[time-nuts] GPS / GNSS front-end board

[Michael Tharp]

On 06/05/2012 11:03 AM, Peter Monta wrote:
> I've been working on a front-end board suitable for GPS and other GNSS
> systems.  It might be of interest to time-nuts given the application
> to timing receivers.

Very impressive. Since I discovered time-nuts this is exactly what I 
wanted to make, and you've gone and done all the hard stuff. If you're 
planning on doing a bulk run, count me in.

What is the function of the clock/PPS inputs? Of course I'm primarily 
interested in turning this type of frontend into a robust timing source, 
particularly if all of the GPSDO functionality can be fit into the 
existing FPGA.  This can be done either by clocking the GPS receiver 
from the disciplined clock (e.g. Trimble Thunderbolt), or by using 
independent clocks and estimating the resulting quantization error (most 
timing-oriented receiver-only modules). In a from-scratch design I don't 
think there's any reason not to use the disciplined clock, but there 
could be lurking correlation problems. In any case, I'm curious as to 
why these are piped into an ADC instead of being used to clock the 
system (10mhz) and as an async input to a hypothetical phase comparator 
in the FPGA (PPS).

I'm not entirely savvy as to what additions would be required to make 
this design more useful for an embedded timing system. A bus that could 
bring the data stream to a companion board with FPGA and/or MCU for 
doing fixes without having to pass through Ethernet would be very 
helpful. That way people with interesting ideas can just fab a board to 
stick on a header and do their thing without a full gigabit Ethernet 
stack, which essentially means a FPGA is required on the other side.

Disclaimer: I know very little about actually implementing a GPS 
receiver, and about RF in general, but I know a cool project when I see 
one and this has a lot of the elements needed for a fully open-source 
timing receiver. Looking forward to further developments, and if there's 
another list you're more active on do let me know.

-- m. tharp