think better 6,8Ghz osc, better reference is more economical for my research off course You can disagree about it. also a better mechanical arrangement is interesting to research because RB lamp and RB filter need a different condition (temp for example).btw Very good design I like it.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 08, 2020, 03:23:46 pm
content: Thanks!I have 11Vpp 2nd harmonic level during lock. I did play with high gain amp design but I did not expect that much of a blast!Altigether, it still amazes me that hand-drawn single layer boards wired together with random unshielded wires worked. And worked well after almost 50 years.LeoQuote from: Theboel on January 08, 2020, 03:13:20 pm

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 11, 2020, 09:38:53 pm
content: I have married two of my favourite companies together by replacing internal Efratom FRK 10MHz OCXO with external Wenzel one. This is not the OCXO I am planning to use eventually, but as the proof of concept it worked out just fine.Lock loop needed a bit of tweaking and inverting the sign of control voltage and few other things on the sytnth board tuning to please 1973 TTL ICs. I can't wait to go full on digital on the frontend servo board.This should bring the phase noise level and stability below 10sec down.CheersLeo

author: edpalmer42
author_rank: Super Contributor
timestamp: January 11, 2020, 09:50:22 pm
content: Can you measure before/after phase noise and/or Allan Deviation?Ed

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 11, 2020, 10:50:55 pm
content: This project needs tonnes of work before any measurements can be conclusive and will make sense. I don't want to get distracted - the PN kit needs the whole evening to set up and run verification tests.I'd use modded Wenzel OCXO with increased crystal drive - to get lower PN in exchange for worse aging.LeoQuote from: edpalmer42 on January 11, 2020, 09:50:22 pm

author: Theboel
author_rank: Frequent Contributor
timestamp: January 24, 2020, 03:41:21 am
content: Hi Leo,maybe this paper can be Your next Home work. my phase noise measurement capabilities not enough to measure 6,8GHzhttps://www.researchgate.net/publication/224912146_A_low_phase_noise_microwave_source_for_atomic_spin_squeezing_experiments_in_Rb-87Anton

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: January 24, 2020, 05:52:00 am
content: Hi Guys,Making Allan deviation measurements before and after the mods will let you know if you are doing any good.A good Quartz to measure against will allow you to test up to a Tau of over 100 Secs as the Quartz will be over 10X better than an FRKL and the same for an FRKH except at 100 Sec. where it will be 5X better.Like a doctor "do no harm" should be your motto!The resonance cell is most likely what's limiting your performance not the electronics.By the way I've stopped installing filters in the 5065A as they seem to effect the long term stabilities (over 1000 Sec) and after a year or so performance wanes.Cheers,Corby

author: KE5FX
author_rank: Super Contributor
timestamp: January 24, 2020, 10:52:42 pm
content: Quote from: 5065AGuru on January 24, 2020, 05:52:00 amInteresting. What data do you have that supports this? I'm pretty sure it hasn't gotten significantly worse over 6 years in either of my two units that you modified in 2013:I no longer have the PHM around here at this point, so can't reproduce the earlier tests exactly. But you can infer that if Rb1 (or Rb2 for that matter) had become much less stable over the last 6+ years, the dark blue trace would have been markedly higher than the broken red trace.Meanwhile, the t=1s stability of both units is right where it has historically been, at 5.4E-13 for Rb1 and 6.7E-13 for Rb2.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: January 25, 2020, 01:14:18 am
content: Well I stand corrected at least for your units!It's good to see that they are still performing well.I had to remove the mod on one of the original units a while back as it had degraded.My recent yields on trying the filter mod is about 30% the other 70% I can't get to respond well.I do know that when I originally did the testing on the first "supers" that I had to select lamps to get some of them to respond. So the other parts of the instrument have to be working well.The excellent AD at 1 Sec is indicative of what the noise reduction will do (at least for a 5065A)Cheers,Corby

author: KE5FX
author_rank: Super Contributor
timestamp: January 25, 2020, 01:29:30 am
content: I sure wish someone would come up with an economical laser-diode solution to replace the lamp. A couple of people have looked into it over the years, but nobody seems to have any results to share, so it must be harder than it seems at first.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 25, 2020, 12:06:48 pm
content: I don't know much about diode lasers.What's their spectral line width?Can its frequency be wiggled by modulating power?Can its width be widened with square wave OOK modulation?Leo

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: January 25, 2020, 05:16:50 pm
content: To realize the best performance a laser scheme locks the laser to another Rubidium cell. You have that additional cell, an oven controlled laser diode + driver and extra optics. It's very hard to justify the complexity and size when a regular lamp is so cheap! A search online will find lots of neat experimental systems! So far long term reliability also is not the best! The FRK units regularly lasted 15 years or so and when they were still available a new lamp screwed right in! The PRS10 advertises 20 year lamp life. I would LOVE to find a laser system that would slide in and replace the 5065A lamp assy. but I'm not optimistic! Anyone playing at this Please share your data.Cheers,Corby

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 25, 2020, 07:02:08 pm
content: Original FRK (pre -L nad -H) documentation states that you can replace the Rb lamp while it's powered up and live.I can't see much point since loss of 2nd harmonic will cause the sweep controller to swing the OCXO lock to lock. It would have been nice if lamp DC voltage loss would hold the OCXO voltage stable for a bit of "holdover." Maybe live lamp change is a gimmick to keep the RF ouput going, despite few kHz sweep.But as you said, FRKs don't fail for decades. I have PRS10 but I am not running it contunuously - I have heard quite a few people commenting on their poor reliability - just a few years until they need servicing.Leo

author: james_s
author_rank: Super Contributor
timestamp: February 28, 2020, 05:42:09 pm
content: What necessitates replacement of the lamp? Isn't it a sealed glass capsule with no electrodes to degrade? I recall on some of the commodity Rb standards some people heat old lamps with a heat gun to push the Rb back to the cooler pinch seal where it belongs.

author: notfaded1
author_rank: Frequent Contributor
timestamp: February 28, 2020, 06:02:36 pm
content: Quote from: Leo Bodnar on January 25, 2020, 07:02:08 pmThis is good news... I'm not sure I would run it continuously for months but I just bought an -L that was shown outputting rock solid 10MHz before I bought it. It looks like it has the option for external frequency adjust which I thought seemed like plus.I was reading in the manual... good old school documentation. I wish I had docs like this for my Datum 4040A if onlyBill

author: thinkfat
author_rank: Unknown
timestamp: February 28, 2020, 06:37:07 pm
content: Quote from: james_s on February 28, 2020, 05:42:09 pmYes, that works if you have Rubidium splattered around in the lamp or if you have it form a metallic coating in the light path, reducing the output. However, there's supposedly also a possibility for Rb atoms migrating into the glass itself, lowering the vapor pressure and degrading the output of the relevant spectral line. In that case, no heat treatment can restore the lamp.

author: james_s
author_rank: Super Contributor
timestamp: February 28, 2020, 07:28:44 pm
content: Quote from: thinkfat on February 28, 2020, 06:37:07 pmHmm I wonder if you could bake it out of the glass at a higher temperature? I'd be concerned about the internal pressure rising to the point of the bulb exploding though. If the lamp is worn out to the point of being unusable though there seems little to lose.