[time-nuts] Phase Noise of 74AC gates - application

Bob Camp lists at cq.nu
Sat Feb 20 17:36:10 UTC 2010


We do indeed make some assumptions when talking about frequency distribution.

If the need it to drive the standard input on a couple of conventional counters and an "economy class" signal generator, that's one end of the "output" spectrum.

If I want to use the signal to drive one side of a high resolution heterodyne system that's another end of the output spectrum.

If I'm driving it out of an OCXO that was cheap when it was brand new that's one end of the input spectrum. 

If I'm driving it with a hydrogen maser ....

That's a pretty big space, even looking at two basic dimensions. There certainly are a number of other dimensions you could toss in. For a lot of people a hand full of logic gates and some coils will do a very nice job. For others anything short of a full discrete design just isn't going to cut it. 

Different problems will indeed tend towards different solutions.  


On Feb 20, 2010, at 11:46 AM, Arthur Dent wrote:

> 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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