Comparators have very wideband, high gain inputs with typically high noise figures. The effective input noise is determined by the noise figure and the comparator bandwidth and the fact the the comparator only utilizes a few mV of the input signal. If you are trying to square up a 10 MHz signal, and noise from DC-1000 MHz is affecting the comparator switching time, you have unnecessarily added a bunch of noise above 10 MHz. You can't filter this noise back out after the comparator output. That's the theory of it.
1/f noise is not the issue. CMOS gates have lower input noise IN RELATION TO THE SIGNAL LEVEL involved. Comparators only use a few mV of your signal. That's why the high gain is bad. The ideal circuit is a bandpass linear amplifier that makes a large filtered 10 MHz sine wave, which is then passively clipped with diodes at the logic levels you need. This is based on the paradigm described by John Dick (of JPL) in his 1990 PTTI paper on zero crossing detectors (someone posted that paper I think; anyone know the URL?). It is clear IMHO that a comparator is just about exactly the opposite of what Dr. Dick prescribed. In any event, if you actually test real comparators, you will find them to be universally lousy. I will be happy to be proven wrong if someone is aware of a good comparator. It's just that I have never met I comparator I liked :-) Rick Karlquist N6RK Didier Juges wrote: > Rick, > > Can you explain #2? > > I understand ECL has more jitter, so I understand excluding ECL based > comparators, but why excluding ALL comparators? It seems to me the > comparators allow tighter control of the threshold, so it sounds as if it > would help at very low frequencies, unless the higher 1/f noise of the > compartor dominates other factors. > > How does the 1/f noise of a CMOS gate compare to an analog comparator? > > Didier KO4BB > >> -----Original Message----- >> From: [EMAIL PROTECTED] >> [mailto:[EMAIL PROTECTED] On Behalf Of Rick Karlquist >> Sent: Thursday, July 31, 2008 3:14 PM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] What is a Time-Nut grade Zero >> Crossing Circuit? >> >> Two things NOT to do: >> >> 1. Do NOT use ECL. CMOS is much lower jitter. >> >> 2. Do NOT use a comparator to square up the sine wave. >> Especially don't use a ultrafast ECL based comparator. >> > > _______________________________________________ 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.