Eric S. Raymond writes: > Is "unbiased and has a (relatively) white spectrum" equivalent to > looking like symmetrical digital white noise around actual UTC, if you > knew what it was?
Yes, if you knew the error exactly, then looking at it as a signal in its own right. The task of the PLL is to steer the error to zero and the filtering that allows it to do this without undue overshoot or even oscillations oscillations necessarily has a few assumptions about the possible forms of error signal baked in. "Unbiased" means that various forms of averaging should converge to zero. "Relatively White Spectrum" means that there shouldn't be any concentrations of energy at specific frequencies within the loop bandwidth of the PLL (equivalently that the Fourier spectrum in that bandwidth is "flat"). Together these two conditions ensure, among other things, that the average error converges to zero smoothly and that the autocorrelation for the error signal stays close to zero for all time lags. Viewed from the other side: if you had a biased error signal, the PLL would converge to a fixed offset to UTC that was representative of that bias. If the spectrum was not white, then the PLL would develop a time-variable offset around UTC (which could end up as an oscillation). > (I'm asking this question because my inituitions about analog-level > signal processing are still weak.) In this case we're talking about digital signals all the way, but it doesn't make much of a difference for your question fortunately. Regards, Achim. -- +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+ Factory and User Sound Singles for Waldorf rackAttack: http://Synth.Stromeko.net/Downloads.html#WaldorfSounds _______________________________________________ devel mailing list devel@ntpsec.org http://lists.ntpsec.org/mailman/listinfo/devel
