EEVblog #457 - Oscillator Calibration Followup

[olsenn]

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

If you're working with digital electronics and don't want to decompile your firmware into asm and count clock cycled explicitly, having a counter synchronized to your MCU clock can be helpful. Sometimes this is impractical, in which case a stable and accurate frequency source comes in handy. If nothing else, it gives you an idea of just how bad your crystal is.

[EV]

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 also two other not so good LPRO. One with 42 uHz std deviation and the other with 165 uHz std deviation. My two FE-5680A rubidium standards have std deviation of 73 uHz and 77uHz.

I use this Trimble thunderbolt only to adjust the rubidium standards to correct frequency. It is good for that purpose. The accuracy is good enough for me.

[NiHaoMike]

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.

Why don't they design it so that the screw is connected to ground?

[SeanB]

The screw typically is grounded, but the approach of the trimmer to the other plate is still a capacitive operation.

[rf-loop]

I use this Trimble thunderbolt only to adjust the rubidium standards to correct frequency. It is good for that purpose. The accuracy is good enough for me.

Yes, for this purpose it is very good and simple (and of course it can use as reference directly if know its limits and this is enough for needs!

[Dr. Frank]

Hello Dave,
this device already will make a Time-Nut out of you, as soon as you install an antenna on the roof top of your house, and start to measure the stabilities of GPSDO and Rb standard.
Be careful, it's highly infectious!


In this thread, there's a whole lot of misinterpretation of the behavior and characterization  of Time Standards / clocks.

Stability of a clock is always referenced to the time scale, where this is measured, i.e. short term of 0.01, 0.1, 1, 10 sec (measuring time), mid term from 10 .. 1,000 sec and long term > 1000 sec. This is described / measured by the so called Allan Variance, a special statistic, that separates the different time areas of stability.
The simple variance calculation does not do the job correctly, as it will mix all the different time domains.

XTals in general in most cases have very good short term stability, i.e. 10E-9.. 10E-12.
Also the mediocre XTAL of Daves counter will have quite good short term stability, I suspect.
But its midterm and longterm stability is really bad, additionally, XTALs can be calibrated to 0.1 ppm resolution only, as demonstrated.

OCXO (running freely) have very good short term and midterm Allan variance stability, from 10E-11 down to 10E-13 for some special designs. They also can be trimmed much finer, < 10E-9, that means at least 100 times finer than the XTAL in the 54131A.
And they stay tuned on the order of of 10E-10 for weeks and months, depending on the design and vintage of the device.

OCXO were beaten only by Hydrogen Masers, which have 10E-13 .. 10E-16 from short to mid term (one does not need necessarily a Pulsar for that stability). They need only be calibrated once against a Cs clock and stay there for years.

Atomic standards as Rb, Cs have very bad short term stabilities, due to small interrogation times of the Cs beam, or the small path in the Rb bulb, and due to their measuring principles as such.
Their mid and especially long term stabilities are extremely high, 10E-11 .. 10E-13 can easily be achieved on small, commercial devices.

They normally contain also an OCXO, which is disciplined with > 100 s Time Constant, so those combinations have very good short term stability from the OCXO and even better mid/long term stability from the atomic standard.

Btw.: The american GPS system currently relies mainly on Rb based clocks: http://tycho.usno.navy.mil/gpscurr.html
The Cs clocks on board are either out of order (shorter operation time), and/or Rb has better midterm stability, and can easily be corrected by the ground stations.

GPSDO systems like Thunderbolt, or the one Dave has acquired, have a similar performance  like atomic standards, but due to the noisy transmission path, it takes longer to reach a similar  mid/long term stability.

<10E-11 is easily achieved for short to midterm, and for 10E-12 .. 10E-13 stability and uncertainty, it takes 1 to 10 days of averaging.
Therefore that GPSDO can be used to calibrate the PRS10 Rb standard within 1 day to about 10E-12.

Several of the Time-Nuts (e.g. John Miles) have Allan Variance diagrams and explanations online, for all the mentioned types of clock, so everybody can understand easily the concept of Clock Stability :
http://www.ke5fx.com/rb.htm
http://www.ke5fx.com/timelab/readme.htm

Conclusion: Daves Time Standard is really a very nice device, and I would not sell it.

Frank

[hairykiwi]

Dr. Frank - thanks for the really interesting background info in your post and the links.

There's a good description of frequency meter measurement techniques here

Cheers grumpydoc - also interesting reading.