[time-nuts] GPSDO and oscillator steering - EFC vs DDS schemes?

Attila Kinali attila at kinali.ch
Tue Dec 8 18:26:40 EST 2015

Ciao Azelio,

On Tue, 8 Dec 2015 19:36:37 +0100
Azelio Boriani <azelio.boriani at gmail.com> wrote:

> Given that until now good (maximum stability) OCXO are much less than
> 100MHz, from the OCXO we exploit its high stability and we impose
> accuracy from a coordinated source: the OCXO+EFC method uses the
> built-in stability and disciplines the accuracy.
> The DDS method virtually can start from any oscillator, apply a
> suitable correction function giving the same result, transferring the
> hardware characteristic of a BVA (for example) into the driving
> function.
> Can a DDS be driven with the speed necessary to correct the output so
> that it results in the same stability as a BVA, starting from a given
> unstable oscillator?
> Or, how much unstable can be the 100MHz starting oscillator so that I
> can obtain after the DDS+suitable_driving_function  the same final
> stability as an ordinary 10MHz OCXO?

No, no matter what kind of steering you use, you will not make the
clock more stable. The BVA is a stable oscillator by and in itself.
Even if you do not lock it to anything, it rivals several comercial
atomic frequency standards in stability up to several dozen to hundreds
of seconds. If you start with an "normal" OCXO, you will never get that
stability, no matter how hard you try to compensate for all environmental
factors. But that's not the point of my question.

The point of my question was, why people use EFC control with all its
problems and its "low" resolution, when there is better methods known.
Or, whether I missed some important point that is not obvious from reading
an otherwise good paper.

			Attila Kinali
Reading can seriously damage your ignorance.
		-- unknown

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