[time-nuts] 50-ohm PPS driver

[Stewart Cobb]

The "standard cheap" way to make a 1 PPS 50-ohm output is to use a 74ACT04
hex inverter chip powered by 5V. The chip is specified for 24 mA from each
output pin. Connect 5 inputs together to your PPS signal. Run each of the 5
outputs through a 220 ohm series resistor, then tie them together as your
PPS output. This will drive about 2.4 volts into a 50-ohm load, with crisp
edges. It will easily trigger most instruments.

The chip is specified up to 6V supply voltage, so you can get an output
pulse of almost 3V if you have a 6V rail in your design.

You can use the sixth inverter as your PPS input, with two paralleled
100-ohm resistors as the load (and ESD guard) at its input pin. The
switching point for the 'ACT version of the chip is around 1.4V, so the
2.4V output signal will easily drive it.

This circuit may not be perfect, but it shows up in a lot of professional
gear.

Cheers!
--Stu