[time-nuts] FLL errors

Bob Camp kb8tq at n1k.org
Sat Aug 29 08:34:26 EDT 2015


Hi

There are many ways to implement just about anything. There are lots of features 
that can be added into any design. Multi-lock loops are something that goes back at
least into the 1940’s.

> On Aug 28, 2015, at 6:37 PM, Azelio Boriani <azelio.boriani at gmail.com> wrote:
> 
> If FLL: something that links frequencies,
>  PLL: something that links phases,
>  frequency detector: output proportional to frequency error,
>  phase detector: output proportional to phase error (XOR),
> and the original question (about FLL) was how to implement a simple FLL,
> can an FLL be made by a phase detector?

No it can not with a real phase detector. The advantage of an FLL is that it can tolerate
a frequency error and still be locked. That’s it’s advantage. If you have a phase detector,
it will run into a limit and crash with a frequency error. If you have a system that needs 
tolerate a long term frequency offset, that’s one of many reasons to use an FLL.


> Can a PLL be made by a
> frequency detector?

Again, not with a real world part. Any real detector will have a zero error. That error 
will be fatal if you are trying to do phase with a frequency detector. 

Bob

> Phase_detector_XOR + RC + VCO-> FLL: equal frequencies, phase linked
> to VCO odds,
> in this case the frequency error is 0.
> 
> On Fri, Aug 28, 2015 at 7:14 PM, Charles Steinmetz
> <csteinmetz at yandex.com> wrote:
>> 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
>> <https://en.wikipedia.org/wiki/Phase_detector>
>> <https://jaunty-electronics.com/blog/2012/09/exclusive-or-xor-gate-based-phase-detector/>
>> <http://memo.cgu.edu.tw/jtkuo/files/eelab%202014(iii)/1230_lab12_expxx_phaselockedloop.pdf>
>> and many, many, many others.
>> 
>> 
>> Best regards,
>> 
>> Charles
>> 
>> 
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