Hi
> On May 23, 2015, at 12:37 AM, Charles Steinmetz <csteinm...@yandex.com> wrote: > > Bob wrote: > >> The simple answer is that a biased fast CMOS gate will do a better job >> ADEV wise than your signal sources will. > > Maybe or maybe not, at tau ~1 second. Trouble is, as tau gets larger, the > gate performs *worse*. The switching threshold of all MOSFET logic devices > varies all over the place with temperature and supply voltage as well as > random drift. At tau >10 or 100 seconds, these effects become more and more > pronounced and xDEV gets worse, even if you take pains to keep the circuitry > out of drafts. Gates are not a good way to square sine waves if you care > about stability at longer tau. Yes indeed, if you have a clock that goes below 1x10^-15 at 1 second and drops linearly with tau from there, you will have issues. If you do not have such a clock. The gate probably will do just fine. The delta on the gate turns out to be a delta time (as in delta ns / ps / fs). As you go out in tau, the impact (parts in 10^whatever) of that time delta drops linearly with tau. So: what sort of clock (that you have) are you proposing to look at? Bob > > Most of what has been said against comparators on this thread are indictments > of mistakes made in applying them, NOT deficiencies of comparators per se. I > don't have the time nor energy to go into it in any depth right now, but: > Properly applied, comparators can work better than pretty much anything else > when the job is squaring a 1 to 100 MHz sine wave. > > A few "Do's" and "Do not's": > > Do use a comparator with split supplies for the input section, so you can use > actual ground as the reference voltage. Do not use inputs biased to > mid-supply. Most especially, do not use separate voltage dividers to bias > the two inputs, because the divider noise is uncorrelated and adds. If you > must use inputs biased to mid-supply, use one good, low-noise voltage > reference (LM329 or LM399) to bias both inputs so the bias noise is low and > is common-mode (make sure to keep the time constants equal at the two > inputs). But just don't use inputs biased to mid-supply in the first place. > > Do use a comparator with properly-designed internal hysteresis of a few mV > (e.g., LT1719). Do use a good, modern comparator (again, e.g., LT1719) that > was designed since chip-level thermal flow analysis became standard practice, > to avoid the mysterious drift, instabilities, and metastabilities that > comparators from the bad old days (mid-'90s and earlier) were famous for. > > Do not rely on a comparator to work with inputs from mV to 10s of volts. You > wouldn't expect that with a logic gate, why in the world would you expect it > with a comparator? Adjust the input level with amplifiers or attenuators to > the optimum value for the comparator you are using at the frequency you are > operating. > > A 5 or 10Vp-p sine wave at 10MHz slews fast enough at zero-cross not to need > bandwidth-limited clipping amplifiers (a la Dick and Collins). Those > techniques were designed for squaring audio-frequency sine waves, such as the > mixer output(s) of a single- or double-mixer system. If you feel the need, > you can increase the zero-crossing slope of the input signal by starting with > a larger input signal than is optimum for the comparator in use and using > diode clamps to limit the peak amplitude. > > There are many other best practices, but the ones above are enough to avoid > the major application mistakes and have a reasonable chance of designing > something that works to a high standard. > > Best regards, > > Charles > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
