Attila, I concur with you, what Azelio described is a standard off the shelf PLL. An XOR for a Type I phase discriminator, characterized by a 90 degree phase lock, and with more complicated logic, a Type II PLL which locks at zero degrees. In a well designed loop, in both cases over the long term the frequency is exact, what it does have to a large extent, is phase jitter.
So how does someone measure an error to 2 parts in a hundred billion? Or is that a 2 cycle slip in 100 gig cycles? Thanks to all that replied. Bob >>> -----Original Message----- >>> From: time-nuts [mailto:[email protected]] On Behalf Of Attila >>> Kinali >>> Sent: Thursday, August 27, 2015 4:51 PM >>> To: Discussion of precise time and frequency measurement >>> Subject: Re: [time-nuts] Chinese GPSDO 10 MHz error >>> >>> On Thu, 27 Aug 2015 17:19:34 +0200 >>> Azelio Boriani <[email protected]> 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. >>> Metastability is the mind-killer. >>> Metastability is the little-death that brings total obliteration. >>> I will face my metastability. >>> I will permit it to pass over me and through me. >>> And when it has gone past I will turn the inner eye to see its path. >>> Where the metastability has gone there will be nothing. Only I will remain. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
