10Mhz Frequency Standard  - Page 1
Forum section: electronics / testgear

author: Anson
author_rank: Regular Contributor
timestamp: July 04, 2013, 10:55:55 pm
content: I would like to have a fairly accurate and reliable 10Mhz in on my frequency counter. I am kinda lost over the easiest and cheapest method. Sure it would be nice to have a rubidium but I don't know if I need it. I mean how much more accurate is it and does it really make a difference when I'm only using it for hobby use. I realize technically I don't even need a reference. But my counter is old and I feel it would give me more confidence in its readings. I have seen some trimble crystal oscillators on ebay for $25. Are these a good substitution for my needs and can anyone point me in the right direction as far as what I need to get it hooked up to my counter. I know I will need to make a board with some power circuitry but I am not sure on the rest. Any help would be appreciated.

author: trophosphere
author_rank: Frequent Contributor
timestamp: July 04, 2013, 11:15:29 pm
content: One thing that came to mind my mind after reading your post is the following: If you aren't sure about your requirements and just want more confidence in the readings of your frequency counter then why not just get it calibrated? That would give you the confidence and you wouldn't need to mess with any frequency standard. I realize it will drift and maybe you want something that you can just "set and forget" but (as you know)everything drifts. It's the amount of drift over some unit of time that you will need to determine based on what you will be using it for.Besides that,thisshould have some good information for you.

author: Stonent
author_rank: Super Contributor
timestamp: July 04, 2013, 11:28:22 pm
content: I think I'd go rubidium, but mainly for the snob factor. (In your most snobbish accent) "Oh, you use a precision oscillator? Well that's quaint. It might be ok for some classes of people, but I trust nothing but pure rubidium."

author: jonese
author_rank: Contributor
timestamp: July 04, 2013, 11:51:57 pm
content: Thought about a Trimble ThunderboltThey are going for a couple of hundred US dollars or so, sometimes less.

author: Ziltoid
author_rank: Contributor
timestamp: July 05, 2013, 12:46:21 am
content: Gerry had a similar problem, he compared a OCXO with a rubidium freq standard. Alslo he has alot of Information gathered!http://gerrysweeney.com/i-need-10mhz-how-hard-can-it-be/

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 01:07:34 am
content: Anson - I've got a 12V 10MHz OCXO laying around that someone gave me that I synchronized perfectly to my rubidium standard. If you would like this, I'll ship it to you friend. I'll fire it up and let it warm for 30 minutes along with my rubidium and make sure it is still synchronized before I send it. Just let me know. It isn't a frequency standard, but the few times I've had it powered and warmed up, it was VERY accurate. I think it is a double oven, it is about 1.5" cubed or so.

author: Anson
author_rank: Regular Contributor
timestamp: July 05, 2013, 03:36:45 am
content: Quote from: alank2 on July 05, 2013, 01:07:34 amHey that sounds great! Thanks!What will I need to get it up and running? I assume I will need a wall wort and dc input jack a regulator a filter cap? Then a bnc for output a small box and I'm done?

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 03:41:54 am
content: Quote from: Anson on July 05, 2013, 03:36:45 amSounds about right. I'll get it calibrated tomorrow and will let you know how much current it needs at start up and when fully warmed up.Thanks,Alan

author: ddavidebor
author_rank: Super Contributor
timestamp: July 05, 2013, 05:37:01 am
content: I suggest you to build a little box whit back up battery and power supply.You should turn this things on for a few days, even week before calibrate it.30 minutes warmup is utter useless.I've a trimble frequency standard, i can calibrate it if you're near italy and if you want.

author: jpb
author_rank: Super Contributor
timestamp: July 05, 2013, 09:47:19 am
content: I'm glad to see the OP is now sorted.The Rubidium standards are getting rather pricey on e-bay and there is no guarantee as to how long they will last (i.e. how near end of life they actually are), OCXO are a lot cheaper and use less power.What I'm doing is using a cheap OCXO which I've further thermally insulated and calibrating it using the 1pps from a GPS module (also pretty cheap). I was going to build a disciplined oscillator but I think the drift should be small enough for just periodic manual adjustment. But then I'm not really trying to get down to 10^-12, 10^-10 should be fine.I was interested to see the effect on current draw on further insulating my OCXO. When it is just on the bench it draws around 310mA (after warming up). When I partially insulated it (put it in some polystyrene) the current draw dropped to 230mA. I was tempted to try putting it in a vacuum flask but decided that was a little over-the-top!

author: jpb
author_rank: Super Contributor
timestamp: July 05, 2013, 10:22:43 am
content: Quote from: mojo-chan on July 05, 2013, 09:49:25 amI've been looking at those too, but I've not yet bid on any. The main thing that puts me off is that a lot of them were designed for the days of pre-GPS and analogue television broadcast. With the switch over to digital there is a danger of broadcasts moving. I thought that one way to avoid this would be to go with the 60kHz Rugby signal (which I think now comes from Cumbria) but doing some searching on the internet it seems that standards based on these don't work so well at certain times of day (reception/noise issues).I've seen some standards being sold on e-bay based on old satellite systems which are no longer operational!There is an interesting kit one available based on Radio 4:http://blog.g4ilo.com/2011/05/off-air-frequency-standard.htmlbut this relies on BBC Radio 4 long wave which may not go on forever!I am using this little GPS module from Adafruit via Proto-Pichttp://proto-pic.co.uk/ultimate-gps-breakout-66-channel-w-10-hz-updates-mtk3339-chipset/it is less than £30, has a pretty accurate 1pps output (certainly if averaged over a period) and a connection for an external antenna.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 05, 2013, 11:28:07 am
content: Quote from: jpb on July 05, 2013, 09:47:19 amBe careful when adding extra insulation. The oven controller could be designed for a certain range of heat flow. If you reduce the flow too much, the controller might become unstable. Also, if you add too much insulation, the heat from the oscillator and it's support circuits could be enough to cause the unit to overheat.Ed

author: jpb
author_rank: Super Contributor
timestamp: July 05, 2013, 12:18:25 pm
content: Quote from: edpalmer42 on July 05, 2013, 11:28:07 amI wondered a bit about this - I was going to find out by experimentation. Perhaps I should attach a thermocouple and see how the temperature varies in and out of the insulation.It runs very hot just sitting on the bench so I guess it is designed to run at around 70 or 80C.Presumably the controller is just a simple affair with a bit of hysteresis built in? Would it be better to attach it to a thermal mass (i.e a lump of metal) and then insulate this?The fact that it is still drawing 230mA I would have thought meant that the heater was still being used, what would the typical current flow be for just the oscillator and its support circuits?My main concern is the polystyrene getting too hot and giving off fumes. If I blow up the OCXO then it will be sad but it wasn't expensive.

author: G0HZU
author_rank: Super Contributor
timestamp: July 05, 2013, 01:15:37 pm
content: Quote from: mojo-chan on July 05, 2013, 09:49:25 amAbout 15-20yrs ago I designed and built my own 10MHz offair standard locked to BBC 198kHz. I only ever made the one ugly prototype and still have it today. It's been housed in a small cardboard box most of this time but recently I've upgraded it to a plastic ice cream tubThe aerial is inside so the box needs to be non conductive.It was never a great design but it can lock within a minute or so and I can get to 10MHz +/-0.02Hz stability with a few averages or even better if I average across several minutes. A GPS reference is obviously much, much better but I've been happy with the ugly homebrew offair std.The BBC keep threatening to close the service on 198kHz which is partly why I've never made a tidy version in a proper boxI've also got various 10MHz OCXOs here and the better ones (matchbox sized) can keep within +/- 0.2Hz of 10MHz over several months and the warm up time is about 4 minutes. Something like this should be fine as a frequency counter reference for nearly everybody and you can buy decent OCXOs on ebay for low prices now. The ones I have here were state of the art 10 years ago in terms of fast warmup and ageing etc and you can buy similar surplus OCXOs on ebay today

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 01:51:28 pm
content: Ok guys, I've got the ocxo and the rubidium up and running, warmed up for 45 minutes now. The ocxo starts with 350mA and settles down to about 130mA. I've got the rubidium signal on CH1 (top) and the ocxo signal on CH2 (bottom). I am triggering on the CH1 (rubidium) so it is stable. The bottom signal is moving to the right ever so slightly, 20nS timebase, 10 divs in movement in 13.16 seconds. I have NOT tried to adjust the ocxo yet to match perfectly, but will. Here is my question, what formula would I use to calculate the ocxo frequency?Is this the correct formula to figure out the error?(movement_divs * timebase * frequency) / movement_seconds10 * 20nS * 10 Mhz / 13.16 --> 200nS * 10 Mhz / 13.16 --> 2 / 13.16 --> 0.15198 HzIf the ocxo waveform is moving to the right, is it slower, meaning it can't keep up with the rubidium 10 MHz? So in this case, the ocxo is running 10MHz - 0.15198 Hz or 9999999.84802 Hz ?Thanks,Alan

author: jpb
author_rank: Super Contributor
timestamp: July 05, 2013, 01:58:36 pm
content: You haven't said what counter you have, but since it is to be a reference for the counter, why not just connect it as such and measure the other and adjust until the reading is exact? You only need agreement to be within the accuracy of your counter.

author: baljemmett
author_rank: Unknown
timestamp: July 05, 2013, 03:16:48 pm
content: Quote from: G0HZU on July 05, 2013, 01:15:37 pmI think they're running out of valves, but the service will probably stay up at least long enough to finish rolling smart meters out across the country... There's a fun little trivia question hiding in there

author: ddavidebor
author_rank: Super Contributor
timestamp: July 05, 2013, 03:28:27 pm
content: Quote from: jpb on July 05, 2013, 09:47:19 amSorry, but maybe 10^-8 is more realistic for a few days calibration with 1pps

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 03:35:04 pm
content: Hi,ocxo 350 mA at startup, 130 mA when warmed up350ma from 0s to 257sBetween 350ma and 130ma from 257s to 660s130mA (very close to being stable) from 660s to 765sas stable as it seems to get at 765s (12 min 45 sec)At this point it takes 15.11s to move 1/5th of a div at 20ns.1/5 * (20/1000000000) * 10000000 / 15.11 = error of 0.00264725 HzMoving to the left, so would that mean it is faster than the rubidium? If so, and assuming the rubidium is 10 Mhz (which is a whole different assumption), it should be 10000000.0026475 Hz if I am right.Thanks,Alan

author: jpb
author_rank: Super Contributor
timestamp: July 05, 2013, 04:16:07 pm
content: Quote from: ddavidebor on July 05, 2013, 03:28:27 pmI can't argue (yet) because I've not finished my experiments, but the gps module claims 10nsecs jitter on the 1pps, and on my TF930 the reading for 100second gate is very steady. The Morion mv89a OCXO is a double oven version and claims short term drift of around 2x10^-12.Perhaps 10^-10 is a little ambitious but 10^-9 I think is doable. I'll let you know how I get on.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 05, 2013, 04:32:44 pm
content: Quote from: jpb on July 05, 2013, 04:16:07 pmA GPS standaard is on short domain 1 sec, onbly 10E-8, the 10 to 100 nSec jitter is valid.Only a measuring period over 1000 or 10.000 seconds is usefull.So i use a GPS standard and the BBC standard to compare. An two rubidium clocks to validate.It has no use to set a OCXO to the right frequency, you need to know what its drift is over long period.Even HP OCXO ( 10E-10 a month), have to be recalibrated every three months. If you know the drift and the start calibration freq.you can predict its drift over long period. And never turn it off. Becuse turning of OCXO, shift its freq. also.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 05, 2013, 05:47:21 pm
content: Quote from: jpb on July 05, 2013, 12:18:25 pmAll good questions, but very hard to answer without having the schematic.The thermocouple would have to be installed inside the oscillator to be useful. If you attached it to the outside, it would tell you how much the temperature of the case rose, but not what's happening to the crystal.The controller probably doesn't have hysteresis. It's likely some form of PID controller that maintains a precise temperature controlled by a thermistor in a bridge arrangement. That's the most common type. Adding thermal mass would make the heater work harder. It might help. It might not.The heater will be drawing some current at all times to keep the temperature up. That's why it's there in the first place. The crystal itself is designed to have little or no frequency shift at a particular temperature. The controller is set to maintain that exact temperature. If the oscillator has seperate power pins for the oscillator and the heater it's easy to measure the oscillator power. Otherwise, you might be able to get an idea by raising the temperature of the oscillator. As you raise it, the current will drop as the oven cuts back. At some point the current won't drop any more. That's the point where the oven has shut off completely. But, you might see the current start to fluctuate before that point if the controller becomes unstable. I've never tried that so I'm just guessing. I suspect that different oscillators would react differently.About all you could do is try things and measure the effects. But you will probably find that the results don't justify the amount of work required.Ed

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 06:02:01 pm
content: Quote from: Wim13 on July 05, 2013, 05:23:38 pmI'm not talking about drift - I'm talking about how close I can get it synchronized to match my rubidium...

author: Wim13
author_rank: Frequent Contributor
timestamp: July 05, 2013, 06:13:29 pm
content: Quote from: alank2 on July 05, 2013, 06:02:01 pmmaybe i dont understand, but if you write: At this point it takes 15.11s to move 1/5th of a div at 20ns.i realy read heremove, and not a steady difference...

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 06:19:07 pm
content: Quote from: Wim13 on July 05, 2013, 06:13:29 pmI may not understand either. I've got my rubidium on CH1 and it is triggering on it so it is stable. Then I am feeding the OCXO into CH2 and looking at it alongside the rubidium. If the OCXO were at the same exact frequency of the rubidium, then both waveforms will be stable and not move in relation to each other. If however, the OCXO were actually at 9999999 Hz instead of 10 Mhz, then its waveform will be moving in relation to the rubidium waveform.If my formula is right (and I'm not sure it is, I was hoping someone would come along with a formula for comparing two frequencies next to each other on a scope), it would move 5 divs per 1 second if it were 9999999 Hz...My goal is to synchronize the OCXO to my rubidium as close as possible.

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 06:26:01 pm
content: Are you guys aware of the NFT utility written by a KKP at the avr forum:http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=82842&highlight=nfthttp://n1.taur.dk/nft/nft.pdfmanual with screenshotshttp://n1.taur.dk/nft/nft.exewin32 exe

author: Wim13
author_rank: Frequent Contributor
timestamp: July 05, 2013, 06:28:18 pm
content: Quote from: alank2 on July 05, 2013, 06:19:07 pmyour calculation is correct, if the drift is constant, i was thinking it was speeding up 4 nSec per 15 sec.

author: alank2
author_rank: Super Contributor
timestamp: July 05, 2013, 06:30:20 pm
content: Quote from: Wim13 on July 05, 2013, 06:28:18 pmGood; it doesn't seem to drift much. I can adjust it so it is barely moving forward or barely moving backward in relation to the rubidium signal, and then it stays doing that.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 05, 2013, 07:33:13 pm
content: Quote from: alank2 on July 05, 2013, 06:30:20 pmAn other way of calculating ,10 Mhz has a period time of 100 nSec, the OCXO was 4/15 , 0,26 nSec offThat means it wil catch 1 cycle every 100 div by 0,26 is 384 seconds.So if you look at the scoop for 384 seconds it will be shifted 1 cycle, so the difference is 1/384 HZ is 0,0026 Hz

author: G0HZU
author_rank: Super Contributor
timestamp: July 05, 2013, 07:38:33 pm
content: QuoteAll agreed, that's why I still keep using my old Droitwich locked offair std every few months to check all my various 10MHz ovens. Eg HP8568B, HP8566B, Marconi 2024 with HS oven, Quintenz 10MHz OCXO, plus the oven in my Anritsu counter. It has something like 5e-10 ageing per day but I find that because I turn them off when not in use, the drift becomes less predictable.I did study the performance of some of the OCXOs and also a GPS reference on a fast updating 11 digit counter and all of them suffered short term step changes in stability if you simply tilted the OCXO in its axis. So even the GPS oven couldn't stay accurate eg it short term shifted the 10MHz by a few mHz if tilted. Obviously it locked again a short while later but it wasn't able to flag itself as being off frequency.I need good 10MHz reference accuracy for the test gear that goes to >20GHz but I'm more than happy with the performance the ovens give and I can't see the need for a GPS reference because to justify it I'd have to leave all the ovens running and check/adjust them fairly constantly either manually or with ATE.The Anritsu counter has the top option for the OCXO from Toyo that is the size of a small coffee mug and it takes >45Minutes to warm up (for even basic accuracy) so I usually use my Quintenz OCXO as an external reference. It takes less than 5 minutes to stabilise and offers very good performance. If I'm doing something critical I'll check the OCXO against the offair standard. Good enough for me

author: dfmischler
author_rank: Frequent Contributor
timestamp: July 05, 2013, 08:04:08 pm
content: Quote from: G0HZU on July 05, 2013, 07:38:33 pmI would think the best way to use the 10 MHz GPSDO is to use it as an external reference. Then you can turn off the equipment you don't need to use.Anybody else concerned about an uncalibrated 10MHz rubidium secondary standard being off by a few Hz and having no way to check it? There arerumors of thishappening. Remember that 10 MHz is not the natural frequency of rubidium, so this is a synthesized or disciplined frequency.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 05, 2013, 08:14:23 pm
content: Quote from: dfmischler on July 05, 2013, 08:04:08 pmMy experience is that a GPS is not so reliable, to much jitter on short term. 10 - 100 nSec ( per Sec )Better use a Rubidium, i check the rubidiums now and then, against a GPS and second source BBC4 ( only Europe )The rubidiums i have are stable, better then 10E-11 a month, so they only need adjustment once a year.No need to synchro direct with GPS or else.

author: dfmischler
author_rank: Frequent Contributor
timestamp: July 05, 2013, 09:18:31 pm
content: Quote from: Wim13 on July 05, 2013, 08:14:23 pmI can buy that, but I would say your complaint is that some GPS frequency devices are not very stable on short scales. Many of the manufacturers give you the option to have a TCXO, OCXO or rubidium stabilized oscillator as the disciplined oscillator.QuoteYes, but you apparently have something to compare them to. My concern is for the people who buy a rubidium standard with no way to know if it is really accurate and stable when it claims it is locked in. Almost all are probably fine, but how does Joe TimeNut know about the one he bought?

author: G0HZU
author_rank: Super Contributor
timestamp: July 05, 2013, 09:41:30 pm
content: QuoteFrom a technical point of view that would be a neat solution but when I borrowed a Quartzlock E8-Y GPS reference I found I had to deploy the remote aerial outside and also run the Motorola software to make sure I was getting good reception from lots of satellites. I also found that it took a long time to settle. The spec sheet said something like 30 minutes but I found it took less than this. But it was still a chore to use it so I'd have to leave it running all the time.http://www.quartzlock.com/downloads/datasheets/E8-Y_4pp.pdfI nearly bought one of the rubidium references on ebay a while back but I saw common sense and realised that I didn't need the extra performance and I'd still end up checking it against the offair std anywayIt's a really hot and humid night here in the UK so for a quick test I fired up the Quintenz OCXO and gave it 5 minutes to settle and it was within 0.02Hz of the 10MHz from the offair standard if I averaged out any short term drift on the offair std. Not anywhere near as good as a rubidium std but still very impressive. It's temperature spec is something like -40 to +70degC and it was a 'special' high performance version of this oscillator here:http://www.quintenz.de/pdf/xo/QO2736.pdf

author: Wim13
author_rank: Frequent Contributor
timestamp: July 06, 2013, 07:21:17 pm
content: This is how i measure and calibrate GPS, BBC4 and the RubidiumOn the DSO, put the Rubiduim on the Ext trigger, BBC4 on channel1and the GPS on channel 2. ( See picture )Nice thing of Digital scoops, they can remember on screenOn this picture the measure time is 1 hour, the jitter on the GPS is about 40 nS averageduring this hour, so al within 1.1 E-11Second source is the BBC4 Quartzlock receiver, you can see jitter of it about 100 nS,due to path changes of 100 * 0,3 is 30 meters.On an other occasion i also discover night an day shift on GPS, of about 100-200 nSec,If you synchro GPS on an OCXO this is something to think about.

author: G0HZU
author_rank: Super Contributor
timestamp: July 07, 2013, 10:38:01 am
content: Looks goodMy homemade offair standard isn't as good as yours. I designed and built it in a hurry and it is a very simple design. It works best during the day where it can reliably deliver 1e-8 to 1e-9 in the short term with some averages. At work we have been using the Quartzlock offair standards (198kHz) for about 20 yrs and the newer models seem to work much better than the originals. I think we disposed of all the early ones as they were a chore to use because they were so fussy in finding a reliable lock and they weren't that stable either.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 07, 2013, 03:28:36 pm
content: Quote from: G0HZU on July 07, 2013, 10:38:01 ami have model 2a of the Quartzlock 198 khz receiver, what is a design error i think is the antenne on the back of the unit,i removed it fro the back, made a metal base for it and placed the antenna 4 meters away, that solved lots of problemsThe antenna on the back sometimes oscilates if not proper tuned. When removing it, it has a much better tuning.On day time it is very good, at night, the receiving path is shifting al the time

author: Wim13
author_rank: Frequent Contributor
timestamp: July 07, 2013, 03:31:55 pm
content: Quote from: mojo-chan on July 07, 2013, 03:11:01 pmDivider jitter is so easy to measure, so that already wiped out...And yes the output of the GPS, is a 10 Khz signal, which has a low rise.You can also use sinus waves, so rise time is no issiue.

author: jpb
author_rank: Super Contributor
timestamp: July 10, 2013, 09:21:07 pm
content: My OCXO only outputs 0.7Vrms so I had to wire up a small amplifier to get it up to 1.11Vrms for my counter's external ref in, which delayed my experiments a bit but I now have it setup on the bench.I have a 5k pot to adjust the tuning voltage on the OCXO. Adjusting this by hand to give me 2.777V gives a average period reading over 100s of my GPS 1pps of 1.000 000 000 occasionallychanging by 1 in the last place.Obviously this isn't very scientific - I need to put my amp in a box and do a lot of tidying up before I'm happy to leave things on for a long time but my conclusion is that it is reasonably easy to use a cheap OSXO off e-bay with a bit of adjustment with a gps module to get a standard to within around 10^-9. This is as close as I need with a 10digit counter.Obviously a properly disciplined oscillator or a rubidium source will be rather better but 1 part in a billion doesn't seem bad. It is also in accord with what GOHZU reports.Edit: I adjusted the Voltage up to 2.77871V and the period measurement is sitting at 1.000 000 000 with no changes in the last digit. The OCXO (which I have further insulated) is only drawing 184mA but seems to be stable so I am cautiously optimistic.Edit again: I somehow had it on frequency instead of period. Switching to period and reducing V to 2.77825V gives a period reading oscillating between 999.999 9998 msecs and 1.000 000 000 secs which is a few parts in 10^-10. The counter uses a 50MHz clock so the difference is one count in the 100 second gate.

author: Spark
author_rank: Newbie
timestamp: July 14, 2013, 12:18:45 pm
content: In a previous life I used to design these things. The main problem with using off air standards directly is they all suffer from short term jitter. The long wave BBC signal has a very low frequency data phase mod signal on it (used to control elect meters I believe). They claim this averages to 0 ( from memory over 1 sec.). However from practical experience it takes a lot longer (about 1min.) to get below 1^10-9 jitter. If you are using this as a calibration sig then no problem, but if being multiplied up to microwave freq. then not so good. The best option is to use a good ovened osc. either xtal or rubidium (the osc of which is xtal anyway). and cal this against an off air std. as often as needed for you application. Or you could just buy a GPS disciplined rubidium std. from eBay.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 14, 2013, 01:43:22 pm
content: Quote from: Spark on July 14, 2013, 12:18:45 pmBecause you also mentioned, a GPS has also jitter, and even day and night shift, so thats why a dont like GPS connected Rubidiums,because this jitter of GPS drifts the Rubidium very slowly. A rubidium who has been calibrated and not connected against off air standards acts much better,and does not have this drift.But is more work of course, you have to find out the trend of the rubidium.I

author: jpb
author_rank: Super Contributor
timestamp: July 14, 2013, 04:07:58 pm
content: From my brief experiments, it seems that the degree of hassle/expense is related to how accurate you want to be.I can hand adjust my OCXO to within 1 nsec on a 1 second period and it stays that way for 12 hours or so. The jitter on the gps seems to even out to less than a nanosec when I use a gate of 100 secs so getting a solution to this sort of accuracy is cheap (£15 for OCXO from e-bay, £7 for good quality pot., £28 for GPS module and £5 for antenna I think - though I am relying on my counter to provide the measurement and I had to spend several quid on building myself an amp as the OCXO signal was too low for my counter then I had to get a little power supply for the +-5V needed by the amp ......).One problem I found with using my counter directly is that when the period is one count low i.e. 999.999 999 8 msecs (50MHz clock in counter) then the accuracy is 0.2 nanosecs but when it is one count high I lose the last place and it would only read 1.000 000 000 seconds so I don't know if it is one, two, three or four counts high.A way around the asymmetry would perhaps to use a divide by 99 counter then it would read 98.9 999 999 8 or 99.0 000 000 2 seconds and the accuracy could be measured to the order of 2x10^-10.Rubidium sources now seem expensive on e-bay. They start at £65 and at the cheap end you have the gamble that their lamp voltage is low and they are near end of life. Additionally, as I can't measure to below 2x10^-10 accuracy having 10^-11 would be invisible to me. I only want this to provide an external 10MHz for my counter and a function generator (when I eventually get one) so even 10^-9 is 1000 times better than the built-in TCXO.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 14, 2013, 04:24:19 pm
content: @ jpbThats why i use a time interval counter for precise measurements,on 1 channel the GPS and the other the Rubidium, and then plot a graph for some hours.measure the difference in nano or pico over hours, and also the trend.Or these days you can do it also with a DSO. See picture somewhere above.So i discover the day and night shift on the GPS.to discipline a rubidium with a GPS you need a very clever algorithm, that takes days.

author: G0HZU
author_rank: Super Contributor
timestamp: July 14, 2013, 05:10:54 pm
content: This is a good threadWith my homemade offair standard I can see some of its short term jitter on my old Anritsu counter because the counter can display 10 digits per second or 11 digits in 10 seconds.So during the day I typically see +/- 20mHz of jitter between counter refreshes but sometimes it jumps more than this for the odd count or two.To overcome this I wrote a quick Windows app that reads the counter at every refresh and it builds up a long term average. For quick and dirty checks I average it over a minute or so but when checking it against a GPS reference I set it to 1mHz resolution and average over something like 30 minutes. It usually gives very good results over this time frame. I know the jitter is in the offair std because I get a very stable reading from the counter if I connect a decent OCXO to the counter input.It is a very crude design and it doesn't use any kind of crystal filter. I designed it hoping I could get 10e-8 over a few seconds because that was the limit of my counters at the time.It uses a hard limiter and some very high Q resonator sections and a very low loop bandwidth. At the time (early 1990s) it appeared to have similar performance to the Quartzlock 2A offair stds we had at work but the big bonus is that it always locks (during the day) whereas the QL models were a chore to use. Now I have the Anritsu counter I can see the short term jitter far more easily and I'm actually quite pleased with the offair std performance as long as I use a few averages.

author: jpb
author_rank: Super Contributor
timestamp: July 14, 2013, 06:52:25 pm
content: Quote from: Wim13 on July 14, 2013, 04:24:19 pmMy attempts at the DSO method have not been very successful. If I trigger on the 1pps from the GPS and look at the jitter on the OCXO output, from one second to the next it is around 5 to 10 nsecs which is what I'd expect. But if I leave persistence on at infinity it spreads over a wide range. I guess that the frequency is slightly out. If it is losing just 2 nsecs a second then over 50 seconds it will spread over an entire wavelength.Unfortunately I don't have the remote control option on my scope (it is a WaveJet 300 series not the 300A series and LeCroy want £400 for the option). I'll have to manually look at 1024 saved waveforms I guess.

author: jpb
author_rank: Super Contributor
timestamp: July 15, 2013, 09:36:09 pm
content: I did some DSO measurements by hand. It was very tedious so I only did about 125 out of 1024 possible points. The jitter shows quite well but there is probably too few points to get an accurate measurement of the slope. The slope as fitted is around -0.1nsecs per second so the period of the nominal 10MHz is around 10^-10 short (the frequency is slightly too high).It is very annoying, but my scope doesn't allow an easy saving of the history of all 1024 waveforms at once, instead they need to be saved one at a time. If I get enthusiasm up I'll repeat the experiment with more points when I have more time.Attached is the plot of the results - each point represents a 1pps and the relative offset of the 10MHz zero crossing following the 1pps transition (rising edge).OK I know phase isn't measured in nanosecs - you need to divide by 100 nsecs and multiply by two-pi to get it in rads.

author: Wim13
author_rank: Frequent Contributor
timestamp: July 20, 2013, 08:00:36 pm
content: Here a plot, off a GPS 1 sec against a Rubidium 10 mhz,after 1 hour warmup of the Rubidiummeasured over 4000 sec, it changes about 50 nSec, which is 1.2 E-11and within specs.Is you look over 2000 seconds, is it almost zero.But measured over longer times, is also different, so using a GPSas a reference is not easy.

author: jpb
author_rank: Super Contributor
timestamp: July 20, 2013, 08:28:45 pm
content: Those results are interesting in the extent of GPS jitter. So GPS is good over long time periods but very noisy in the short term.This is quite an interesting comparison between GPS disciplined OCXOs and Rubidium sources:http://www.ptsyst.com/AppNote2.pdfI'm still working on the analysis of 1024 seconds of data for my modern GPS with my OCXO. I also have a Trimble Resolution T which I want to compare it to.A lot of GPS jitter seems to come down to the low clock frequency of the GPS receivers - I wonder if it is possible to overclock them.I also wonder if there is some way of setting up multiple GPS antennas in a sort of phased array. Unfortunately GPS data and systems are rather complex.

author: edpalmer42
author_rank: Super Contributor
timestamp: July 20, 2013, 10:01:30 pm
content: Here's some measurements that I've made on various GPS devices:--------------------------------------------------------------GPS Devices -- Performance of 1 PPS OutputDevice ............... Std Dev (ns).... Range (max-min)(ns) ... Device TypeNavsync CW12 ......... 4 - 5 .......... 20 - 25 ............... GPS Rcvr (1) see belowMotorola UT+ ......... 40 - 55 ........ 95 - 110 .............. GPS Rcvr (2)Rockwell Jupiter ..... 10 ............ 50 ................... GPS Rcvr (3)Motorola M12M ........ 10 - 15 ........ 40 - 60 ............... GPS RcvrTrimble Thunderbolt .. 0.4 - 0.5 ...... 2 - 4 ................. GPSDOHP Z3801A ............ 0.1 - 0.2 ...... < 1 ................... GPSDOHP Z3817A / CW12 ..... < 0.1 .......... < 1 ................... GPSDO (4)Jackson Labs GPSTCXO . 0.3 - 0.4 ...... 2 - 3 ................. GPSDOResults are based on multiple runs of ~ 1000 measurements each.Sawtooth correction has not been used for any of the GPS receivers.GPS Rcvr measurements made with HP 5372A. GPSDO measurements made with HP 5370B.All units were connected to the same antenna system.1. Sawtooth correction not supported.2. Most 'range' results were in this group, but there were a few at 20 - 30.3. Only one test run.4. Requires external 1 PPS input. Equipped with E1938 oscillator.------------------------------------------------------------------------------Note that sawtooth correction gives a big improvement to the performance of a GPS receiver. If a unit supports it, you would normally use it. These measurements only looked at the raw hardware. Newer units tend to give better performance over short time intervals because higher clock speeds allow the 1 PPS pulse to be placed with better resolution. Sawtooth correction helps compensate for the lower clock speed.Ed

author: G0HZU
author_rank: Super Contributor
timestamp: August 11, 2013, 02:11:46 pm
content: Hope nobody minds me digging up this thread again...I calibrated my Anritsu counter yesterday and then used it to measure the short term stability of my homemade offair standard.The counter can display to 10mHz on a 1 second gate time and this is able to capture some of the short term instability of the offair standard.If I use a longer gate time and display to 1mHz then it tends to hide the worst of the jitter.Obviously this level of performance is poor compared to GPS or a Rubidium standard but you can see why I'm generally pleased with the offair standard performance (for home use) as long as I use some averaging.I gather the data using this simple Windows app that takes data from the counter via GPIB

author: jpb
author_rank: Super Contributor
timestamp: August 11, 2013, 03:19:42 pm
content: Quote from: G0HZU on August 11, 2013, 02:11:46 pmI certainly don't - it makes much more sense to keep everything in one thread especially as there are not an excessive number of posts.Quote from: G0HZU on August 11, 2013, 02:11:46 pmHaving 10^-10 accuracy seems to be a good target for the home lab as it matches 10 digit counters etc. If your standard is 10^-12 then you need an expensive 12 digit counter to admire it properly!10^-10 is many times better than in-built function generators which are typically 1ppm or worse. I suppose if you're into GHz RF it matters more but even there you're within Hz on a 1GHz signal.I need to get back to my experiments - I'm slowly developing a home-made GPS disciplined OCXO but progress is proving rather slow as I get distracted onto other things!

author: davorin
author_rank: Unknown
timestamp: October 08, 2014, 09:53:48 am
content: Have also built now a GPSDO prototype based on this design:http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htmAs I also want to have a display showing the exact time, I added a 18F4550 microcontroller testwise with a HDSP-2111 display...The Jupiter-T GPS module delivers a status and timing message every second (@@Ea command)...What suprises me though is the fact that the delay between the message and the 1Hz pulse is different from time to time...mostly short oafter the 1Hz pulse, sometimes shortly before the 1Hz pulse...and in rare cases the 1Hz pulse lies within the message...So hard to guess to which 1Hz pulse the message is referring to...

author: jpb
author_rank: Super Contributor
timestamp: October 08, 2014, 10:31:26 am
content: Interesting. Though I guess the difference is always less than half a second?From a practical point of view (for say synching time) you could take the message time, add 1 second to it and then wait 500 msecs and take the next pulse after that.Adding a second would get over the problem of the message sometimes coming after the pulse.From the point of view of just displaying the time, unless you lead a very frenetic life being accurate to within 250 msecs in the display is probably ok!

author: davorin
author_rank: Unknown
timestamp: October 08, 2014, 10:35:51 am
content: Actually I was planning to do a nice display with 1/100 of a second ;-)Maybe I just try to contact this Miller guy who did the original design....At one point I received from the GPS module time information which was around 15 seconds ahead of atomic time...

author: rf-loop
author_rank: Super Contributor
timestamp: October 08, 2014, 12:05:42 pm
content: UTC, GPS, TAI and so on are good to know.Also some GPS (I think most) have settings (commands) related to these things. (least my Trimble TB and HP Z3801A and Symmetricom 58503A have settings for this)Here more about this "confusion"

author: davorin
author_rank: Unknown
timestamp: October 08, 2014, 12:38:40 pm
content: Terve sveitistä (o;Seems to be accurate now to UTC....guess it stored the last @@Aw command (UTC time)...though the message still comes shortly after the 1Hz pulse....Interesting pointer....was never awayre that GPS time is not accumulating leap seconds (o;

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2014, 01:15:05 pm
content: Quote from: jpb on July 20, 2013, 08:28:45 pmMost GPS receivers generate their 1 pulse per second output from their clock which is itself not phase locked to GPS time leading to jitter of up to one clock period. If you have a good time interval counter, then you can measure the jitter on every pulse and determine the receiver's clock frequency accurately enough to use the GPS receiver as a temperature sensor.GPS receivers intended for timing applications either phase lock their clock to GPS time or calculate the difference on every pulse and make this number available (sawtooth correction) so that correction may be applied externally.