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

Bob kb8tq kb8tq at n1k.org
Sun Jan 7 10:08:36 EST 2018


> On Jan 6, 2018, at 9:12 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> One point about oscillator design I've not yet seen mentioned is this: the
> limiter
> must not degrade the resonator Q when in action.  Hence, a pair of diodes
> connected in parallel back to back, across a shunt resonator, would be a bad
> thing to do from the perspective of low phase noise. A differential
> amplifier
> that limits by running out of current on peaks, driving a shunt resonator,
> is
> a much better way even though one pays a price in having more transistor
> noise in the circuit.
> I've long wondered if a very slow AGC might avoid the nonlinear mechanisms
> issue except, of course, for things happening within the AGC loop's
> bandwidth.
> Is anybody reading this aware of what the truth really is?

If you have a “very slow AGC” it will only take care of AM noise *inside* it’s 
bandwidth. You very much want to eliminate that noise rather than just let
it roar on down the road. AGC plus limiter works mainly because you are 
trying to take care of close in noise with the AGC and you let the limiter
handle the rest. If you build an AGC plus limiter oscillator it’s pretty simple
to adjust the AGC into various regions. It’s a “look at the data and adjust”
sort of process. 


> Dana
> On Sat, Jan 6, 2018 at 4:29 PM, Magnus Danielson <magnus at rubidium.dyndns.org
>> wrote:
>> On 01/06/2018 10:31 PM, Joseph Gwinn 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.
>> Regardless what non-linear mechanism in play, this remains a non-linear
>> mechanism that achieves the goal. Choose wisely.
>> Cheers,
>> Magnus
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