Rubidium Standard 5680B unstable? Possible? - Page 1
Forum section: electronics / projects

author: Lightages
author_rank: Unknown
timestamp: April 11, 2015, 05:57:40 am
content: I just recently received a FE 6580B standard from an ebay seller. It is one that does not supply 10MHz from the output but rather 1pps. There has been a thread here on modifying it to provide 10MH using a MC74HC393 or variants thereof.The item I received never provided a 1pps output that I could see. This did not concern me as I want a 10MHz reference. The ebay seller even referenced the EEVBlog threadhttps://www.eevblog.com/forum/testgear/fe-5680b-1pps-to-10mhz/about the mod as part of his selling points.I verified that I could see 20MHz coming from where it was indicated it would be, and then added the 393 to divide by 2 to get the 10MHz that I wanted. Sure enough I got 10MHz from the divider like I wanted and was happy. Happy until I watched the output for more than a minute. It varies from 9.99985Mhz to 10.0001MHz with a period of around 30 seconds. WTF?I see this variation on my DS1054Z, DS1025SDG, and my frequency counter. If I observe the output of my SDG1025 it is stable on the order of hours when measured by my DS1054Z and my frequency counter.So is it possible for a rubidium standard to provide an unstable output? The item was described as used so it was a pull from a cell phone tower, probably. If it was pulled it was pulled for one of a number reasons. One, perhaps it was unstable. Two, perhaps it was obsoleted. Three, perhaps it was never installed because it was faulty. Four, perhaps it was never installed nor tested.

author: rdl
author_rank: Super Contributor
timestamp: April 11, 2015, 06:33:55 am
content: I was reading a web page just the other day where someone described a very similar problem and fixed it by turning the thing upside down. Something about the earth's magnetic field being oriented differently between the northern and southern hemisphere. I think he was in Australia.

author: indy
author_rank: Newbie
timestamp: April 11, 2015, 06:40:19 am
content: Yes, it is a feedback controlled oscillator and component aging or failure could happen, Read herehttp://www.vk3um.com/Rubidium%20Standard.html

author: cyr
author_rank: Frequent Contributor
timestamp: April 11, 2015, 07:33:14 am
content: Does you unit have a "locked" signal output you can check? Because it sounds like it never achieved lock, the output frequency will sweep like that while looking for the resonant point but should stop within a few minutes or so.

author: dom0
author_rank: Super Contributor
timestamp: April 11, 2015, 09:59:00 am
content: When the rubidium lamp reaches it's end of life similar things (frequency output, but failure to lock) happen, too...

author: Dragon88
author_rank: Regular Contributor
timestamp: April 11, 2015, 12:06:34 pm
content: What frequency counter do you have, and with what type of timebase?Do you get a lock signal on the rubidium?

author: rdl
author_rank: Super Contributor
timestamp: April 11, 2015, 02:53:59 pm
content: I just remembered, it was not a web page, where I read about this problem. It was in the April 2015 issue of Everyday Practical Electronics. The author of the article, Jim Rowe, describes a problem he he had with a couple of FE-5860A units that sounds exactly like what yours is doing.QuoteQuoteand yes, turning the unit upside down was the fix.

author: grumpydoc
author_rank: Super Contributor
timestamp: April 11, 2015, 04:04:36 pm
content: QuoteIf there's no 1PPS signal chances are it isn't locking, because the 1PPS output isn't enabled until it has locked.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 11, 2015, 04:31:50 pm
content: Quote from: rdl on April 11, 2015, 02:53:59 pmIf I had a device that needed turning upside down to work I'd say that there is something serious wrong with it.

author: Lightages
author_rank: Unknown
timestamp: April 11, 2015, 04:56:49 pm
content: Turning it upside down does sound like a joke but I will try it. I did run it upside down when it was open for modifying but I don't remember seeing 1pps then neither.

author: rdl
author_rank: Super Contributor
timestamp: April 11, 2015, 05:03:30 pm
content: It was not a joke. According to the article, these devices are extremely sensitive to magnetic fields. They are tightly sealed inside solid mu-metal boxes to shield them. Apparently if one has been installed and operating in a fixed location for a number of years it can still pick up a small degree of magnetization from earth's magnetic field. If it is then moved to a completely different location where the earth's magnetic field is oriented differently it affects its ability to lock on. At least, that was the theory.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 11, 2015, 05:04:54 pm
content: These units do run pretty warm, and could probably benefit from a heatsink or fan.I have one unit that drops in & out of lock periodically - not investigated further as I have another one.

author: Lightages
author_rank: Unknown
timestamp: April 11, 2015, 05:06:00 pm
content: Hmmmm, my shop is in a shipping container. It might have its own magnetic field.

author: rdl
author_rank: Super Contributor
timestamp: April 11, 2015, 05:30:34 pm
content: I just realized that the EPE article was originally published last year in Silicon Chip. Here is a link:http://www.siliconchip.com.au/Issue/2014/April/A+Rubidium+Frequency+Standard+For+A+SongIt says you can view 27 pages free.

author: Lightages
author_rank: Unknown
timestamp: April 11, 2015, 07:42:09 pm
content: The magnetic field in my container is the same orientation as ambient. I have put the FE-5680B in different orientations and it never locks. There is a trim pot accessible through a small hole in the side of the case. Does this adjust anything worth trying?Edit:Perhaps this trim pot sets the center of the search range?

author: rdl
author_rank: Super Contributor
timestamp: April 11, 2015, 09:15:30 pm
content: Here is a good source of information about those devices.http://www.ko4bb.com/dokuwiki/doku.phpLook a little ways down under "Precision Timing".It says in the faq that control does nothing on some versions of the FE-5680.

author: Lightages
author_rank: Unknown
timestamp: April 11, 2015, 10:07:09 pm
content: Thanks for all the help. I adjusted C217 which adjusts the search range, and it was already maxed out. I get 10.00068MHz maximum on my SDG1025 with TXCO mod to 9.999849MHz minimum. It is obvious that this is defective and/or too old. I am waiting for the ebay seller to respond.

author: lincoln
author_rank: Regular Contributor
timestamp: April 15, 2015, 09:00:51 pm
content: Quote from: mikeselectricstuff on April 11, 2015, 05:04:54 pmNO, NO, NO, NO,Sorry Mike, I love your video and much respect, but these need to run hot. Adding a heat sink will just make their short life pass quicker. These things have a expected life of about 10 years. Its like adding a heat sink on a OCXO...Link

author: Lightages
author_rank: Unknown
timestamp: April 15, 2015, 09:04:58 pm
content: The seller has sent me another unit free and without me needing to send mine back.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 15, 2015, 11:23:48 pm
content: Quote from: lincoln on April 15, 2015, 09:00:51 pmWould't they normally be running in forced air in their original comms equipment?Surely the only part that cares about temp is the rubidium cell - the rest is electronics, which in general doesn't like heat.

author: lincoln
author_rank: Regular Contributor
timestamp: April 16, 2015, 01:00:16 am
content: Quote from: mikeselectricstuff on April 15, 2015, 11:23:48 pmfrequency stuff generally no, or at least the products my coworkers worked on IE Symmetricom TimeSource 2500, those have a very nice SRS Rb inside.Quote from: mikeselectricstuff on April 15, 2015, 11:23:48 pmright....... but the electronic in the box are designed to take it. cooling the out side of the box still will cause the filament work harder. Will it cause a measurable difference? ya, Can most people buying the surplus gear be able to measure it? Nope. probably not. The e-bay oscillators are generally running on barrow time anyway.anyway, we all loved your tear down video. one friend did tech support for datum / Symmetricom Rb for years.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 16, 2015, 01:51:11 am
content: Quote from: mikeselectricstuff on April 15, 2015, 11:23:48 pmUsually, yes they're running with some kind of forced air.Inside the unit there are two temperature-controlled ovens. One for the Rubidium lamp (~100C) and a seperate one for the Rubidium cell (~75C). There could be a third one for the quartz crystal in the oscillator, but most Rb standards don't bother with that. So there's a tradeoff between keeping the ovens toasty and keeping the electronics from melting. Most units have temperature specs for the baseplate ( i.e. < XX degrees C). So some cooling is good, but more isn't better, it just makes the ovens work harder and draw more power.Ed

author: Bud
author_rank: Super Contributor
timestamp: April 16, 2015, 03:01:28 am
content: Mine is one massive heatsink as you can see. No fan inside. The unit gets warm to touch when runs.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 16, 2015, 03:27:13 am
content: And against the above, and to Dave's dismay, I keep mine completely sealed up in a plastic ABS box...(Yes I took the lid off) XD

author: Vgkid
author_rank: Super Contributor
timestamp: April 16, 2015, 04:58:54 am
content: ^^^ I'm intrigued, tell us about it.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 16, 2015, 05:17:05 am
content: Quote from: Bud on April 16, 2015, 03:01:28 amYes, but 'usually' doesn't mean always.Now that I think of it, I've never seen how those things are mounted in an official Lucent installation. Are they convection cooled or is there a fan tray somewhere blowing air through the entire bay? Anyone know?I susupect that skinny Rb standards like the FE-5680 and the Datum SLCR often end up bolted to a card and stuck in a card cage with fans blowing air vertically through the bay. The 5680 that I got was still bolted to the Motorola card that it was installed on when it was fresh from the factory. It was bolted onto a 340 x 185 mm copper area on both sides of the board that served as a heatsink.Ed

author: kjs
author_rank: Regular Contributor
timestamp: April 16, 2015, 05:52:06 am
content: Had the same issue with the 5680 and got some Lucent ones instead ($159, already in a box). They were still in the original Lucent packing material with seal and have never been installed.The problem I have with the majority of these Rb standards (including the Lucent) is that even very expensive ones use pretty poor OCXO's to lock to the RB source and usually with far to much loop bandwidth. That means that they are great for time keeping but not as a reference for normal lab use. The short term stability is far to lousy if you use it as a time base in the sub 5 sec range. Sure, they are great for time keeping over long periods of time (months) like in a submarine which has to have the perfect timing after 3 months under water.As a lab reference a high quality GPS disciplined oscillator is far better if it was build with a crystal oscillator with excellent short term stability. Now, if you would lock this same crystal oscillator with a loop bandwidth in the milli-Hertz range to an Rb standard you have the best of both worlds.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 16, 2015, 08:08:41 am
content: Quote from: lincoln on April 16, 2015, 01:00:16 amThese units (at least the one I took apart) do not have a filament - it's an electrodeless lamp

author: edpalmer42
author_rank: Super Contributor
timestamp: April 16, 2015, 03:06:25 pm
content: Quote from: kjs on April 16, 2015, 05:52:06 amCorrect. The 'telco' Rb standards like the 5680 or the LPRO were used for long term stability for timing or holdover situations, not short term stability or low phase noise. For example, they likely wouldn't work well as a frequency reference that is then multiplied up to the Gigahertz region. They may or may not be usable as a short-term frequency reference, depending on the amount of precision required. Each user should evaluate their own situation.By the way, if you look inside an LPRO, you won't find an OCXO at all. All they use is a bare crystal which, oddly, has a heatsink attached! To meet their specs, that's all they need since the oscillator frequency is controlled by the Rb.I've heard people talk about using a clean-up oscillator to improve the performance of one of these Rb standards, but I don't remember seeing a project.Ed

author: TSL
author_rank: Regular Contributor
timestamp: April 16, 2015, 06:51:02 pm
content: Quote from: edpalmer42 on April 16, 2015, 03:06:25 pmIncorrect.. you have that backwards. RB gives you short term stability with long term drift, GPSDO's the opposite. The short term jitter from GPSDO's can be annoying if enough birds are not in view.The best secondary standards use GPSDO disciplined RB's with OVCXO's providing final output. i.e. a Symmetricom with a PRS10.A couple of quotes from the LPRO manual..."The LPRO is designed for ease of integration into time and frequencysystems because of its low profile and single circuit board design.""Linked with a GPS receiver, the LPRO provides the necessary timing requirements forCDMA cellular and PCS systems. The low temperature coefficient and excellent frequency stabilityextend holdover performance when the GPS signal is not available."As for phase noise both the FE5680 and the LPRO101 are exceptional units that easily match some of the better know GSPDO's such as the Trimble Thunderbolt.QuoteBoth the 5680 and the LPRO are being used, and have so for a long time, as a base reference for many microwave transverters. The LPRO is such a clean oscillator that its been the RB of choice for such things for many years.Quoteincorrect... an RB standard is locked solid and stable in under 5 minutes. A GPSDO can take 20minutes or longer to achieve that stability, more so if its been moved and it has to do a "site survey"QuoteThe projects you've heard about are predominately to do with the FE5680's. There are some 25 different models of this unit, some with fairly raw output and others with multi-pole bandpass filters that produce excellent sine outputs suitable for direct use as local oscillator sources for microwave applications.The more raw FE5680's have an almost triangular wave form output the benefits from an outboard bandpass filter of some nature to remove the harmonics and clean up the waveform.Also the RB5680's a very susceptible to noise on the power supply, and artifacts from the +15v line will easily find their way onto the output if care is not taken.the LPRO less so but has this to say in the manual..."The Rb atomic frequency source uses a modulation/demodulation lock-in amplifier schemewith a modulation frequency of ~152 Hz. Inherent in this approach is sensitivity to noise at multiplesof the modulation frequency. This noise is coupled through both the heater and electronic powerlines to cause modulation spurs on the output frequency. Care should be taken to avoid the modulationfrequency and its lower harmonics (roughly up to the tenth harmonic)."I've done much work with FE5680s owning 3 models myself and using them to lock transceivers for use at 144MHz and higher.An excellent comparison of RB units for phase noise and stability can be found here...http://www.ke5fx.com/rb.htmuse of FE5680's as a rig locking mechanism and other useful FE5680 stuff can be found here...http://www.vk3um.com/Rubidium%20Standard.htmlFor good reading on things RB you should consult the LPRO manual here...http://www.wa6vhs.com/Test%20equipment/FREQUENCY%20STANDARDS/man-lpro.pdfregardsTim

author: edpalmer42
author_rank: Super Contributor
timestamp: April 17, 2015, 02:42:18 am
content: Quote from: TSL on April 16, 2015, 06:51:02 pmWe were comparing the telco Rb standards to a good OCXO, not to a GPSDO. Most telco Rb standards are inferior to a good OCXO as far as short term stability (AKA phase noise) is concerned. That was simply not a requirement for the telcos so the vendors didn't spend money to provide it. Kjs commented that the best of both worlds would be a GPS disciplined Rb standard with a good OCXO. You correctly described the PRS10 as filling most of that requirement. I do not consider the PRS10 to be comparable to the 5680 or LPRO. It's much better than they are. It uses an ovenized SC-cut crystal to provide exceptional performance. Of course, the price, even second-hand on ebay, reflects that.Quote from: TSL on April 16, 2015, 06:51:02 pmReally? The site you listed shows that the FE5680 and LPRO are much worse than the Thunderbolt. Compare these two graphs:http://www.ke5fx.com/rb/PN.pnghttp://www.thegleam.com/ke5fx/tbolt/tbolt_pn.pngAt 10 Hz offset, the 5680 and LPRO measure about -95 dBc/Hz while the Tbolt is about -115. The 5680 never gets close to the Tbolt. The LPRO does eventually reach the same level as the Tbolt. Whether the difference is signicant depends on each users application.Quote from: TSL on April 16, 2015, 06:51:02 pmCheck outhttp://www.ka7oei.com/10_MHz_Rubidium_FE-5680A.html"As a 10 MHz reference for microwave transverters, I've found the "barefoot" FE-5680A to be unsuitable for microwave transverters due to low-level audio-frequency phase modulation on the output.To use the FE-5680A with a microwave transverter it is necessary to "clean up" its output by locking another crystal oscillator to it."So with extra work, the 5680A can be cleaned up. But you have to do that work. Considering how inexpensive they were a few years ago, the extra work might be reasonable.Quote from: TSL on April 16, 2015, 06:51:02 pmAgain, GPSDOs weren't the topic of this discussion. OCXOs, stand-alone Rb, and GPSDOs (whether OCXO or Rb based) have distinct, but overlapping areas of use. If you need absolute frequency accuracy, you need a GPSDO. If you need lowest possible phase noise, a precision OCXO is the solution. You may have to combine solutions. Each user has to decide which solution is best for their specific application.Ed

author: TSL
author_rank: Regular Contributor
timestamp: April 19, 2015, 04:07:34 am
content: Quote from: edpalmer42 on April 17, 2015, 02:42:18 amShort term stability is != to phase noise. Short term stability is measured over periods of 0.1 second to maybe a day. Phase noise is measured over shorter time scales and looks more like unwanted modulations than a wandering frequency.That being said I agree with the premiss of your statement in that ultimately the cleanest output comes from so form of crystal oscillator.Quote from: edpalmer42 on April 17, 2015, 02:42:18 amFair enough in that comparison, and those are interesting measurements and particularly stellar performance from a Thunderbolt that is exceeding its published specs by 10dB or more !Manufacturers specs for both the TB and FE are much closer, i.e.For the FE5680...Phase Noise (fo=10 MHz)@ 10 Hz: -100 dBc@ 100Hz: -125 dBc@ 1000 Hz: -145 dBcFor the Thunderbolt...@10 Hz -120 dBc/Hz@100 Hz -135 dBc/Hz@1 000Hz -135 dBc/HzQuote from: edpalmer42 on April 17, 2015, 02:42:18 amAnd I certainly don't dispute that for an FE5680 revision A - their output is nearly triangular and the spurs plentiful. This not the case for a FE5680 revision B. There are approximately 25 different models of this RB, Revs A - D each with multiple options, and they are certainly not all created equal. Some don't even have an RF out with only a 1pps timing signal.Quote from: edpalmer42 on April 17, 2015, 02:42:18 amAgreed!regardsTim

author: Marc_Und
author_rank: Newbie
timestamp: April 24, 2015, 05:52:02 pm
content: Hi,I have just bought via ebay (from China) what I thought was a Fe-5680A, but when I opened the case, forum it was actually a FE-5680B.Does anyone know I can tell what options this unit will have? Or where I can find a manual for it, as there appears to be very little information available for this version on FE-5680.I was planning to create a precision frequency reference, as described in the EPE magazine (which others have referred to). Is this still possible with this unit?Regards,Marc

author: TSL
author_rank: Regular Contributor
timestamp: April 24, 2015, 07:42:38 pm
content: Quote from: Marc_Und on April 24, 2015, 05:52:02 pmHi Marc,Nearly everything about FE5680's can be found here, including the manuals...http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faqDespite this link referring to the A model, most of it is still applicable to the other models.A picture or two of the outside and inside would go a long way to determining which particular model you actually have.regardsTim

author: Marc_Und
author_rank: Newbie
timestamp: April 24, 2015, 08:21:46 pm
content: Thanks Tim,I will look at that link.I have attached some photos like you have suggested, which I hope someone can identify the model from.Regards,Marc

author: TSL
author_rank: Regular Contributor
timestamp: April 25, 2015, 02:31:18 am
content: Interesting! its is the first Rev B I've seen without an SMA output for the 10Mhz. Not entirely unusual that can just mean one of the possible options per model I mentioned.Take the top lid off and photograph that PCB. The top PCB contains the DDS, 10Mhz filter and maybe the Max232 for the serial out. On some models the MAX232 is on those empty pads next to the bolt hole in your photo.To take the top lid off you find an additional screw under the label. There is only 2 screw securing it to the central support bar.Once you have the lid off you should see a coax or wire going from the board at the back to pin 7 of the DB9.The synthesizer board sometimes contains the MAX232 and the 10Mhz output, see hires pic here...http://www.insideelectronics.co.uk/wp-content/uploads/2013/04/DSC07534.jpgregardsTim

author: Marc_Und
author_rank: Newbie
timestamp: April 25, 2015, 08:51:03 am
content: Hi Tim,I have taken some photos of the top pcb (see attached) - an overview, plus some close-ups.The unit doesn't have a DB9 connector - it is a DB15 (the VGA triple-row type).I hope they help, identify what the unit has.Regards,Marc

author: TSL
author_rank: Regular Contributor
timestamp: April 26, 2015, 02:06:10 am
content: Quote from: Marc_Und on April 25, 2015, 08:51:03 ama DB15OK... you've successfully found a version of the FE5680 I've never seen before!!I've no idea what the pinouts would be for that.I've spent half a day trawling though my notes and links and I cant find any mention of _this_ particular model.I'd be going back to the seller and asking for pin outs and more info. I can see a 9 pin output next to the DB shell in your photos and I cant see a Max232 anywhere in the photos. In fact it appears to have been left off since there is blank pads where it should be.Talk to the seller if its not what you expected, they should refund or replace.regardsTim

author: Vgkid
author_rank: Super Contributor
timestamp: April 26, 2015, 02:42:04 am
content: Have you tried contacting the company. They seem to still be in bussiness, despite having no technical e-mail.

author: TSL
author_rank: Regular Contributor
timestamp: April 26, 2015, 04:02:26 am
content: Quote from: Vgkid on April 26, 2015, 02:42:04 amIf you're talking about contacting FE, then that's not going to help. I've been down this path with them and the fact is that the 5680 is OEM'd to client specs and they wont share the individual configs with you. At last count there were 57 know variants spread across 4 different models ( A thru D), so it can be a bit of pot-luck trying to find a "good" one that will do what you want.Not all the Chinese sellers are clued into what units they have, RFbuy, fluke.i and a couple of others do.FYI here is the Option list from the docs I have and a unit can have more than one option applied..01 50.255055 MHz Sine Wave02 Remote Digital Control - RS-232; Resolution: 1.8 x 10-7Hz03 5 MHz-FEI Standard Frequency04 15 MHz-FEI Standard Frequency05 13 MHz-FEI Standard Frequency06 2.048 MHz-FEI Standard Frequency07 10.23 MHz-FEI Standard Frequency08 Customer Specified Frequency –1 Hz to 20 MHz.09 Square Wave Output : TTL16 improved Spurious: -80 dBc at ±5 MHz from carrier.18 Conformally - coated PC Boards21 Increased RF output level of 1.0 Vrms (+13 dBm).22 Unit is foamed for operation in severe shock and vibration environment.25 Operates on a single +22 to 32 Vdc supply voltage instead of +15.26 Loop Lock Indicator reversed < 1Vdc=Unlocked >3Vdc=Locked28 Improved Drift Stability:4 x 10-12/day and 5 x 10-10/year29 Improved Drift Stability:2 x 10-10/year after 1 year30 Analog Tuning: 0 to 10V31 Short Term Stability: 5 x 10-12??32 Frequency vs. Temperature : ±1 x10-1035 Analog Tuning: 7 x 10-9 range36-44 Extended temperature ranges from -55? to +71?45 Drift Stability: 1 x 10-11/month.46 Reverse Voltage Protection.48 Frequency vs. Temperature :±5 x 10-11.50 Special Configuration # 155 Special Marking57 Special Configuration # 2Notice some numbers missing ? one of those numbers is probably the OEM types that pop up like this that has a DB15 instead on DB9.regardsTim

author: Vgkid
author_rank: Super Contributor
timestamp: April 26, 2015, 04:32:17 am
content: ^^^ Same issue I ran into trying toget information on a Rakon oscillator. That was still new in box, but built to manufacturers specifications. grrr

author: Lightages
author_rank: Unknown
timestamp: April 26, 2015, 04:50:10 am
content: Actually that FE5689B is the same as mine. It only has a 1PPS output possibly. Mine shows a 10MHz sine wave on pin 11 for a few seconds and then it disappears. This could be that it actually doesn't output that siganl or it doesn't until it locks. Mine does not lock and show a 1PPS signal. The replacement is on the boat.Information on this is here:https://www.eevblog.com/forum/testgear/fe-5680b-1pps-to-10mhz/msg556800/#msg556800

author: Marc_Und
author_rank: Newbie
timestamp: April 26, 2015, 12:15:48 pm
content: QuoteIt looks like, I may be out of luck as well.As I can't get a 1PPS signal out of my FE-5680B. It is Locking on within a few minutes, but I can't detect any 1 pps signal on my oscilloscope (it is an basic old-fashioned type (not digital))Any suggestions? - Can it be fixed? Or more usefully hack/modify it get a 10MHz signal out of it?I have contacted the Ebay seller (in China), and awaiting a response.Regards,MarcAdditional Note: To add extra annoyance, the seller has asked to it to be returned, before I get a refund.

author: Lightages
author_rank: Unknown
timestamp: April 26, 2015, 06:50:53 pm
content: The link I provide has the information to get 10MHz.

author: Marc_Und
author_rank: Newbie
timestamp: April 26, 2015, 07:55:12 pm
content: QuoteI tried looking at that, but the 20Hz signal at the 20MHz test point, was so weak, that I am sure if I am not just seeing noise, rather than a proper signal.So didn't want to waste effort on a probably duff / dying unit.Otherwise, I was seriously considering doing just that.

author: Lightages
author_rank: Unknown
timestamp: May 23, 2015, 05:12:18 am
content: I received my replacement 5680B today, no charge and did not have to return the old one. I tested before doing anything else and it locks on within 2 or 3 minutes. I transferred the extra circuit needed to get 10MHz out of the thing from the old one to the new one and now I have a nice stable 10MHz reference.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 23, 2015, 06:40:52 am
content: Hi Lightages,congratulations, and welcome to the time-nuts!You have to assemble your unit on a big cooling plate, otherwise it will overheat and you'll get reduced lifetime of the electronics and instabilities.Check, that the case temperature of the units base plate stays at about 40..50°C max. That's also the specified limit, but the necessity of cooling is not explicitely explained at FEI, that's a pity.Check instead other Rb manuals, which explain that, and the physical and electrical principles, in a better way, like these from EFRATOM:http://www.ko4bb.com/manuals/index.php?dir=02_GPS_Timing/EfratomWhat others have written here, concerning not cooling these boxes at all is complete nonsense, sorry! They will wreck their own boxes.The trimmer inside these boxes will change the internal magnetic field around the Rb lamp, to trim the resonance frequency.If these lamps age, either the trimming range for a lock is exceeded, or the internal pre trimming resistors may have drifted apart.That will be the reason, why you didn't get a lock on the first box.You may try to recover that trim range by changing this internal pre trim resistor.Also check other sites about Rb standards:http://www.ke5fx.com/rb.htmhttp://febo.com/pages/oscillators/rubes/Frank

author: Lightages
author_rank: Unknown
timestamp: May 23, 2015, 06:52:19 am
content: Thanks for the pointers Dr. Frank. I will get it mounted on a heat sink of some sort and keep an eye on the temperature for a bit.I did try to play with the other one but I forget what trimmers I tried to tweak. I will look again.Time nut? Wow I feel honored!This is probably one of the less expensive things you can get accurate with. Now my signal generator and frequency meters will be pretty accurate. The TXCO in my Siglent SDG1025, the one I installed, was pretty good compared to the 5680B. It appeared to be only around 0.2ppm out from the 5680B.

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2015, 12:13:24 pm
content: There's an article on the web by VK3UM about restoring the operation of the rubidium lamp, just search "rubidium lamp repair".Apparently because it's a contactless lamp (inductively energised sealed bulb) it seems to be a matter of re-positioning the deposited Rubidium on the glass using a hot air gun. Might be worth a try on your faulty unit as you've little to lose.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 23, 2015, 01:51:39 pm
content: Quote from: Gyro on May 23, 2015, 12:13:24 pmBefore doing that, better check other root causes!The rejuvenation of the Rb ampule (emitter) is necessary only, if the monitoring voltage indicates an unhealthy status.For other Rb standards, this monitoring voltage is on the connection block, and it is specified, between which levels the ampule is fine.The FEI 5680 is not as well documented, as most of the other Rb clock.But for the EFRATOM, the LPRO, and others, there are very detailed schematics and even circuit descriptions.These all have their similarities, as all function to the very same principle.There are also repair descriptions, pointing out that some crucial resistors or capacitors often fail, or drift outside their spec, causing loss of lock of the PLL.So that is the first thing to look for!By the way, these drifts and failures are caused by the high temperature inside these clocks, and their probability would increase greatly w/o appropriate cooling of the base plate!It's the downside of those compact Rb standards, that the electronics should be as cool as possible (< 50..60°C), but the 106°C and 75°C ovens should not draw too much energy.So the physics package is thermally isolated as good as possible against the electronics, but the cooling is required anyhow.Frank

author: bson
author_rank: Unknown
timestamp: May 23, 2015, 08:43:12 pm
content: What is the 1PPS behavior on these 5680 variants? I was considering rigging something where a 10MHz OCXO output is counted locally, and this counter is read by a PIC which also receives the 1PPS. Over a period of time it would control the OCXO drift by adjusting its temperature. I was thinking for precision/repeatability to have the 1PPS latch the counter output, and the PIC reads the latched value. Whenever it changes it has a new interval sample. It would also need to interrupt the PIC so it can detect no-lock. Obviously, this can be no better than the PPS input... Is a GPSDO like a Tbolt going to work better? (BTW, my interest is in a low-phase noise freq std, not precision timekeeping.)

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2015, 08:58:26 pm
content: IK0OTG did pretty much exactly this (albeit from a GPS 1pps source), might be worth a look....http://www.ik0otg.net/index.php?option=com_content&view=article&id=53&Itemid=59&lang=en

author: TSL
author_rank: Regular Contributor
timestamp: May 24, 2015, 06:06:29 am
content: Quote from: bson on May 23, 2015, 08:43:12 pm99% of the 5680's don't have PPS out, it wasn't in the original spec and only found its way into the very latest models. I have the original Tech Manual for the Series 2 units and there's no mention of PPS anywhere in it, not even as an optional extra. I've yet to find an update of the manual that covers those that do have PPS. I suspect that such a doc doesn't exist outside of the OEM dept where the PPS units came from.Quote from: bson on May 23, 2015, 08:43:12 pmAs Gyro has pointed out, there is a few people who have already done this sucessfully.Quote from: bson on May 23, 2015, 08:43:12 pmProbably. The specs for the Thunderbolt as a low phase noise frequency ref is exceptional. It is the go-to GPSDO for those who build transverters and such fo the Microwave bands and I know of guys here in Oz that are using it as a basis for gear up to 78GHz.Some have made their Tbolt even better by removing the OXCO and replacing it with the venerable HP 10811DNotes of such mods and resultant tests can be found here...http://www.ke5fx.com/tbolt.htmregardsTim

author: paulie
author_rank: Frequent Contributor
timestamp: May 24, 2015, 02:03:29 pm
content: Quote from: bson on May 23, 2015, 08:43:12 pmYou maybe got the idea from that great (but recently departed) genius, philosopher, and humanitarian dannyf:https://www.eevblog.com/forum/projects/toyo-tco-919-%27high-stability%27-crystal-oscillators/Not only 1/20th cost of alternate units but more capabilities.

author: Bryan
author_rank: Frequent Contributor
timestamp: May 24, 2015, 08:57:50 pm
content: If you want 1pps from 10Mhz May want to have a look at PD9 and PD10. Cost you about $2.00 if you have your own programmer.http://www.leapsecond.com/pic/picdiv.htm

author: bson
author_rank: Unknown
timestamp: May 24, 2015, 11:12:42 pm
content: Quote from: paulie on May 24, 2015, 02:03:29 pmI hadn't seen that! I think he's making it a bit hard on himself though, by taking the XO output and dividing it down to a 1PPS signal, which is then compared to a reference 1PPS input. Instead, feed the OCXO to a counter, which can be internal to a PIC, or external, and use the reference 1PPS signal to read the counter delta from the previous second. If it's <10M the OCXO is running slow, if it's >10M it's running fast. Adjust the PWM output based on e/Tc, where e is the count offset (visavi 10M) and Tc the desired convergence time. Keep a running average to detect when e is evenly distributed around 10M within that window; at that point it's locked. Sum e and e^2 to produce a windowed error bound and std dev, and display on the front panel. Obviously with an 8- or 10-bit PWM output a trimmer needs to be set to bring it to ballpark to begin with, and the PWM control should only adjust within a few degrees, as it's going to lack precision otherwise. Anyway, that's what I had in mind...My suspicion is the phase noise (windowed average of |e| while in lock) is going to dominate at some point. This method can only discipline it to the 1PPS signal, it can't fix phase noise... But it at least allowsmeasuringit, which is a good start...

author: paulie
author_rank: Frequent Contributor
timestamp: May 24, 2015, 11:29:08 pm
content: I think TCXO phase noise is minimal which is one of their main advantages. And I think his idea is excellent way to actually obtain a 10mhz reference. Not to mention virtually any other frequency is available independent of the oscillator or GPS but just as stable.Regarding your other point that is close to what I've attempted using an AVR instead of PIC. My conclusion is the dannyf way might be better but I won't be sure until finishing both.BTW note that getting clean 10mhz from 1pps is extremely difficult compared to getting 1pps from 10mhz. XOR gates are good for multiplying couple octaves and ringing tanks a little more but beyond that PLL can be huge jitter bugs.

author: bson
author_rank: Unknown
timestamp: May 24, 2015, 11:40:42 pm
content: Well, I bought a Thunderbolt with both 10M and 1PPS outputs... Let's see what can be done with this. Maybe I'll also get an HP 10M OCXO and experiment.