[time-nuts] Noise Floor

[Bob kb8tq]

Hi


> On Mar 24, 2020, at 10:01 AM, Attila Kinali <attila at kinali.ch> 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 <kb8tq at n1k.org> 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.
> 
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