Hi, Yes, indeed, so for many purposes the 6957 is probably good enough, and actually better than many classical approaches (i.e. direct comparators). It is when you design for a fixed or very narrow range of frequencies that you should consider rolling your own, assuming the performance of the 6957 becomes a limit to what you can achieve.
Cheers, Magnus On 2019-07-27 15:49, Bob kb8tq wrote: > Hi > > Assuming we are still talking about a test instrument that needs to handle a > variety of levels > and a range of frequencies, the 6957 is probably as good as anything. > > With a “full up” Collins style circuit, you very much need to optimize for a > specific input. > Change that and you change the circuit. 1 MHz, 10 MHz, and 100 MHz will each > “want” > a very different set of parts. Change levels 10:1 and that has an impact …. > > Even if you *do* get a circuit up and running, take a look at the TC of the > caps in all those > filter stages. Matching all that up for a valid test is going to be a bit > hard. You have a wide > range of values and (likely) a range of capacitor types. Not an easy problem > to solve without > ovenizing the whole beast. Do that and you no longer have a “simple” box … > (and no guarantee > a basic oven will solve the problem …) > > Bob > >> On Jul 27, 2019, at 6:32 AM, Magnus Danielson <[email protected]> wrote: >> >> Hi, >> >> On 2019-07-27 12:07, Attila Kinali wrote: >>> On Sat, 27 Jul 2019 18:21:50 +1200 (NZST) >>> Bruce Griffiths <[email protected]> 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 -- [email protected] >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
