On Fri, Jan 11, 2008 at 06:46:46AM -0800, Johnathan Corgan wrote: > David I. Emery wrote: > > > A hack I have thought about adding to the USRP FPGA code (but not > > implemented yet) would allow collection of the count in a > > continuously rolling 64 MHz counter driven by the current 64 MHz > > clock on a rising or falling edge of a 1 PPS signal brought in on > > some spare pin and stuff this value in a register that could be read... > > I think would be very easy to do in the Verilog. Is there a simple > (read cheap) PPS generator I could purchase?
Yes, several... Many GPS receivers (notably the Motorola Oncore series and it's successor the M12/M12+) have a highly accurate 1 PPS output when they are locked to the satellite constellation. Typical accuracy of the 1 PPS when seeing good satellite coverage in timing mode is around 20 ns from true UTC. Motorola Oncore family receivers are often available on Ebay used or NOS for around $20-50. The M12+ is available new for reasonable prices as well. Many other GPS devices output a 1 PPS pulse of varying accuracy and stability, most are accurate to under 1 us though many of those will have several hundred ns jitter and wander on the 1 PPS. And for those who want something better, there are lots of surplus time and frequency reference boxes that show up on eBay surplus from CDMA cell sites. These contain a 10 MHz OCXO or rubidium standard disciplined by GPS to typical accuracies in the part in 10^10 to 10^11 or better area and usually provide precision 10 MHz with low phase noise, high short term stability and very good long term accuracy when GPS locked. They also provide a 1 PPS locked to GPS derived from the 10 MHz and usually ASCII RS-232/422 output of the time of day once a second. Small units of this sort - notably made by Datum, Trimble (Thunderbolt and successors) and Symmetricom are quite often available on Ebay for prices in the $100-$400 area. The Z3801A/Z3816A made by HP (now Symmetricom) is very popular with hams and available on eBay and sometimes at Hamfests - these contain one of HP's best OCXOs... (the 10811 family). > Is the standard PPS output compatible with the GPIO pins on the USRP > (voltage, drive level, rise time, etc.)? Most GPS 1 PPS is 5 volt TTL level. As such I think they will work with the USRP FPGA, but I defer to experts on the exact rules of signals for that (too lazy to look it up).... usually the 1 PPS is fairly heavy drive current and can drive significant lengths of 50/75 ohm cable (though that varies - the Oncore receivers don't have as mogey drivers I don't believe). Any interested in this topic should look up the time-nuts mailing list at febo.com... -- Dave Emery N1PRE/AE, [EMAIL PROTECTED] DIE Consulting, Weston, Mass 02493 "An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in celebration of what could have been, but wasn't and is not to be now either." _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio