Nailed it.

The TXB0104 can't drive wire. Adding a 74LS244 to buffer the TTL from the TXB' 
has stabilised the circuit. The noise on the 5V rail is gone, as is the noise 
on the data stream. The GPS chip is talking to the PC, and is currently 
tracking 8 satellites (not bad considering the antenna is a cheap little 5V 
Trimble mag-mount patch antenna).

And now onto the fun part of the project: syncing a 10MHz oscillator to the 
GPS 1PPS signal.

Thanks,
-- 
Phil.
piclist@philpem.me.uk
http://www.philpem.me.uk/
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