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

donald collie donaldbcollie at gmail.com
Sun Jan 7 07:02:11 EST 2018


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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