Hi, On 2019-07-27 12:07, Attila Kinali wrote: > On Sat, 27 Jul 2019 18:21:50 +1200 (NZST) > Bruce Griffiths <bruce.griffi...@xtra.co.nz> wrote: > >> The LTC6957 is a better choice for squaring up sinewaves: >> http://www.ko4bb.com/getsimple/index.php?id=phase-noise-and-other-measurements-with-a-timepod > If you want to have a single component ZCD, then I agree. > Otherwise, a multi-stage Collins like ZCD can perform better. > Especially, if the input waveform has known properties, then > the multi-stage approach can properly optimize for those. The LTC6957 is a multi-stage device with only 4 different bandwidths to optimize for, so you can do better. It may however be good enough for many purposes. > >> Comparators are almost always noisier. >> Oliver Collins wrote a paper on optimising such sine to square converters. >> I extended the analysis to allow optimisation when the input noise of the >> cascaded stages arent equal. > There is one important point with Collins' analysis that hardly gets > mentioned: His analysis assumes that the output signal of a stage is > trapezoid. While this is true for high gain settings, it is not for > low gain settings. Ie in his example with 6 stages, the first three stages > have a total gain of 23, ie the signal has still significant curvature. > Thus Collins' analysis the noise contribution of these three stages contains > significant erros. See the attached paper for details.
The trapetzoid model is a simplification which is better than sine or square, but not perfect. Another thing with Bruce noticed was that it assumed the same noise from all op-amps, but you can choose different op-amps with different noise and slope-rates and then you need different formulas, which Bruce produced. > > Additionally, in a multi-stage ZCD, it is very important to keep the > duty cycle at 50%, as otherwise the even harmonics give rise to an increase > of flicker noise due to noise up- and down-conversion. See [1] for details. This effect has been seen by NIST for dividers, which made them conclude one needs to end with a divide by 2. Cheers, Magnus > Attila Kinali > > [1] "A Physical Sine-to-Square Converter Noise Model", by Attila Kinali. 2018. > http://people.mpi-inf.mpg.de/~adogan/pubs/IFCS2018_comparator_noise.pdf > > > _______________________________________________ > 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. _______________________________________________ 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.