[time-nuts] What is a Time-Nut grade Zero Crossing Circuit?

Rick Karlquist richard at karlquist.com
Thu Jul 31 16:47:33 EDT 2008

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: time-nuts-bounces at febo.com
>> [mailto:time-nuts-bounces at febo.com] 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.

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