[time-nuts] IC for 1 PPS Output

John Ackermann N8UR jra at febo.com
Fri Apr 7 08:15:35 EDT 2006

Brooke Clarke wrote:
> Hi:
> I've got the TVB 1 PPS PIC circuit working and would like to add an 
> output IC to drive 50 a couple of Ohm cables.
> Need two outputs, the normal 1 PPS and also the 10 kHz output since I 
> think it would work with the SRS app note for making 1,000 TI 
> measurements in one second to better see small offsets.
> I seem to remember that there may be an internal resistance already in 
> some chips so putting a 47 or 50 Ohm resistor in series may result in an 
> impedance above 50 Ohms.  Any thoughts?
> Thanks,
> Brooke Clarke

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.


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