[time-nuts] FLL errors
Charles Steinmetz
csteinmetz at yandex.com
Fri Aug 28 03:03:39 UTC 2015
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
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