[time-nuts] Z3810AS PPS to a Raspberry Pi

Dan Watson watsondaniel3 at gmail.com
Mon Mar 23 12:20:48 EDT 2015


I'll start by saying that I only very recently found time-nuts and got bitten by the "bug". It started with wanting to know more about the accuracy of the clocks driving my microcontroller projects. It quickly grew from there and I now own an HP 5334B, Agilent 53131A, a few GPSDOs, several OCXOs, and the list goes on. So thanks for the great sharing of information that goes on in this mailing list. It has helped a lot.

I purchased a Z3810AS/KS-24361 around the same time I was experimenting with Raspberry Pis as NTP servers. Naturally I wanted to try getting the unit's PPS signal into a Pi to compare it against the other GPS modules I was testing. The differential signal and very short duration of the pulse proved a bit of a hassle. I'm sure there are many ways to do this, but I decided to take my own stab at it. The solution I came up with is very simple but works well. I capacitively couple the signal into a MOSFET which triggers a monostable 555 circuit. This generates a nice clean ~100ms pulse that I send to the PI, and an LED. Because who doesn't like blinking LEDs?

When I prototyped on the breadboard, Pin 1 of the PPS connector referenced to ground pin 7 worked best. However, on the actual PCB, connecting to pins 1 and 6 provided the best results. I will have to fix that on the silk screen if I make more, as well as a few other things.

I laid out a small PCB and ordered a few boards so that I can drop it right onto the GPIO header of the Pi. I have attached a picture of the board, as well as a capture of the input and output waveforms. The blue trace is the PPS signal from the GPSDO on the gate of the MOSFET, and the yellow trace is the output of the 555. After a couple days of settling, NTP stats on the Pi seem comparable to other GPSs I've tried, but I'll have to collect more data to really be sure of the performance. I think this circuit could also be used to translate PPS signals from other items of equipment to proper logic pulses. Lots more experimenting to do...

Anyway, if anyone is interested in more information about the circuit or board let me know. And comments are welcome.


Dan W.
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