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.
