[time-nuts] Noise Floor

[Attila Kinali]

Hoi Bob,

I see you got your hands on a PhaseStation. Color me jealous! :-)

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.

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).


				Attila Kinali
-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.