[time-nuts] Chinese GPSDO 10 MHz error
alex at pcscons.com
Thu Aug 27 18:16:13 EDT 2015
it is a bit more complicated FLL need circuit which is sensitive to
frequency difference, it looks always, PLL need a phase detector and
has a capture range, which is depend mainly on the bandwidth of the loop
there are combined phase /frequency detectors, which are sequential
circuits/logic, usually with some uncertainty --and therefore more phase
noise-- at zero phase difference
On 8/27/2015 1:50 PM, Attila Kinali 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
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