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Re: [Discuss-gnuradio] Frequency Offsets in RFX 2400

From: David I. Emery
Subject: Re: [Discuss-gnuradio] Frequency Offsets in RFX 2400
Date: Fri, 11 Jan 2008 16:42:41 -0500
User-agent: Mutt/1.4.1i

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

        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, address@hidden  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."

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