[time-nuts] FLL errors

[Bob Camp]

Hi

> On Aug 28, 2015, at 6:16 AM, Azelio Boriani <azelio.boriani at gmail.com> 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.
> 

Quite to the contrary, the XOR is a perfectly acceptable phase detector and 
it behaves with an adequate level of performance to be used as a phase 
detector in a LOT of places.  

An XOR with an RC is pretty much the classic “gain only” loop configuration 
of a PLL. It has a static phase error, but no frequency error. 

Bob

> On Fri, Aug 28, 2015 at 5:03 AM, Charles Steinmetz
> <csteinmetz at yandex.com> wrote:
>> 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 -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.