# [time-nuts] Chinese GPSDO 10 MHz error

Azelio Boriani azelio.boriani at gmail.com
Thu Aug 27 19:36:26 EDT 2015

```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.

On Thu, Aug 27, 2015 at 10:50 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Thu, 27 Aug 2015 17:19:34 +0200
> Azelio Boriani <azelio.boriani at gmail.com> wrote:
>
>> The simplest form of a frequency locked loop is the XOR gate, when the
>> driving signals are 50% square waves. To achieve lock, the phase
>> difference will be proportional to the voltage needed to the VCO to
>> generate the desired frequency. Start with a 5V digital gate, suppose
>> your VCO needs 2.5V to be in frequency: the XOR output will be at 50%
>> duty cycle to generate, out of an RC, 2.5V and the phase difference
>> (between the reference and the VCO) will be 90 (or 270) degrees. The
>> difference will be more or less than 90 if the required voltage is
>> more or less than 2.5V (positive EFC) or will be more or less than 270
>> if the VCO has a negative EFC.
>
> This is the description of a XOR gate based PLL, not an FLL.
>
> The basic difference between PLL and FLL is very very simple:
> A PLL measures phase, a FLL measures frequency.
>
> The control loop then steers the measured value to be as close as
> possible to a predetermined constant. As this steering loop is not
> perfect, there will be a small error. Depending on what is measured,
> it's either a phase or a frequency error.
>
>                         Attila Kinali
>
> --
> I must not become metastable.
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>
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