[time-nuts] Chinese GPSDO 10 MHz error

Bob Benward rbenward at verizon.net
Thu Aug 27 21:25:26 EDT 2015

I concur with you, what Azelio described is a standard off the shelf PLL.
An XOR for a Type I phase discriminator, characterized by a 90 degree phase
lock, and with more complicated logic, a Type II PLL which locks at zero
degrees.  In a well designed loop, in both cases over the long term the
frequency is exact, what it does have to a large extent, is phase jitter.

So how does someone measure an error to 2 parts in a hundred billion?  Or is
that a 2 cycle slip in 100 gig cycles?

Thanks to all that replied.


>>> -----Original Message-----
>>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Attila
>>> Kinali
>>> Sent: Thursday, August 27, 2015 4:51 PM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] Chinese GPSDO 10 MHz error
>>> On Thu, 27 Aug 2015 17:19:34 +0200
>>> Azelio Boriani <azelio.boriani at gmail.com> wrote:
>>> > The simplest form of a frequency locked loop is the XOR gate, when the
>>> > driving signals are 50% square waves. To achieve lock, the phase
>>> > difference will be proportional to the voltage needed to the VCO to
>>> > generate the desired frequency. Start with a 5V digital gate, suppose
>>> > your VCO needs 2.5V to be in frequency: the XOR output will be at 50%
>>> > duty cycle to generate, out of an RC, 2.5V and the phase difference
>>> > (between the reference and the VCO) will be 90 (or 270) degrees. The
>>> > difference will be more or less than 90 if the required voltage is
>>> > more or less than 2.5V (positive EFC) or will be more or less than 270
>>> > if the VCO has a negative EFC.
>>> This is the description of a XOR gate based PLL, not an FLL.
>>> The basic difference between PLL and FLL is very very simple:
>>> A PLL measures phase, a FLL measures frequency.
>>> The control loop then steers the measured value to be as close as
possible to
>>> a predetermined constant. As this steering loop is not perfect, there
will be a
>>> small error. Depending on what is measured, it's either a phase or a
>>> frequency error.
>>> 			Attila Kinali
>>> --
>>> I must not become metastable.
>>> Metastability is the mind-killer.
>>> Metastability is the little-death that brings total obliteration.
>>> I will face my metastability.
>>> I will permit it to pass over me and through me.
>>> And when it has gone past I will turn the inner eye to see its path.
>>> Where the metastability has gone there will be nothing. Only I will
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