EEVblog #457 - Oscillator Calibration Followup

[mikes]

Mmh, i never think that temperature is so important for a rubidium clock

Don't assume it's all in the Rb - both will shift with temperature change. The GSPDO will correct itself, but there's a time constant. Rbs are adjusted by changing a magnetic field (the C-field). So, any temperature sensitivity in the circuitry which controls that will affect the frequency.

The FRS does have provision for temperature correction, and I've got it set pretty close. Note that the changes are 2nd order, depending more on the rate of change than the actual temperature.

[rf-loop]

Mmh, i never think that temperature is so important for a rubidium clock

Probably more important for Efratom FRS Rb!?

Thunderbolt is sensitive for temp! (lot of) also it is not very frequency stable becouse in default parameters are optimized for time. So, it use quite big freq chances for run time error as small as possible.

Efratom FRS is sensitive for many things, also temperature.
Same for LPRO etc. (and some individual unit may drift lot of more than other.
Same for FEI
All these frequencies are walking around  as enviroment change.
What more... GPS  .. all times up and down.. but long time mean is ok. (this is why example 1pps discipline time constant need be long enough.

But then need ask how accurate it need be.

[JackOfVA]

Mmh, i never think that temperature is so important for a rubidium clock

Don't assume it's all in the Rb - both will shift with temperature change. The GSPDO will correct itself, but there's a time constant. Rbs are adjusted by changing a magnetic field (the C-field). So, any temperature sensitivity in the circuitry which controls that will affect the frequency.

The FRS does have provision for temperature correction, and I've got it set pretty close. Note that the changes are 2nd order, depending more on the rate of change than the actual temperature.

Looking at your plot, the time slope is not inconsistently correlated with the temperature slope. Generally, declining temperature corresponds to declining time but the sharp temperature snaps show opposite signs as the sharp drop in temperature corresponds to an increase in time.

That suggests several possibilities, including one that says the temperature change is not just inside the shop but rather reflects outside conditions and that the fast temperature excursions reflect storm fronts or diurnal shift and that causes a small change in atmospheric conditions, such as the refraction index which in turn causes a small but still significant shift in the accuracy of the GPS  timing signal and that the abrupt shifts in time represent corrections by the GPS reference, not the  Rb.

400 ns time shift corresponds to a path length change of ~ 400 ft or 130 meters. That seems within the realm of a change in refractive index due to atmosphere moisture content or temperature.

[mikes]

That suggests several possibilities, including one that says the temperature change is not just inside the shop but rather reflects outside conditions and that the fast temperature excursions reflect storm fronts or diurnal shift and that causes a small change in atmospheric conditions, such as the refraction index which in turn causes a small but still significant shift in the accuracy of the GPS  timing signal and that the abrupt shifts in time represent corrections by the GPS reference, not the  Rb.

Nah. It's just the furnace coming on.

[sergey]

Quick question about tools to be best used for calibrations. Is there are difference between plastic and ceramic screwdrivers and if so which affects less on variable capacitors and LC filters?

[SeanB]

The ceramic are slightly better for some, as they can have a lower dielectric constant. However they are very brittle, and break easily. Best is to have a PTFE one with ceramic tips, lowest loss and most likely to have a constant detuning effect on the circuit. longer ones are better as well to reduce the stray capacitance from you affecting the circuit.

[sergey]

The ceramic are slightly better for some, as they can have a lower dielectric constant. However they are very brittle, and break easily. Best is to have a PTFE one with ceramic tips, lowest loss and most likely to have a constant detuning effect on the circuit. longer ones are better as well to reduce the stray capacitance from you affecting the circuit.

Nice to know. Thanks!

[PA0PBZ]

I noticed Dave and others say 'dielectric', but isn't it just that you increase the plate size on one side of the capacitor? I know that the screw of these trimmers is connected to one side of the cap, and isn't dielectric supposed to be between the plates?

[JackOfVA]

I noticed Dave and others say 'dielectric', but isn't it just that you increase the plate size on one side of the capacitor? I know that the screw of these trimmers is connected to one side of the cap, and isn't dielectric supposed to be between the plates?

There's also a small stray capacitance from the adjustment screw and associated plate to the enclosure and ground and the rest of the circuit. It's only a tiny amount, but a small fraction of a pF is enough to shift the oscillator a few Hz. Hence anything that alters the stray capacitance will also shift the oscillator frequency.

[SeanB]

Depending on the type the dielectric is either air, air and plastic, air and mica or air and air. The trimming operation alters the ratio between the first and the second, either it is all air, all the second or a combination of them. There is also capacitance between both and the trimming tool, the surrounding board, the case and pretty much the rest of the universe, and most have both a lousy tempco and drift with applied voltage and time. Heat a pin on one without otherwise touching it and the value will change. Big ones are not so bad ( 200pF to 1n) but anything below 50pF is going to drift a lot. The art is to get the value close enough with NPO ceramics then use the trimmer to get the final value, preferably as the bottom part of a capacitive divider with a larger trimmer value.

[max-bit]

Frequency meter, has an input 10 MHz, better connect the output frequency standard, the counter inputs.
I am at home I have a GPS receiver (Trimble Thunderbolt) display (that shows the status of the receiver) and four 10 MHz output.
You can get STABILITY about 1x10-11, the more I would not count
Cesium clocks gain of about 1x10-13, 1x10-15's best
The most accurate clock is made ??in Poland pulsar clock about 1x10-16
http://en.wikipedia.org/wiki/Pulsar_clock
Cool the frequency standard although a little big

[EEVblog]

Cool the frequency standard although a little big

Yes, I'm temped to gut the rack and put into a smaller custom box, maybe with an LCD status display or something.

[max-bit]

as a status display suggest:
http://www.m1dst.co.uk/2013/02/announcing-trimble-thunderbolt-monitor-1-0-netduino/
(so as not to create from scratch)
although I do not know whether it will be compatible with your device
It is designed for GPS Trimble (thunderbolt)

Probably not, then you will need to write code from scratch
Or mount frequency standard Trimble

[rf-loop]

Frequency meter, has an input 10 MHz, better connect the output frequency standard, the counter inputs.
I am at home I have a GPS receiver (Trimble Thunderbolt) display (that shows the status of the receiver) and four 10 MHz output.
You can get STABILITY about 1x10-11, the more I would not count
Cesium clocks gain of about 1x10-13, 1x10-15's best
The most accurate clock is made ??in Poland pulsar clock about 1x10-16
http://en.wikipedia.org/wiki/Pulsar_clock
Cool the frequency standard although a little big

And you have some data about Thunderbolt stability?
(I have followed many Thunderbolts, HP Z3801's (what is far over TB stability) Good OCXO's and some Rb's. )

Please do real mesurement so that take continuously exaple 100 second averages from TB and and other source (example from other TB or other more short time stabile source)  then compare these and show me statistics, example 1k pairs compared.  After then tell me about what you mean with this:

You can get STABILITY about 1x10-11

  How big peak-peak differencies you find in this test example between  two TB and what you believe about stability after this imagined test. (I have not exatly this but I can quess with my other experience that you do not claim then even 1x10-10 or do you trust even 1x10-9 if take short time (example 1-100s)  freq average.
Also it depends how you have set your TB. How accurate your antenna position really is, how you have adjusted optimal parameters for your individual TB (example discipline). Or do you run with factory defaults. Also there is big differencies between TB's. Some have more bad OCXO than others... some may have really bad and some may have acceptable good individual OCXO from Vectron if have good luck. 

Long time average is good but is it stability?

What about if look what TB itself is thinking about itself, then, everybody have rigtht to his own religion withoud doubt ( smile ).

Then

in some publication:
"The widespread belief that pulsars are the best clocks in the universe is wrong, say physicists."
look attached image

Btw, If I want measure example frequancy. I hope I can do measurement in some minutes but I do not want wait very long time.

Poland pulsar clock about 1x10-16

But I do not want wait 100 year integration period. It is not so handy.

And it is good to think it is Allan variance... it do not mean...etc..

[EEVblog]

I had no idea what this "Tunderbolt" thing was.
Google helped:
http://www.trimble.com/timing/lab-kit.aspx

[max-bit]

Specifications Trimble:
http://www.trimble.com/timing/thunderbolt-e.aspx
There is further
http://trl.trimble.com/docushare/dsweb/Get/Document-383329/022542-010B_Thunderbolt-E_DS_0807.pdf
10 MHz accuracy . . . . . . . . . . . . . . . . . . . . 1.16 x 10 - ¹² (one day average)
I gave the value of 1x10-11 (especially a little worse)
For our purposes can be measured with an accuracy of 1 Hz to 100 GHz  

[rf-loop]

On surplus markets these are very common models.
http://www.ebay.com/itm/Thunderbolt-PRECISION-GPS-10mhz-FREQUENCY-TIME-Standard-/180399458965?pt=LH_DefaultDomain_0&hash=item2a00a54a95#ht_8267wt_1056

Mostly they are salvaged from from old 911 systems.

Mostly they are good, and many times more than enough for  hobby use freq reference use.

But it need know its limits. Specially for short time accuracy.

[rf-loop]

Specifications Trimble:
http://www.trimble.com/timing/thunderbolt-e.aspx
There is further
http://trl.trimble.com/docushare/dsweb/Get/Document-383329/022542-010B_Thunderbolt-E_DS_0807.pdf
10 MHz accuracy . . . . . . . . . . . . . . . . . . . . 1.16 x 10 - ¹² (one day average)
I gave the value of 1x10-11 (especially a little worse)
For our purposes can be measured with an accuracy of 1 Hz to 100 GHz  

Yes of course one day average.  And if all other things are optimally. It is just "up to" value.
It is totally different case than short time stability!
How long time you use for example measure some frequency. 10s interval... 100s interval..   if you blindly trust trimble is sure better than 1x10-9 momentarily. It is religion.
Of course it may be. But this is other case and if you  have not other realiable reference you do not know if it is accurate or far away.  It is good to remember.

"there is always exactly accurate time if there is alone and only one clock." It include more wise than it looks like first.. 

[EV]

Here is some statistics with my Trimble Thunderbolt. In the attachment is 8 measurements of 36 samples with 10 minutes gate time. So one measurement takes 36 x 10 minutes (6 hours).

[alm]

People shouldn't be too religious about rubidium oscillators sourced from a random eBay source either. Working rubidium oscillators have a very stable frequency, but it's not a primary standard, so no guarantee of its accuracy unless you calibrate it to another source.

A GPSDO is referenced to very accurate (not just stable) frequency standards in GPS satellites, so there is some assurance of absolute accuracy. The short term stability is limited by the (usually) OCXO, however.

[max-bit]

For amateur uses a huge quality
The frequency is calibrated to GPS
A GPS satellites are built Cesium clocks
Of course, you should be aware of the pros and cons.

In Europe they were used patterns such as relying on the DCF77 signal
In Poland, in 80's, the standard frequency signal was transmitted from:
http://en.wikipedia.org/wiki/Warsaw_radio_mast

[max-bit]

http://www.ko4bb.com/Manuals/05)_GPS_Timing/Datum/LPRO/Datum_LPRO-101.pdf
hmmmm 

[max-bit]

[rf-loop]

Here is some statistics with my Trimble Thunderbolt. In the attachment is 8 measurements of 36 samples with 10 minutes gate time. So one measurement takes 36 x 10 minutes (6 hours).

Yest, this same kind of what I have get with littlebit different timing and setup some years ago.

But then short time frequency from Trimble Tb, example 10s or 100s, it is different case.

More long average and GPS DO reach of course more and more GPS system accuracy.

Btv, EV, you have good LPRO individual and perhaps also its temperature is quite stabile.
(some LPRO's are more and some less sensitive for temp.)

I have many long records from test where was reference from good individual HP Z3801 as reference for HP53131A and under test some LPRO.  53131A was connected to printer and result is only there. ( It was for find good and bad individuals from "lot" of LPRO's and also for fine adjust them back to factory limits  5x10-11 after they was used variable time in some systems. After they arrive these was drifted (or mis aligned) so that they was randomly inside +-1x10-9. and some of units also failed...  but most of them need only adjust some internal things as described in service manual.)

In this case I find that trimble can not use for calibrating becouse it takes too long time to wait averaging for enough reliable accuracy.  If have only one what need adjust and have time for it, no problem. Trimble can use. But if need adjust many nearly as work... no way.

Of course there is much what can do for make trimble better. But in this time I did not know all things for this.
And becouse this time I have two very good Z3801 individuals running for ref, it was better solution.



 

[max-bit]

I do not see the problem
The precision of 1 (0,1) ppb is sufficient for applications amateur
and so no quartz clock will not be better ....
And the rubidium clocks from ebay and so there are new so ... we do not know what generates