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.htmlThe 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.htmhttp://www.ke5fx.com/timelab/readme.htmConclusion: Daves Time Standard is really a very nice device, and I would not sell it.

Frank