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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 6 09:25:13 EST 2018


I think loaded Q is being used as term these days for the effective Q of
the resonator as loaded by the support amplifier.

The Leeson model only models how noise types gets created, not how a
physical design actually works.

The modified Leeson model starts to approach the actual design.


On 01/06/2018 03:19 PM, Bob kb8tq wrote:
> Hi
> 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 ….
> Bob
>> 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
>> ZL4GX
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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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