[time-nuts] Notes on tight-PLL performance versus TSC 5120A

[Bob Camp]

Hi

You can also compare an oscillator to astronomical phenomena and then debate
weather they are actually oscillators or not. Comparing clocks to solar
observation was the "way to do it" for many centuries.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Didier Juges
Sent: Thursday, June 03, 2010 12:49 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Notes on tight-PLL performance versus TSC 5120A


---- Steve Rooke <sar10538 at gmail.com> wrote: 
> ...
> Lets explore frequency measurement in a way that we all can
> understand. No oscillator can be measured in isolation, it has to be
> measured against another standard oscillator.

The last part of your statement is not true. An oscillator's period can be
compared to a delay. A delay is the fundamental component of an oscillator,
but without a number of the components that can add their own noise and
instabilities.

You can test the stability of an oscillator by mixing it's output with a
delayed version of it's output. You obtain a delayed version of it's output
by running it through a transmission line. An integer number of wavelength
plus 90 degree nominal phase shift gives you optimum phase sensitivity (and
optimum amplitude noise rejection). If the temperature of the transmission
line is kept stable (which may be easier said than done when dealing with
very high stability oscillators, but somewhat easier to do when dealing with
short measurement periods), the output from the mixer can be used to
characterize the stability of the oscillator.

This method requires a mixer and a piece of coax. By adjusting the length of
coax, it works for any frequency where transmission lines and mixers are
available.

It is particularly useful to measure the phase noise of amplifiers, by
comparing the input and output phase, using the delay line to compensate the
first order phase delay through the amplifier and add 90 degrees. Using that
method, the phase noise of the source can be essentially eliminated.

One significant advantage of this method over the PLL methods, or any method
that requires a reference oscillator, is that it is immune from a lot of the
long term noise and drift effects observed in oscillators (crystal jumps for
instance).

Didier


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