Bill I agree with what you say. I am a fan of Brook Shera's loop and have used it with a Rb and between a Rb and a D 1000. I did modify the input to "fool" it to get smaller steps and the only problem is the AD 1861. Availability is not the problem, they are readily available at a price much less than the original catalog price. The problem is its performance. It was never intended for what we use it, but it was perfect for us at that time and it has no DNL specification. Richard Mc Corkle has run some tests and DNL is 8 bits which is not good for any digital loop let alone a loop with very long time constant. A 16 bit LTC 1655 would be perfect for Rb's with resolution, tuning range and aging over 10 years. I like to use my D 1000 as clean up for particular tests and since those tests are time limited again the 1655 would be perfect. I whish Brooke would consider a software change or authorize some one to do so. I am now a TB fan and use it as a frequency source for all my equipment and even as a 1 PPS GPS receiver. It will take some time but I plan to compare TB with the best affordable GPS receiver controlling a Rb when I find a way to get around the AD 1861 or some one else's loop. In the mean time I am looking forward to some one publishing test results for a Rb TB combination. But without a clean up OCXO I for one do not consider it an improvement as you pointed out. Bert Kehren Hi David,
Just to clear the AIR, all Rubidium frequency standards have a crystal oscillator as the primary signal source within the Rubidium device. The Rubidium portion of the standard is just a very high Q filter whose properties can be controlled such that it's filter's center frequency has extremely small drift. That small drift factor is, typically, way less then the resulting factors that control drift in a Quartz resonator. In order to gain the properties of the Rubidium's longer term stability and the short term noise properties of a very good Quartz oscillator you would need both items. You select a very good Quartz device and phase lock it to a really good Rubidium (with its own Quartz oscillator). You would adjust the loop constants to correct at a very slow pace consistent with the quality of the very good Quartz oscillator. To get to the next level (connection to the Nation's reference), you would discipline the Rubidium against a GPS device with an even slower loop. So, in the end you have two separate loops with three separate devices. This is not your "Nickel & Dime store" plug-and-play set up. It would have to be set up with care and some experimentation to get it right. For a reference on the basic process, you should read the QST article on Brooke Shera's GPS disciplined oscillator system. Contained within it is a description of the loop process I referred to above. To utilize his method would require upgrading his circuit design (some parts not available any longer) and some software upgrading as well to account for those changes. To obtain the QST article go to Shera's web site at http://www.rt66.com/~shera/ Also click on the "more information line" for further reading. Bill....WB6BNQ _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
