Since I have not found a strong definition for the FLL, I assumed: if PLL= zero phase error (and so zero frequency error) the FLL= same frequency, random phase. The XOR with RC is a perfect fit for this: same frequency all the time but phase determined by the EFC needed to have that frequency. The phase = constant, in the XOR/RC is true as long as the VCO is stable and the EFC has not to be altered to steer the VCO, that constant is not a design parameter but walks with the VCO frequency movement.
On Thu, Aug 27, 2015 at 10:50 PM, Attila Kinali <[email protected]> wrote: > On Thu, 27 Aug 2015 17:19:34 +0200 > Azelio Boriani <[email protected]> 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 remain. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
