Hi
> On Mar 24, 2020, at 10:01 AM, Attila Kinali <[email protected]> wrote: > > Hoi Bob, > > I see you got your hands on a PhaseStation. Color me jealous! :-) If you recall all my grumbles, back in a bit I mentioned my pile of ever more broken test gear. Keeping it all in repair had become so time consuming that nothing else was getting done. A secondary issue was the sort of bench space things like a three corner setup with HP 3048 era gear takes up ….. > > The noise floor data is impressive! For reference: Expensive DMTD > systems for metrological applications are usually at 1e-13 @ 1s > and a lot more expensive. > > I see I have to pester John more on how he designed the PhaseStation. > > One intersting thing to note is, that the noise floor does not have > an exactly 1/τ slope. Which suggests that some additional effect > of higher order is affecting the measurement. This can be seen from > the phase data, which shows a quite prominent kink around 50ks and > is (almost?) linear before and after. It would be interesting to > know what caused this. (which is why I included the phase data ….) The mixer was hooked to the inputs with some pretty good / short microwave cables. They both are as identical as Pasternack Enterprises could make them. The guys at Mini Circuits did their best to deliver a splitter (via eBay). None the less, they are by no means perfect. My guess is that the bump is an artifact of temperature change / mechanical relaxation on either the cables or the splitter. I ran similar “floor” data on a TimePod back when I had one at work. My vague recollection is that it did not go quite as far before “ripples” set in. Since that was with different cables / splitter / environment it’s not clear how they would compare directly. On the TimePod data run I could fairly easily map the ripples to the changes in room temperature. > > On Tue, 24 Mar 2020 08:03:31 -0400 > Bob kb8tq <[email protected]> wrote: > >> One of the nice things about devices that work like a DMTD is that measuring >> the floor is a matter of driving the two inputs through a power splitter. >> With a single mixer setup (which *is* much easier to build) the floor is not >> as simple to estimate. The same is true of some (but not all) counter based >> setups. > > Be careful here. DMTD and DMTD-like systems have a dependence of the noise > floor on the relative phase of the input signals. With the lowest noise floor > being at when both signals have the same phase. To trully assess the noise > floor, you have to shift the relative phases through 2π, while making sure > that the phase shift, however you implement it, does not degrade the signal. > And because you are shifting the singal, that the short term noise on the > signal is lower than the noise floor of the measurement system (in laser > systems is called the correlation length). Yup, but at least you *have* a way to do it….. Bob > > > Attila Kinali > -- > <JaberWorky> The bad part of Zurich is where the degenerates > throw DARK chocolate at you. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
