[time-nuts] AM vs PM noise of signal sources

Bob kb8tq kb8tq at n1k.org
Fri Jan 5 10:56:17 EST 2018

```Hi

If I pass both a sine wave tone and a pile of audio noise through a perfectly
linear circuit, I get no AM or PM noise sidebands on the signal. The only way
they combine is if the circuit is non-linear. There are a lot of ways to model
this non-linearity. The “old school” approach is with a polynomial function. That
dates back at least into the 1930’s. The textbooks I used learning it in the 1970’s
were written in the 1950’s. There are *many* decades of papers on this stuff.

Simple answer is that some types of non-linearity transfer AM others transfer PM.
Some transfer both. In some cases the spectrum of the modulation is preserved.
In some cases the spectrum is re-shaped by the modulation process. As I recall
we spend a semester going over the basics of what does what.

These days, you have the wonders of non-linear circuit analysis. To the degree
that your models are accurate and that the methods used work, I’m sure it will
give you similar data compared to the “old school” stuff.

Bob

> On Jan 5, 2018, at 6:27 AM, Attila Kinali <attila at kinali.ch> wrote:
>
> On Tue, 2 Jan 2018 23:34:18 +0100
> Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>
> [About AM noise being of equal power as PM noise]
>
>> Now, for actual sources this is no longer true. The AM noise can be much
>> higher, which is why it can be a real danger to the PM noise if there is
>> a AM to PM noise conversion. One source of such conversion can be the
>> amplification stage, but another could be a mistuned filter, which have
>> different amplitudes of the side-bands, which can create conversion as
>> the balance does not balance the same way anymore.
>
> Yes, exactly. I am currently trying to understand how noise affects
> circuits an how input and circuit noise get converted to output noise.
> First assumption that needs to be dropped is that the noise processes
> is purely additive and independent of the signal. This means that a
> noise process does not anymore produce equal AM and PM power.
>
> I think I have a 90% solution of the noise processes and conversions
> in a sine-to-square converter (aka zero-crossing detector, aka comparator).
> But there is one process that keeps puzzling me. I think I know where in
> the circuit it must come from, but I have no explanation as to how it happens.
>
>
> 				Attila Kinali
> --
> It is upon moral qualities that a society is ultimately founded. All
> the prosperity and technological sophistication in the world is of no
> use without that foundation.
>                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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