[time-nuts] Re: Types of noise (was: Phase Station 53100A Questions)

[Attila Kinali]

On Sun, 13 Feb 2022 17:31:11 -0500
Joseph Gwinn <joegwinn at comcast.net> wrote:

> A better word than multiplicative is parametric, the varying 
> parameters being path loss and path group delay.  This is as seen at 
> the phase noise test set.

That is one way how noise can enter a system. Not the only one though.
And once it is in the system, non-linear elements can increase it. 
Look at the derivations in [1] (and to a lesser extend [2]), which
shows how the non-linearity of an element leads to up and down conversion
of noise and thus turns additive noise into multiplicative noise.
Unfortunately, whatever measurement system you use, will have non-linear
elements in it. Starting from the input amplifier and going to the
sampling system. So this kind of noise amplification is unavoidable.
(Yes, I know that I am blowing my own horn, but these are the only
publications that I am aware of that explain this phenomena at all.
Yes, I find this odd too.)


> > Amplitude and phase noise are looking at noise from two different
> > perspective. One is how large the variation of the peak of a sine
> > wave is, the other is how much the zero crossing varies in time.
> > Note that all natural noise sources will be both amplitude and
> > phase noise. 
> 
> Hmm.  One case I'm interested in is where the path attenuation varies 
> according to a random telegraph waveform, due to for instance a loose 
> connector or cracked center conductor rattling under heavy 
> vibration.  

That's an interesting noise model. And one that is oddly specific.
Did you see that in some application? If yes, could you explain a bit more?

> In this, the electrical length does not change.

I would suggest to be careful with this assumption. 1ps is equivalent
to 140µm to 200µm of distance in a cable/connector. And with phase
noise, we are looking at effects that have the equivalent time-deviation
of a few fs or a few 100nm of distance. Even if the contacts do not move much,
they still move. And that will have all kinds of effects (including change
of capacitive coupling and thus impedance and thus reflection).
I would suggest you do some back of the envelope calculation to see
whether that movement would have some measureable effect on your system
or not.


>  While 
> the source of the carrier whose PN is being measured will have some 
> mixture of AM and PM characteristic of that source, the residual 
> (added) PN will be characteristic of the transit damage encountered 
> between source and PN test set.  So wouldn't this randomly varying 
> attenuation yield mostly residual AM PN and little residual PM PN?  

What is the PN here? Phase noise? If so, then there is no AM PN.
Amplitude modulation modulates the amplitude. The zero crossing
does not change. Thus there is no phase noise. Phase noise comes¨
from phase modulation (or equivalently time modulation).

Sure, there are elements that turn AM into PM and vice versa, but
that is a different topic and I don't want to complicate the
discussion more than necessary. Though with the on/off modulation
you have, that might be something you need to look into.

I am not quite sure what kind of system you are trying to measure
and what you hope to see there. So I'm a bit careful in suggesting
things until know better what your setup is.

			Attila Kinali


[1] "A Physical Sine-to-Square Converter Noise Model"
http://people.mpi-inf.mpg.de/~adogan/pubs/IFCS2018_comparator_noise.pdf

[2] "A Fresh Look at the Design of Low Jitter Hard Limiters"
http://people.mpi-inf.mpg.de/~adogan/pubs/IFCS2019_collins_isf.pdf
-- 
The driving force behind research is the question: "Why?"
There are things we don't understand and things we always 
wonder about. And that's why we do research.
		-- Kobayashi Makoto