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

Bob kb8tq kb8tq at n1k.org
Sat Jan 6 22:05:54 EST 2018


> On Jan 6, 2018, at 4:31 PM, Joseph Gwinn <joegwinn at comcast.net> wrote:
>> ------------------------------
>> Message: 2
>> Date: Sat, 6 Jan 2018 09:19:31 -0500
>> From: Bob kb8tq <kb8tq at n1k.org>
>> To: Discussion of precise time and frequency measurement
>> 	<time-nuts at febo.com>
>> Subject: Re: [time-nuts] AM vs PM noise of signal sources
>> Message-ID: <DDEF34DD-AD21-44C6-9612-D877881078E5 at n1k.org>
>> Content-Type: text/plain;	charset=utf-8
>> 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.
> Hmm.  Not strictly true.  One can also use an AGC loop, like a wein 
> bridge oscillator.  That said, some kind of softish limiter is commonly 
> used.
> Joe Gwinn 

…. and a “soft” limiter (like an AGC)  likely will result in some very interesting noise showing up 
on the output.


>> 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
>> End of time-nuts Digest, Vol 162, Issue 10
>> ******************************************
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