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

Joseph Gwinn joegwinn at comcast.net
Sat Jan 6 16:31:31 EST 2018


> ------------------------------
> 
> 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 



> 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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