[time-nuts] Noise Floor

[Bob kb8tq]

Hi

Since the unit is still in warranty, I’m a bit hesitant to crack it open and start
poking around :).

While it’s not 100% obvious, the ADC clock comes out the back and then 
returns to the unit via a SMA jumper. At least in theory you *could* supply
your own clock to the unit. How well that would work …. no idea. 

The internal ADC clock also is under program control !! You can move it around
from TimeLab if you think you might have a spur issue. Pretty neat feature. 
I always wondered what I’d do if I had to test something that had a harmonic
at / near the clock in the TimePod. I never had a problem so it’s very much
a “who knows?” sort of thing to me. 

ADC wise, there are at least 4 channels into the device. You can do a four 
device cross comparison. How well that works … no idea. Fiddling with it 
*is* on the list. 

Isolation channel to channel is very good. I had issues early on with (phrase 
edited out) cables and questionable fixture wring. Channel to channel isolation
issues inside the box… not so much. At 10 MHz it was below the noise (at -130 
dbc) when I last looked. 

The three corner hat math works, but with the “normal” limitations. Comparing 
one ultra stable device (say a 5065a)  to two not so stable parts (say a pair of 
10811’s at 10K seconds) can result in strange plots.  Not the fault of the device 
(what ever you are using). It’s the limitation of the math involved. When one 
is “decades” more stable it falls apart. 

Just as with the good old 3048 ( and the TimePod), you can edit the FFT 
segment table. If you are after spurs in a specific region or have other application
specific needs, that’s a nice feature. It’s one thing  that kept the 3048’s going long 
after newer gear had mostly replaced them. 

Lots of fun !!!!

Bob





> On Mar 24, 2020, at 12:26 PM, Anders Wallin <anders.e.e.wallin at gmail.com> wrote:
> 
> Thanks for posting these!
> 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.
> 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!?
> 
> It would be interesting to know what ADC was used and if there's an
> SDR-board out there that uses the same ADC.
> 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.
> 
> Another paper to look at is the R&S one
> https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/pdfs/FSWP_Paper_EFTF2016_V02.pdf
> 
> I can try to dig out my old comparison-data [1] and include this new data,
> if Bob makes the results (or raw data) available as numbers somewhere.
> 
> Anders
> [1]
> http://www.anderswallin.net/2015/12/spectrum-analyzer-or-phase-noise-probe-for-phase-noise-measurement/
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