GPSDO with Rb and XO

[tkamiya]

Most GPSDO has Crystal based oscillators:  plain Xtal, temperature compensated, ovened, double ovened, etc.  Some comes with Rubidium oscillator.

EXCEPT FOR HOLDOVER case, are there any advantage for having Rubidium?  In case of GPSDO+Rb, loop constant is set so long, it won't be able to deal with short fluctuation.  Plus, all Rb modules I know has XO in it.  I've never seen just plain Rb generating the output.

Can anyone help me out?

[Leo Bodnar]

"Plain Rb" does not generate anything, it responds to external signal sweeping over.  This signal can be generated by XO, OCXO, VCO or anything else that oscillates and can be frequency shifted.  It's, basically, an ocsillator slaved or disciplined to Rb cell response.

Your average telecom grade Rb source has poor short-term stability and awful phase noise because they use cheap convenient oscillator running at seemingly random (chosen out of convenience) frequency and then synthesize required signal digitally, say, using DDS.

If you take good quality Rb from HP, Efratom or Stanford Research - they use high quality OCXO already operating at required output frequency - say 10MHz.  This is why they produce clean pure sinewave with low phase noise and good short term frequency stability (same thing, different chart.)

Leo

Plus, all Rb modules I know has XO in it.  I've never seen just plain Rb generating the output.

[tkamiya]

Leo, nice to meet you, and thank you for your response!  (you must be the famous pulse generator person)

I have Rb from Standard Research, PRS10.  I also have Efratom FRS-C.  As far as I know, they came out of telecom installations.  I know PRS10 has SC cut crystal and FRS-C has AT cut.  I don't recall if they are single or double oven'ed.  Xtal frequency is 10MHz.

These seem to be very common.  These are also ex-telecom.

What do you consider these?  Cheap telecom stuff or good quality  and proper stuff?  Do you know if manufacturers select units post production and send lower grade stuff for telecom use? 

When you say cheap telecom, are you referring to a type with user-configurable output frequency? 

[Leo Bodnar]

Hi Taka,

You are well equipped then!

I am not 100% sure about FRS, but FRK has 10MHz OCXO that is slaved by the servo http://bama.edebris.com/download/efratom/frk/Efratom_FRK.pdf

By "cheap telecom units" I mean FE-5650A and FE-5680A that have flooded the market.  They are really a good budget design but not the best for metrology use.

If you are interested in Rb operation fundamentals, have a look at Bill Wriley's excellent primer.  It's worth a read:
 http://www.wriley.com/Rubidium%20Frequency%20Standard%20Primer%20102211.pdf

Cheers
Leo

[ArthurDent]

It all depends what you're looking for in a GPSDO. There have been a number of Rb units, including the Nortel unit you described previously. Austron OT-20, Odetics 365/565, FEI-Zyfer 385, and others have units that are Rb based or have Rb as an option. Note that the photo of the FEI-Zyfer unit shows it does use an FE-5650A Rb oscillator.  The OT-20 works with either a cell receiver card or a Motorola GPS receiver.

[testpoint1]

GPSDO is for long term correct the center frequency error, for the jitter, RB oscillator (by zeeman effect control)  is very weak; till the phase noise, I checked several test scheme, I believe many of them do not know what is frequency reference, use a TCXO spectrum analyzer (hign dynamic) to test a Rubidium or Cesium oscillator's 1Hz offset peak value, interesting.

[tkamiya]

I went ahead and built such a unit.  Actually, I've been working on it for the past 6 month or more.

GPS receiver feeds 1 pps into PRS-10.  User output comes from PRS-10 through buffer/distribution amplifiers.  Today, I am doing preliminary tests using TICC + LH setup.  Of course, effects of gps control won't be apparent for months but as of now, FTS4040/A (Cs) and my unit are neck to neck.  Adev at 100 sec are 1.x E-11.  At 500 sec, both 5 or 6 10E-12.  I'm rather impressed....

I believe the default loop constant on GPS steering is something like 24 hours.  So it's a very gentle pull.  It has been turned on for about 2 weeks.  My Cs is, unfortunately, quite old.  Other than it locks effortlessly, I don't know it's accuracy or stability.  I'm going to have to trust it's at least stable, since it can maintain lock for as long as I keep it on.

[Leo Bodnar]

Yesterday I have found and opened my old Spectracom NetClock/GPS 9183 with Rubidium option and found they are doing exactly that.
It has PRS10 inside which is disciplined from Oncore GPS receiver.
Cheers
Leo

[thinkfat]

I'm working on something similar, a plug-on PCB that you can mount onto a LPRO-101. It'll have a pin socket for a GPS board which can feed a 1PPS signal, but if there's space enough I'll provide an SMA jack to feed an external 1PPS signal from another timing receiver.

If you want to follow along, I've started a thread here:
https://www.eevblog.com/forum/projects/diy-gpsdo-project-w-stm32-tdc7200/msg2801922/#msg2801922

[tkamiya]

Good luck with your project! 

My project is complete, hardware wise and running.  Trying to test it is another story altogether.  My Cesium standard is old and that stability and accuracy of it is in question.  To make it worse, room temperature fluctuate quite a bit.  I am still contemplating, how am I going to validate this thing I created.

[testpoint1]

Good luck with your project! 

My project is complete, hardware wise and running.  Trying to test it is another story altogether.  My Cesium standard is old and that stability and accuracy of it is in question.  To make it worse, room temperature fluctuate quite a bit.  I am still contemplating, how am I going to validate this thing I created.

use a young Rubidium oscillator, the tolerance will be lower, then you can compare with your old cesium clock, as usual, 4040a also exists the aging rate, if no calibration, there is no meaning for the cesium clock if more than 20 years. I will release several Spectratime clock within one week.

[FriedLogic]

My project is complete, hardware wise and running.  Trying to test it is another story altogether.  My Cesium standard is old and that stability and accuracy of it is in question.  To make it worse, room temperature fluctuate quite a bit.  I am still contemplating, how am I going to validate this thing I created.

One option is to use a good rubidium like an Efratom LPRO and reduce the effects of the environment on it.
The temperature could be controlled or compensated for, and air pressure changes can also be compensated for. I've had more success with temperature control than compensation, and although the air pressure compensation worked quite well with the Datum and Efratom LPROs, it did not seem to work so well with the Temex LPFRS and FEI 5680A.
Some Rb oscillators also have 'features' like digital temperature compensation, which can cause steps in the frequency output.

Another option would be to use a suitable dual frequency GPS and post process the data from it.

[tkamiya]

Whatever it may be, it will have to be well characterized.  So far, all I have is datasheet written 20+ years ago and equipment just as old.  I'm sure it is no longer meeting the same spec it left the factory.  How far it has drifted is quite unknown.

[FriedLogic]

Whatever you do, you'll likely have to characterize it yourself - or spend a lot of money.
If you log a reference rubidium against your DUT and GPS, and also log air pressure and the temperature (sometimes even the temperature at each piece of equipment involved) for a few weeks you should start to get a picture of how they behave.
In general, the more references you test, the more you learn, and the more references you can test against each other at the one time the better.
A good quartz oscillator can be useful for short term measurements like ADEV and phase noise, but you also need gear capable of measuring that too.

[tkamiya]

That's the problem.  I don't have a good "reference".  Each device at this point is DUT.  How do I solve this dilemma?  I have more than 10 Rb.  Slightly less GPSDO with OCXO, one GPSDO + Rb, and one OLD Cesium. 

In fact, first few try, two DUT with one "reference" (that was Cs), I got two identical trace.  That's impossible.   Even the initial lockup Servo movement (up down up down) is identical and in sync.  Obviously, my issue there is measuring method or Cs.

If I can have one trustable reference, my job will be a lot easier.
I guess they don't call this Time NUTS for nothing.

[edpalmer42]

That's the problem.  I don't have a good "reference".  Each device at this point is DUT.  How do I solve this dilemma?  I have more than 10 Rb.  Slightly less GPSDO with OCXO, one GPSDO + Rb, and one OLD Cesium.

All GPSDO/OCXO have a similar ADev curve.  Here's an example:

http://www.leapsecond.com/pages/gpsdo/free-lock-gpsdo4.gif

The graphs with the dots show four different GPSDOs.  Below about 1000 sec. you're seeing the OCXO.  Then they all merge with GPS and look more or less identical.  The hump in the 10 - 1000 sec. range can often be tuned out by optimizing the GPSDO time constant and damping factor.

I've never seen a graph of a GPSDO/Rb unit, but I expect it to look similar.  Eventually, the performance will be determined by GPS.

In fact, first few try, two DUT with one "reference" (that was Cs), I got two identical trace.  That's impossible.   Even the initial lockup Servo movement (up down up down) is identical and in sync.  Obviously, my issue there is measuring method or Cs.

If I can have one trustable reference, my job will be a lot easier.
I guess they don't call this Time NUTS for nothing.

At every value of Tau, an ADEV curve shows you the combined results of the ADEV values of three items:  your measurement system, your reference, and your DUT.  If one of those is significantly worse than the others, it will dominate the results.  It's also typical that you need to use different references for different values of Tau.  A good OCXO is the best reference for low values of Tau.  Medium values of Tau match up with a REALLY good OCXO or a Rb.  At high values of Tau, you might say that Cs is necessary, but ADEV isn't really the right measurement for high values of Tau.  Just use aging instead.

Chasing better references and better measurement systems is a never-ending activity.

If you really need to determine the ADEV of your various devices, there are two ways to do it.  Send one out to be measured by someone who has the knowledge and equipment to do it.  I don't think a cal lab would do it.  The second way is to gather enough measurement equipment and do a Three-Corner Hat measurement.

[FriedLogic]

That's the problem.  I don't have a good "reference".  Each device at this point is DUT.  How do I solve this dilemma?  I have more than 10 Rb.  Slightly less GPSDO with OCXO, one GPSDO + Rb, and one OLD Cesium. 

In fact, first few try, two DUT with one "reference" (that was Cs), I got two identical trace.  That's impossible.   Even the initial lockup Servo movement (up down up down) is identical and in sync.  Obviously, my issue there is measuring method or Cs.

In that case it might be worth just measuring some of the rubidium oscillators against each other for starters. With the PRS10 you can adjust the frequency by precise amounts and make sure that everything measures as it should. The 1PPS can be compared with GPS too.
If the Cesium works at all you can replace on of the rubidiums with it and see how it goes, knowing that your measurement setup has been tested.

The Reference/DUT distinction is a bit irrelevant without a true master reference. That was my point about logging several oscillators and GPS at the same time - they all act as references for each other. It's not perfect, but it's a start. They may not behave perfectly, but other than for environmental changes, they are unlikely to misbehave in the same way at the same time.

If I can have one trustable reference, my job will be a lot easier.
I guess they don't call this Time NUTS for nothing.

That was why I started looking at dual frequency GPS - I think it's probably the way to go now.

[tkamiya]

Lets say I compare two PRS10.  Connect one as timing source and measure another.  Then, the data I get belongs to which one?  We are back to square one. 

Maybe I'll do time-interval measurement?  (I have TICC)

[jpb]

Lets say I compare two PRS10.  Connect one as timing source and measure another.  Then, the data I get belongs to which one?  We are back to square one. 

Maybe I'll do time-interval measurement?  (I have TICC)

The noise is the sum (or rather the square of the noise is the sum of the squares) of both if they are uncorrelated. This is where the three cornered hat approach comes in - measure 3 devices then you have 3 equations (the measurement of each against each other) and 3 unknowns (the noise of each) which is enough to solve for each separately.

The measurements should be taken at the same time to minimize environment variations (if they are all dependent on temperature then hopefully they all vary in roughly the same way and it cancels out).

Such an approach isn't going to be as accurate as a good reference but it is the only approach really available to hobbyists who don't have half a million $ or £ available to buy a hydrogen maser! 

EDIT: half a million is probably a bit of an exaggeration for the cost of an hydrogen maser but they are not cheap. An alternative such as a super low noise OCXO such as the Oscilloquartz BVA which would be a good reference for times up to say 100 seconds are about £30k new and even on ebay are over £1k. (The BVA is no longer manufactured anyway I think.)
https://www.eevblog.com/forum/metrology/oscilloquartz-8600-3-gpsdo-internal-photos/

[tkamiya]

I think Hydrogen Maser starts around 250K USD.  Plus a temperature stable environment, running cost, etc, etc, etc....  It is certainly not in my wallet...  I know about the OQ BVA but going eBay route will put me back to the very problem I want to solve.  Yet, another unknown.

I have 3 T-bolt and more than 3 PRS10.  I also have 4 HP5335A which 3 of them are from same generation.  3 cornered hat is certainly possible.  I can at least pick "the best one" of each type.  I can run them at the same time but I have to think about recording the running results. 

I thought about putting OCXO in an thermos.  I was told it was a bad idea, but BVA is a dewar design.  I could possibly buy a really big dewar to give more thermal mass and try that.  (yup, another unknown!) 

I would love to find a traceable metrology lab that can get involved.  Shipping being a big problem, I don't think that's realistic either.  I'll research the "hat" method.

[edpalmer42]

To do a Three-Cornered-Hat measurement, you must do the measurements simultaneously.  If you don't, the data isn't correlated and the calculations usually fail with zeros or infinities popping up.

Timelab has the calculations built-in so the processing becomes trivial.

It's okay to have an oscillator in a Dewar if it's designed to be in a Dewar.  If you put any old oscillator in a Dewar or just add insulation of any type to it, it can overheat.

[jpb]

The alternative to the 3 cornered hat approach is to have two identical oscillators as you can then make the approximation that the noise is divided between them so that the ADEV is divided by sqrt(2). Of course this is only an approximation.

[Johnny10]

Have you talked to the Cal labs in your area?
Britest or Custom Cal seem to have fairly decent pricing for locals.

As you say you only need one unit calibrated.

[FransW]

Tkamiya,

I try to understand what problem you are trying to solve.
For me, the following holds:
The Planck time (tP) is the unit of time in the system of natural units known as Planck units. A Planck time unit is the time required for light to travel a distance of 1 Planck length in a vacuum, which is a time interval of approximately 5.39 × 10−44 s.

Since NIST is improving one decade per 10 years, we have quite some distance to cover.
Classical estimates put it to approx 300 years.

Or there is something we do not fully understand yet. And remember: there are no absolute measurements.

So, in the mean time we have to rely on NIST. Why not average all your equipment readings?
And monitor it's statistics, mean, average, standard deviation, highest and lowest values?
Hydrogen maser will not bring you nearer to the truth.

Frans

[FriedLogic]

I went ahead and built such a unit.  Actually, I've been working on it for the past 6 month or more.

GPS receiver feeds 1 pps into PRS-10.  User output comes from PRS-10 through buffer/distribution amplifiers.  Today, I am doing preliminary tests using TICC + LH setup.  Of course, effects of gps control won't be apparent for months but as of now, FTS4040/A (Cs) and my unit are neck to neck.  Adev at 100 sec are 1.x E-11.  At 500 sec, both 5 or 6 10E-12.  I'm rather impressed....

I believe the default loop constant on GPS steering is something like 24 hours.  So it's a very gentle pull.  It has been turned on for about 2 weeks.  My Cs is, unfortunately, quite old.  Other than it locks effortlessly, I don't know it's accuracy or stability.  I'm going to have to trust it's at least stable, since it can maintain lock for as long as I keep it on.

That's probably the way to go - just get familiar with logging to TimeLab and LH, and using the results.
Since the TICC has 2 channels, you could use the 4040 as the reference and measure it against a rubidium and GPS to get some idea of its shorter and longer term stability as well as its accuracy. This also effectively logs the rubidium against GPS.
To reduce the GPS noise you could use a GPSDO instead, but it might be best not to use a long time constant in the GPSDO as that can lead its oscillator affecting the frequency.
Even when you do not have a very good reference like a hydrogen maser, you can get a good idea of performance by measuring against several different references like quartz, rubidium and GPS.

If you can then also log the temperatures of the oscillators that you are using, as well as the air pressure and humidity, you will then be able to work out their environmental sensitivity.
In the case of a Rubidium oscillator the ambient conditions often dominate the performance. Ageing may be about 1E-13/day, but air pressure sensitivity could be up to 1E-13/mbar, and the TC could be over 10E-13/°C.