[time-nuts] Upper limit on phase noise from two oscillators.

Bill Hawkins bill at iaxs.net
Wed Apr 27 16:21:43 EDT 2005

Dr. Kirkby,

Mike S's query solidified my resolve to get to the bottom of this.

Searching for "phase noise" at your web site brings up a paper that
concerns the use of lock-in amplifiers to extract 1 microvolt signals
from 3 volts of noise. The setup is described as a modulated laser
and a light receiver. Apparently, the laser is modulated with the same
sine wave as the lock-in (phase detector) amplifier. [The same technique
is used by the ancient Fluke 207 VLF receiver to pull WWVB out of today's
man-made noise. The 207 also compensates for offset.]

It seems that the sensitivity of the receiver would be affected by the
phase noise level of the oscillator. I don't know that. [There was a time
when I thought I was educated. Time has changed that.] If the oscillator
that modulates the transmitter is not the same as the oscillator for the
receiver, then relative phase noise would be all that mattered. At some
point there would not be enough phase coherence to obtain lock, no?

Is that your reason for raising the noise level of this group? When the
full extent of a question is not revealed, it sets off a wave of noisy
speculation that is not far from shared ignorance, in some cases. Not
that I haven't learned anything from all this, you understand.

Kind regards,
Bill Hawkins

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
Behalf Of Mike S
Sent: Wednesday, April 27, 2005 10:28 AM
To: Tom Van Baak; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Upper limit on phase noise from two

At 10:05 AM 4/27/2005, Tom Van Baak wrote...
>Now is low phase noise really what you're after, or
>are you more interested in low short- and long-term
>Allan Deviation?

As a rookie to this, please correct me if my understanding is off.

Phase noise is only really relevant to sine wave signals. If one had a
perfect square wave source, it would have zero Allan deviation, but
significant phase noise, due to it's difference from a pure sine wave.

If one is using the repetitive nature of a signal by triggering at a fixed
level and slope, phase noise may not have a significant impact on the
application, since a figure for phase noise doesn't necessarily imply that
the wave shape varies across cycles, nor does it indicate poor Allan

Worse Allan deviation would always be reflected in worse phase noise
(because it implies a less than perfect sine wave), but not the other way

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