Friday, 21 February 2014

GPS disciplined clock/oscillator

GPS disciplined clock/oscillator

In the electronic wilderness before I re-discovered amateur radio, I have had an interest in mapping from Boy Scouts, and GPS, from boating and 4WD (we belonged to the Mitsubishi 4WD club). I have had GPS devices ever since President Clinton opened GPS for civilian use.

I had an ALDI GPS mobile phone tracking system going, but never used. Pretty neat for $80. While Telstra don't advertise it, there is a $10 per month post-paid mobile access that suits such devices. All use is on top of the monthly charge. The tracking device reports its position when requested by SMS. Expensive if running all the time, but cheap if only used when needed, like locating a vehicle in congested traffic to find an alternative route.

Enough background. Through my local radio club, I heard about GPS disciplined 10 MHz oscillators, so I just had to have a look. They are easy to find on eBay, but not so easy to find information, documentation or software. Given I have a lot of time to wander the ether, I managed to find most of what I needed.

I duly order the GPSDC (Trimble Nortel GPSTM NTGS50AA $120), antenna (Nokia 470290a-101 Timing Reference Antenna $20) and power supply (-48V $25) from China where they disassemble old mobile phone equipment. The fellow running the store was very helpful. With everything arrived over a couple of weeks, I had a last minute panic on the coax connectors; they were odd and I couldn't find what type they were. The man from China said they were a variation of  SMB. So with overnight service from element14 (ne-Farnells), I had SMB to SMA adaptors and on familiar territory.

I wasn't planning to get it going last night, but "Big Bang Theory" was still in its St Valentines recess, so I started connecting it all up. Fortunately the -48 V is floating and the board is grounded. CDMA mobile gear uses either 24 V or -48 V; curious? The GPSDC connects to a PC via a serial port. I have a long-standing (35 years) dislike of serial ports! Running Windows 8 on a laptop with a serial to USB converter, trying to run programs written for Windows 95 or NT is courting trouble. However it eventually worked long enough to detect one satellite, with the antenna indoors, before crashing, verifying that the thing worked. Using "Lady Heather" I was going. Putting the antenna outside and leaving it run overnight, I woke up to a green "Locked" LED. The scope showed the 10 MHz I was after.

Lady Heather, bit squished but can be fixed

That beautiful green light that says locked. Orange one I think is just on/off.

Elaborate mount for GPS antenna. The ladder did fall over in the wind and broke an expensive coax adaptor. 

The board with a CRO showing the locked 10 MHz output, 2 V peak to peak.

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