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

Bob kb8tq kb8tq at n1k.org
Sat Jan 6 09:19:31 EST 2018


The key point missing is the fact that any real oscillator must have a limiter
in the loop. Otherwise it will “create one” by going over the max output of this or
that amplifier. To the degree that the limiter has issues (limits poorly) you will get 
AM noise.

On a practical basis, loop Q is as significant as resonator Q . The various 
elements in the loop degrade the total Q by a significant amount. Getting 25 to 
50% of the resonator Q is “doing well” with his or that common circuit. Yes, there
are even more layers past this ….


> On Jan 6, 2018, at 1:53 AM, donald collie <donaldbcollie at gmail.com> wrote:
> So to be lowest noise, an oscillator should have the highest Q resonator
> possible in its feedback loop, operate in class "A" [for maximum
> linearity], and utilise active amplifier device(s) that contribute the
> least noise [both amplitude, or 1/f], and phase. This latter implies
> operating the active device at maximum output level [ie signal to noise].
> The quality of the power supply effects the amplifier SNR, so in the
> persuit of superlative oscillator phase noise, the power supply should be
> as good as possible.
> Resistors in the oscillator carrying DC make 1/f noise - the best in this
> respect are the metal type, I think - so use metal resistors or WW.
> What are the other conciderations that come into the design, for lowest
> noise of the oscillator itself
> Split, then
> lump...;-).................................................Cheers, de : Don
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> On Sat, Jan 6, 2018 at 1:08 PM, Magnus Danielson <magnus at rubidium.dyndns.org
>> wrote:
>> Joseph,
>> On 01/05/2018 09:16 PM, Joseph Gwinn wrote:
>>> On Fri, 05 Jan 2018 12:00:01 -0500, time-nuts-request at febo.com wrote:
>>>> Send time-nuts mailing list submissions to
>>>> 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.
>>> All the points about the need for linearity are correct.  The best
>>> point of access to the math of phase noise (both AM and PM) is
>>> modulation theory - phase noise is low-index modulation of the RF
>>> carrier signal.  Given the very low modulation index, only the first
>>> term of the approximating Bessel series is significant.  The difference
>>> between AM and PM is the relative phasing of the modulation sidebands.
>>> Additive npose has no such phase relationship.
>> May I just follow up on the assumption there. The Bessel series is the
>> theoretical for what goes on in PM and also helps to explain one
>> particular error I have seen. For one oscillator with particular bad
>> noise, a commercial instruments gave positive PM nummbers. Rather than
>> measuring the power of the signal, it measured the power of the carrier.
>> Under the assumption of low index modulation the Bessel for the carrier
>> is very close to 1, so it is fairly safe assumption. However, for higher
>> index the carrier suppresses, and that matches that the Bessel becomes
>> lower. That's what happen, so a read-out of the carrier is no longer
>> representing the power of the signal.
>> However, if you do have low index modulation, you can assume the center
>> carrier to be as close to full power as you want, and the two
>> side-carriers has a very simple linear approximation.
>> Cheers,
>> Magnus
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