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

Dana Whitlow k8yumdoober at gmail.com
Fri Jan 5 13:42:10 EST 2018


Is this an audio tone, summed with audio noise whose spectrum surrounds
that of the tone?

Dana


On Fri, Jan 5, 2018 at 9:56 AM, Bob kb8tq <kb8tq at n1k.org> wrote:

> 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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