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

Bob kb8tq kb8tq at n1k.org
Fri Jan 5 16:32:00 EST 2018


Hi

It can be either. The easy example is a tone that is outside the entire band of the noise. 
If it is a “real” noise spectrum, that’s never going to be the case. There will always be
*some* noise at the tone frequency in a real system. 

Bob

> On Jan 5, 2018, at 2:49 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> But what I'm wondering, because this is important to the discussion, is the
> tone at a frequency encompassed on both sides by the noise band?  Or
> is the tone outside the noise band?
> 
> Dana
> 
> 
> On Fri, Jan 5, 2018 at 1:35 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
>> Hi
>> 
>> The audio (or RF) tone is summed with “baseband" noise. 1/F noise seems
>> to be the flavor of the day in recent postings. The only reason to use
>> audio
>> in the example is that it is really easy to demonstrate things at audio
>> with
>> a sound card.
>> 
>> Bob
>> 
>>> On Jan 5, 2018, at 1:42 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
>>> 
>>> 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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