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

John Miles jmiles at pop.net
Thu Jul 31 17:22:23 EDT 2008

> 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 :-)

I think you're right about that.  About the best thing you can say for using
a comparator is that it can still be better than feeding a CMOS input that
has insufficient gain by itself.  You definitely get better inband phase
noise from the output of an Analog Devices ADF-series or NatSemi LMX-series
synthesizer if you feed its reference input through an external comparator
such as an LT1016.  Is that the best part for the job?  No, but it still
offers an improvement.

Something I've wanted to do for a long time now, but haven't had enough
spare time to, is to document the additive noise of a large list of
sine->square converters.  There is an extensive table in the manual for the
Lucent rubidium oscillator that was just mentioned on eBay (
http://www.symmetricom.com/media/pdf/manuals/man-lpro.pdf ), but it has
several limitations.  It doesn't cover single-ended versus differential
variations, it only considers 10 MHz inputs, it's not clear how they
controlled for amplitude levels and source impedance, and their LT1016 test
circuit is just embarassing.

It's an interesting topic, especially when you're not trying to go for
absolute optimal performance but just want to choose a quick/simple hack
that won't degrade whatever you're driving.  The literature is biased (no
pun intended) toward more complex circuits that are quieter than most signal
sources and destinations are likely to need.  That's what would be nice
about the Lucent app note, if only it were better realized.

-- john, KE5FX

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