[time-nuts] FLL errors

Charles Steinmetz csteinmetz at yandex.com
Fri Aug 28 13:14:37 EDT 2015

Azelio wrote:

>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.

No, not at all.  I was suggesting that the XOR PLL you were 
commenting on had problems.

First, an XOR with or without an RC is not an FLL at all, of any 
sort.  Second, the XOR gate is a time-honored phase detector for 
PLLs, and if its limitations are understood and accounted for, it 
makes a perfectly serviceable PD for a PLL.  There are other phase 
detectors that are more popular these days, for a variety of reasons, 
but the XOR works just fine in a proper design.

See, e.g.:
Best, Phase-Locked Loops (2007), pp. 16-18
Gardner, Phaselock Techniques (3rd ed) (2005), pp. 245-46
Wolaver, Phase-Locked Loop Circuit Design (1991) pp. 55-59
and many, many, many others.

Best regards,


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