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.
