[time-nuts] FLL errors
kb8tq at n1k.org
Fri Aug 28 11:06:06 EDT 2015
> 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.
> 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,
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