OK: the XOR gate with an RC is a defective PLL and a defective FLL. It is a simple way to have an idea of what an xLL should be but of no serious use.
On Fri, Aug 28, 2015 at 5:03 AM, Charles Steinmetz <[email protected]> wrote: > Azelio wrote: > >> 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. > > > The "x" in "xLL" refers to the parameter that is measured, which the "LL" > attempts -- more or less successfully, depending on the particular > implementation -- to drive to zero. (More correctly, the "LL" attempts to > drive the measured quantity to a constant. Many PLLs do not lock with the > controlled oscillator at 0 phase relative to the reference oscillator, they > lock near 90 or 180 degrees. This includes PLLs with XOR phase detectors, > which lock with the VCO at ~90 degrees to the reference oscillator.) > > An XOR measures the *phase* difference between two oscillators, and an xLL > with an XOR detector is, therefore, a PLL. If it is incapable of locking > stably, that does not make it an FLL -- it is just a defective PLL. > > An FLL measures the *frequency* difference between two oscillators and > attempts to drive it to zero. (As I mentioned in my previous post, because > of systematic biases, the FLL actually drives the frequency difference to a > low value near zero. Carefully engineered dither can be added to > redistribute the error stochastically around zero.) > > Best regards, > > Charles > > > _______________________________________________ > 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.
