GPS Disciplined Oscillator: Part 1

While I don’t do a lot of RF electronics work, the idea of having a lab standard frequency has appealed to me for some time. Initially, I was drawn to a Rubidium (Rb) standard, which can be had off eBay for $100-$150 or so, but these units are used, and the life left in the lamp is obviously unknown. A Rb standard is accurate to about 1*10^-11, or about one millihertz when they’re set to 10Mhz. Not too bad. A GPS oscillator, assuming well designed, should be about an order of magnitude greater, or 1*10^-12 ((That said, the Rb standard on short term (a few seconds) will be more accurate, but longer term, the GPS will be much more stable)). Plus, a GPS oscillator shouldn’t really “age” like a Rb, so there should be no need to replace the unit after so many hours of running. Further, Rb standards run hot, and use a fair amount of power (I believe about 10W). But, they “lock” within about 2 minutes of powering on. GPS standards are generally going to take a bit longer than that to lock, if powered on cold (15 minutes or so from my readings), and obviously they also require vision of the sky, but, power consumption should be on the older of 500mW-1W when running, so my inclination would be to let the unit run 24/7, and as a bonus, use it as a time source for NTP.

All this said, I’ve been keeping an eye out for one of the venerable Jupiter 12 units on eBay, and while a couple miniature ones came up a few months back, I missed out on them, and wasn’t REALLY interested because they used a very small .5mm header. Then just a couple weeks ago, two of the larger units came for sale via the same seller for $50 each. I purchased one, and started looking into creating the Phase-Lock Loop (PLL) needed to lock a 10Mhz OCXO to the 10Khz signal generated by the GPS unit. Also, had to order an antenna, and pigtail for that antenna (I decided to buy an MCX to SMA pigtail because that’s what’s used in the project I’m attempting to imitate). The PLL circuit I went with is the one created by JRMiller that I do believe is derivative, but the board looked good, and the parts weren’t too expensive. Plus, he had all the part numbers for ordering them off Newark/Farnell, which meant I was able to order the PLL board from him, and all the necessary parts in about 10 minutes time. Score!

The board came from James within a week (amazing how fast post is from the UK to the West Coast of the US), and the Newark order took closer to 2, but that’s fine… as they had all the parts in stock. While I hadn’t used Newark before, I’m pretty happy with their stock, service, and prices. They’ll certainly be one I look at from now on. =) I also already had the 10Mhz OCXO, so no need to drop money on one of those.

So, as the items were winging their way toward me, I started considering what I really wanted out of the unit. Sure, it could just be a GPSDO, but that seems like a waste since I have a GPS outputting 1PPS, and a time signal over RS232, why not use that for an NTP server? But if I’m using that signal, I don’t really need the 10Mhz OCXO running unless I’m using the whole thing as a frequency standard. So… here’s my basic goal:

Bench sized unit (in my case, I decided to use this case: HAMMOND 511-0900).
Output 10Mhz Signal when OCXO is enabled
Easy to turn on and off OCXO
Display for easily seeing if the unit is working
RS-232 output of 1PPS signal, as well as time information

That’s pretty much it. Which is why I decided to just use a simple ATMEGA328P, and a cheap LCD display I got off eBay to control both the information given by the GPS unit, and to turn the OCXO on and off via a Logic Level P-channel MOSFET (NDP6020P). I’ll be posting schematics in the next part, as well as continuing to work on doing the board layout. I’m still unsure if I’m going to throw the OCXO and AVR on the same board, or whether I’m going to just put the OXCO on a small daughter card that will hook to the PLL board… think it’ll depend on whether I can get the board layout done.

Stay tuned!

Links of interest: