On Sat, 11 Nov 2017 04:44:53 -0800 "Tom Van Baak" <[email protected]> wrote:
> That said, I want to point out that the latest GPSDO / counter from Stanford > Research continues their tradition of relatively open design. If you have an > hour, go through the very detailed user manual, which includes theory of > operation and BOM and schematics, just like the old days: > > http://www.thinksrs.com/downloads/PDFs/Manuals/FS740m.pdf > http://www.thinksrs.com/downloads/PDFs/Catalog/FS740c.pdf Oh.. very nice. I wasn't aware of this beauty! I only had a quick look at the schematics, but a few things stick out: 1) They use "half" Nutt interpolators for time stamping instead of A-B measurements as is usual with the Nutt interpolator. I was wondering why nobody did that, as it is kind of the obvious thing to do, today. 2) They use simple, time-to-amplitude converter for the interpolation. The only interesting bit there, is the, in my opinion, weird way of starting the charge using an opamp, that at the same time compensates for the diode forward voltage. 3) This is the first time I have ever seen someone even mentioning the possibility of metastability in counters or the Nutt interpolator. And they solve it in a quite interesting way too. 4) Even though they call it TCXO in the manual, the normal oscillator in the FS740 is an ovenized AT cut crystal. As it is built manually on the main PCB itself, it's nothing fancy, but apparently does the job quite well. 5) Every other clock source in the instrument is derived from the 20MHz that this central oscillator produces. Even the 12MHz of the frontpanel controller. 6) The OCXO and Rb options only phase lock the central oscillator with a small PLL at 300Hz BW. That explains that the phase noise performance is constant over all 3 options. Though I would have expected a higher close in phase noise for the rubidium, as it does in the ADEV plot. 7) The Rb only performs slightly better between taus of 100 to 10k than the OCXO. I probably have to spend some time reading the schematics, which by the way are superb. So much love to details, even small ones. You can almost feel what kind of trade-offs the engineers made and for what reason. Attila Kinali -- You know, the very powerful and the very stupid have one thing in common. They don't alters their views to fit the facts, they alter the facts to fit the views, which can be uncomfortable if you happen to be one of the facts that needs altering. -- The Doctor _______________________________________________ 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.
