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

Bob kb8tq kb8tq at n1k.org
Sat Jan 6 22:03:38 EST 2018


Hi

….. except you can decide to use a “25%” design for your oscillator or you can go with a 
“50%” kind of circuit. It’s going to be a bit tough finding a crystal that is 2X higher Q ….
Rick’s papers go through a bit of just *why* you would go with the “25%” circuit. 

Bob

> On Jan 6, 2018, at 9:25 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
> Hi,
> 
> 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.
> 
> Cheers,
> Magnus
> 
> 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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