Adding 1 PPS and 10MHz Outputs to the Z3801A

Make Z3801A GPS-disciplined oscillators more useful by making 1 PPS and additional 10MHz signals available on the rear panel, and providing a 1 PPS signal on the DCD line of the serial connector so the unit could serve as a reference clock for an NTP server. Although I ultimately want to add a distribution amplifier/splitter for the 10MHz sine wave signal, I thought it would be easier to initially add several
Adding 1 PPS and 10MHz Outputs to the Z3801A - schematic

10MHz TTL square wave outputs. This page describes how I did that. Jeff Mock has published a modification that provides a nice RS-232 level 1 PPS signal on the DCD line, so I used that as the starting point. (Note: I`ve learned that Jeff`s web page is no longer there. A mirror is at. ) Jeff taps a 1 PPS TTL level signal off of U5, pin 2 (a Lucent 1141LL, whatever that is). I used that signal to drive my 1 PPS circuit. After probing around on the board, I was unable to discover a 10MHz TTL level square wave anywhere. However, TP5 in the front portion of the main board has a nice 10MHZ sine wave available, which unfortunately is riding on top of a DC bias voltage. I used a piece of RG-174 cable to bring the signal from TP5 back to my circuit. The front pin of the adjacent TP4 complex is at ground, so I connected the RG-174 shield there. The 1 PPS circuit is very simple. I used a 74AC04 hex inverter, with the first section providing an initial inversion so that the end result remains a positive-going pulse. The output is a pair of inverters parallelled together through 51 ohm resistors; this combination provides a nice low-impedance signal that can drive a reasonable length of coax. The unused sections of the 74AC04 must be tied to something; if they float, the chip may oscillate or otherwise find itself in a state of high current draw. I tied the unused inputs to the output of the first inverter, so they are available as...

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