[time-nuts] FLL errors

Charles Steinmetz csteinmetz at yandex.com
Thu Aug 27 23:03:39 EDT 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,


More information about the time-nuts mailing list