[time-nuts] Phase Noise of 74AC gates

Arthur Dent golgarfrincham at yahoo.com
Sat Feb 20 16:46:23 UTC 2010


I wrote: “The input uses a LT1016 10ns comparator as the input stage
that drives several CD74AC00E quad nand gates” and got this response.

“This is a terrible design.  The comparator will add considerable
jitter.  What were they thinking? Rick Karlquist N6RK“

While my post was to show that commercial designs can use 74AC chips
as was being discussed and was not to comment on the use of the LT1016
comparator, here is another point of view.

First, you have to consider what the design parameters of this distribution
amplifier were. The unit I described could use an external input or use
an internal OCXO with a panel mounted pot to adjust EFC. The internal
OCXO is an average single oven unit that I’m sure isn’t that great so
the unit probably wasn’t designed to NIST standards. To use a Rolls
Royce where a Ford would serve the purpose wouldn’t make much sense
so my uneducated guess is the LT1016 worked fine in this application.

Second, the statement that a comparator design is terrible is generic
and may not apply to the LT1016. On this list you can find the following
info from others who have used the LT1016 comparator.     

New TAPR "TADD" kit in the works -- are you interested?
http://www.febo.com/pipermail/time-nuts/2005-December/019773.html

“Each input can be selected to drive either a 74AC14 Schmitt trigger
(TTL level) or an LT1016 high-speed comparator (with trigger point
adjustable from 0 to 5 volts).  The LT1016 has less than 10ns delay,
and initial tests show jitter that's not much worse than driving the
74AC14 directly; it's a good chip.”



      


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