[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.
John
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