[time-nuts] IC for 1 PPS Output

John Ackermann N8UR jra at febo.com
Fri Apr 7 08:31:02 EDT 2006

John Ackermann N8UR wrote:

> Hi Brooke --
> The circuit I use is stolen from Tom Clark's TAC -- paralleling several 
> sections of a 74AC04 hex inverter through 47 ohm resistors.  For the 
> current TADD projects, I'm using three sections that way, so each chip 
> gives me two outputs, and I use a 47 ohm SIP resistor pack to keep the 
> board space down.
> Tom came up with the 47 ohm value (though IIRC he used four sections in 
> parallel) based on compromising for maximum voltage into a 50 ohm 
> termination, and best rising edge.
> The 74AC chips don't have internal source resistors, but I think some of 
> the op amp distribution amp chips might.  For square waves at these 
> frequencies, I'd use 74AC parts over an analog circuit.
> John

I don't think I can post them until later today or tonight, but I 
recently took some photos of the TADD-3 rising edge, using the 
parallelled 74AC04s, on a 400MHz scope.  I don't recall the exact value 
right now, but the risetime is well under 5ns, and the high state is 
more than 3 volts.


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