On Fri, Jan 11, 2008 at 06:46:46AM -0800, Johnathan C. 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."