Hi, On 2020-01-22 01:05, Mark Haun wrote: > On Wed, 22 Jan 2020 00:30:12 +0100 > Magnus Danielson <mag...@rubidium.se> wrote: >>> What are the adverse consequences of using large divisors in the >>> loop, as would be required for my odd OCXO frequency? E.g. on >>> paper, it would seem that I could use 80 MHz / 625 = 16.384 MHz / >>> 128 = 128 kHz PFD frequency. How would this differ from a more >>> "normal" ref clock frequency of 10 or 16 MHz with smaller divisors? >> From my experience, such factors and rate of phase comparator is >> relatively easy to work with and get to work reasonably well for most >> purposes. I recommend you to use a PI-loop. >> >> For a step-up you want to keep the PLL bandwidth fairly large, and >> that helps making it easy. > Why is this? In my primitive understanding, the loop bandwidth sets the > point where the phase-noise characteristics of the reference and the > VCO are "glued together." Because my VCXO has good phase noise, and > lacks only stability (say 0.1 to 1 sec and longer), I would have > thought I would want a small bandwidth---basically I want to preserve > the phase-noise characteristics but keep it from drifting.
With higher bandwidth, it follows the reference tighter. Consider that the bandwidth of the PLL is related to the time-constant for it to react to both the reference and the steered oscillator, and it will low-pass filter the reference and high-pass filter the steered oscillator, and higher bandwidth thus suppress more variations of the steered oscillator, relaxing thermal issues for instance. If you have a loop filter being a low-pass filter rather than PI-loop, then you also get better lock-range, quicker lock-in and smaller phase-errors due to thermal effects or oscillator differences. For a PI-loop you essentially remove it from being an issue anyway and you can focus your bandwidth on phase-noise considerations, but the high-pass vs. low-pass balance remains an issue for compromise. For step-up PLLs, you typically wants a high bandwidth to keep tight phase with the reference, but for a clean-up PLL you want a low bandwidth to filter out as much noise from the reference. As you compare the phase-noise of the reference and steered oscillator, as compared on the same frequency, the optimum bandwidth is usually where they cross each other. This assumes that you actually considered all the variations. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
