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

[Ulrich Rohde]

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[BOOK] The design of modern microwave oscillators for wireless applications: theory and optimizationUL Rohde, AK Poddar, G Böck - 2005 - books.google.com
Delivering the best possible solution for phase noise and output power efficiency in 
oscillators This complete and thorough analysis of microwave oscillators investigates all 
aspects of design, with particular emphasis on operating conditions, choice of resonators 
and transistors, phase noise, and output power. It covers both bipolar transistors and FETs. 
Following the authors' guidance, readers learn how to design microwave oscillators and …
  Cited by 198 Related articles All 6 versions 
 
 
 
 
73 de Ulrich , N1 UL 
In a message dated 1/7/2018 7:07:56 AM Eastern Standard Time, donaldbcollie at gmail.com writes:

 
 Does any limiter, soft or hard, [and perhaps any nonlinearity of power
term 3 or greater in the amplifier of an oscillator] cause the "baseband
1/f noise to translate up to the resonator frequency [a form of
crossmodulation]?. I wonder this because
phase noise vs freq plots look a bit like the 1/f plots of a resistor, or
active device, or power supply. Ceramic caps, and resonators [I`m thinking
of quartz crystals] don`t pass much DC, and as I understand it, 1/f noise
is associated with dc passing through resistors, or semiconductors. So the
best way to go might be to have a very linear amplifier, which exhibits
very low noise [perhaps 150dB below the operating level], with an AGC loop,
that sets the operating levela little below the level at which the amp
starts to clip - this could be done with a thermistor to avoid the AGC loop
altering the [optimised] operating conditions of the amp. Alternatively you
might be able to use a tetrode device like a dual gate MOSFET, and apply
the AGC to the second gate. Thus you could keep the extremely linear amp
extremely linear. [150dB below 1Volt RMS is 0.032uV RMS].
Cheers!........................................................................................................................................................................Don
ZL4GX

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On Sun, Jan 7, 2018 at 3:12 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?
>
> 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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