[time-nuts] Chinese GPSDO 10 MHz error
attila at kinali.ch
Thu Aug 27 16:50:43 EDT 2015
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.
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 remain.
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