[time-nuts] Noise Floor

[kb8tq at n1k.org]

Hi

Another way to check what your measurement system is doing is to 
compare two similar devices. Your guess is then that the floor must be
well below the combination of the two devices. As the difference in the 
ADEV and Mod ADEV plots shows, you also need to stick with one xDEV
as you look at this or that. 

So, what does the ADEV plot actually show? 

It's comparing device Rb 1 (the famous 5065C) to a random eBay 5065
that has taken some significant effort to get running. (So much for
guaranteed
good / working ....). 

The plot shows that at a given point the noise contribution of Rb 1 plus
the noise of Rb 2 plus the noise floor of the system is some magic number. 
Assuming these are all random noise, then normal noise power math 
applies. (two equal signals bump the total by 3db).

When would normal "noise math" not apply? If the noise was something like
a correlated spur (think 60Hz in the US) it could easily add as a voltage
rather 
than a power. Knowing *what* you are measuring impacts how you interpret 
the results. 

The normal setup for a testing bench normally includes a *lot* of A to B
comparisons between a *lot* of sources to find "the best of the best of the
best".  This is true even if one will ultimately use three corner hat
methods. 
That stuff goes much better with sources that are fairly close to each
other. 

Lots of grubby details *do* get into this. A fixture that "only" has 110 db
isolation
gives you a nasty spur at the offset frequency between your OCXO's. It
messes
up the phase noise and ADEV. Figuring out what this or that is and where it
comes 
from is not a trivial task.  Despite not being a lot of fun, it *is* part of
setting this
stuff up.

Bob

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of jimlux
Sent: Tuesday, March 24, 2020 1:21 PM
To: time-nuts at lists.febo.com
Subject: Re: [time-nuts] Noise Floor

On 3/24/20 9:26 AM, Anders Wallin wrote:

> 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.
> 

The early USRPs in particular (I don't have as much experience with the 
new little ones) are really intended for use as an educational tool - 
they don't have particularly high performance in many ways - spurious 
radiated emissions are a big problem (the internal LO leaks and gets 
into the input of your receiver) - cross talk - lack of an easy way to 
time tag data precisely or to have deterministic timing between Tx and Rx.



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