[time-nuts] Noise Floor

[Attila Kinali]

On Tue, 24 Mar 2020 18:26:28 +0200
Anders Wallin <anders.e.e.wallin at gmail.com> wrote:

> The user-manual provides some details on the internals:
> http://www.miles.io/PhaseStation_53100A_user_manual.pdf
> Figure 6, page 35, would indicate that there is no analog mixing stage,
> just direct ADC.

I was in the lucky position that John gave me a quick tour of the
hardware last year at IFCS. The design is basically the same
as in the TimePod (see the schematics in the back of the TimePod manual),
but with everything that John had learned from the TimePod applied.

There are a few details and these details make up quite a bit of
the better performance.

> Maybe it also shows the ADCs clocked at (max) 125 MHz, with some HPF and
> under-sampling going on in order to measure a 200 MHz carrier!?

One of the big changes is that the sampling clock can be changed.
This allows more flexibility in measurements, especially not having
any frequencies at which measurement performance would drop, because
of being too close to integer fraction of the sampling frequency.
Also the mandatory low-pass filter that the TimePod had has been
replaced by a set of filters (or no filter) to make it even more flexible.

> It would be interesting to know what ADC was used and if there's an
> SDR-board out there that uses the same ADC.

Uh.. I remember John telling me what ADC it was, but I forgot, sorry.

> Our experience with the Ettus radios is that although they have 2 channels
> there is a lot of cross-talk between rx1 and rx2 channels, which makes a
> diy SDR-based phase-meter challenging.

Yes. This is the key point here. The Ettus SDRs are just that: radios.
They don't need to have high isolation, because it doesn't matter for
what they are doing. Neither do they need to keep the two signal paths
closely matched. The PhaseStation and the TimePod are measurement
instruments and John designed them as such. When you see the inards
of the PhaseStation, you can tell that.

I am pretty sure I could design something like the PhaseStation as well.
The working principle is easy and can be explained on a napkin in 5 minutes.
But getting it to this remarkable perfomrance? Not without a lot of 
trial and error. And even then, I wouldn't be sure.

Jupp! I need one of those! And I need to learn how exactly they work!
For SCIENCE! :-D

			Attila Kinali
-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes