[time-nuts] Discussion of precise time and frequency measurement

Fri Jul 28 03:04:15 UTC 2006

The other method i use is a homebrew programmable
(thumbwheels) pulse generator (1µs resolution) to
compare my DUT (an Oscilloquartz B540 OCXO) against a
GPS receiver. The pulse generator divides the B540's
5MHz by 5 million to get an arbitrary 1PPS. The arb.
1PPS is then used as the START signal for the
programmable pulse and the end of the pulse is
compared  (using a 4046) against the GPS 1PPS. The
4046 wil then drive (via a pulse stretcher) a LED. If
the pulse is too short (Thumbwheel setting too low)
the LED will blink, if it's too long, (setting too
high), the LED will stay off. I adjust the thumbwheels
to the highest setting that allow LED blinking.

The actual thumbwheel setting is totally arbitrary and
means nothing. However, successive recordings over
time (using nothing more than paper and pen) show me
the actual progression (lead/lag) of my B540 against
the GPS. I also wrote C code to create a simple DOS
program that will calculate the actual frequency error
(using thumbwheel setting progression against elapsed
time) and necessary vernier adjust on the B540. All
manual, old-fashioned, but working!

But, the John's method (interval counter) is far
better (better resolution, easy interval reading).

73 de Normand Martel VE2UM
Montreal, Qc. Canada

--- John Ackermann N8UR <jra at febo.com> wrote:

> Hi Faisal --
> 
> I use a time interval counter to compare 1pps
> signals -- the DUT goes to
> the start input, and the reference (here, GPS) to
> the stop input.  You
> measure the time interval between the two, and use
> the change in
> interval to determine noise and offset.  For the
> LORAN experiment, I
> used my HP 5370B though this application could do
> with a much simpler
> counter.  I use homebrew logging programs under
> Linux using GPIB to log
> and process the data.
> 
> John
> ----
> 
> Phaysal Khan said the following on 07/27/2006 08:28
> PM:
> > John,
> >   At the momet I m trying to do some similar
> stuff. I want to compare PPS output from two
> different rcvrs, the reference being Leica MC500 and
> the subject recvr being M12. but I am having problem
> with my oscilloscope which is a Tektronix one with
> 300 MHZ range. It is not locking on to the signal
> and the signal keeps on sliding sideways (seems to
> be a triggring problem). 
> >    
> >   by the way, could you let me know what scenario
> have you build for comparing your signals and what
> instrument are u using to check the offset between
> the two.
> >    
> >   Regards
> >   Faisal
> > 
> > time-nuts-request at febo.com wrote:
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> > Today's Topics:
> > 
> > 1. Re: time-nuts Digest, Vol 24, Issue 47
> (SAIDJACK at aol.com)
> > 2. CDMA time synch problem in Salt Lake City, UT
> (Dave Andersen)
> > 3. Leap second letters (Bill Hawkins)
> > 4. Re: HP 5370A manual (John Day)
> > 5. Re: GPS jamming (Robert Atkinson)
> > 6. Loran timing experiment (John Ackermann N8UR)
> > 7. Re: Leap second letters (Glenn)
> > 8. Re: Leap second letters (Tom Van Baak)
> > 
> > 
> >
>
----------------------------------------------------------------------
> > 
> > Message: 1
> > Date: Wed, 26 Jul 2006 21:12:12 EDT
> > From: SAIDJACK at aol.com
> > Subject: Re: [time-nuts] time-nuts Digest, Vol 24,
> Issue 47
> > To: time-nuts at febo.com
> > Message-ID: <556.3ec51a9.31f96cec at aol.com>
> > Content-Type: text/plain; charset="US-ASCII"
> > 
> > Hello Faisal,
> > 
> > a small annecdote about GPS jamming from the
> design of our FireFox GPS 
> > Disciplined synthesizers:
> > 
> > We have a broadband synthesizer driven by a GPSDO
> on the same PCB. This 
> > synthesizer would completely swamp out the GPS
> receiption if you set it to 
> > 1574MHz CW output, the on-board noise was so
> powerfull (the output of the unit can 
> > be set from DC to 1640MHz). Our output can go up
> to +18dBm, millions of times 
> > more power than the GPS signal itself...
> > 
> > The effect was that the M12+ receiver would just
> loose lock within a +-5 - 
> > 10MHz bandwidth around the GPS carrier. The
> receiver would show 0 sattelites 
> > being received. As soon as you set the frequency
> outside of this band, 
> > everything was fine. 
> > 
> > We improved this by putting the GPS board into a
> metal shield. So the effect 
> > of noise generated inside the enclosure was
> greatly mitigated.
> > 
> > But the CW power radiated by the BNC connector
> itself on the unit is still 
> > enough to find its way to the antenna 10 meters
> away and about 3m above it, and 
> > swamp the signal!
> > 
> > Putting a small paperclip into the BNC RF output
> connector at +18dBm at 
> > 1574MHz would probably cause a couple of blocks in
> our neighbourhood to loose GPS 
> > lock :)
> > 
> > Never tried this and never will of course.
> > 
> > bye,
> > Said
> > 
> > 
> > ------------------------------
> > 
> > Message: 2
> > Date: Wed, 26 Jul 2006 21:14:34 -0400
> > From: Dave Andersen 
> > Subject: [time-nuts] CDMA time synch problem in
> Salt Lake City, UT
> > To: Discussion of precise time and frequency
> measurement
> > 
> > Message-ID: <44C8137A.40303 at cs.cmu.edu>
> > Content-Type: text/plain; charset=ISO-8859-1;
> format=flowed
> > 
> > I've sent this to EndRun tech support, but since
> they're not around at 
> > the moment, figured I'd run a quick check with
> people on the list. I 
> > don't really expect an "oh, here's your problem",
> but I'd love one if 
> > someone happens to know!
> > 
> > I have a somewhat older Praecis Ct (CDMA cellular
> time receiver) that 
> > seems to be having either general problems or leap
> second problems, or 
> > is locking itself to a bad CDMA station. I'm a
> little baffled.
> > 
> > ntpdc> pe
> > remote local st poll reach delay offset disp
> >
>
=======================================================================
> > *clock.xmission. 155.98.35.100 1 64 377 0.03848
> 0.664292 0.00499
> > =ops.emulab.net 155.98.35.100 2 64 377 0.00032
> 0.629554 0.00421
> > =GPS_PALISADE(0) 127.0.0.1 0 16 377 0.00000
> 0.000439 0.00023
> > 
> > I reset it and tried again a bit later, with more
> ntp servers:
> > 
> > =nist1.symmetric 155.98.35.100 1 64 1 0.03024
> 0.681881 7.93750
> > =clock.xmission. 155.98.35.100 1 64 1 0.03828
> 0.704977 7.93750
> > =ops.emulab.net 155.98.35.100 2 64 1 0.00037
> 0.683862 7.93750
> > *GPS_PALISADE(0) 127.0.0.1 0 16 37 0.00000
> 0.000297 0.43768
> > 
> > (top is nist1.datum.com, which I'm fairly certain
> is good)
> > 
> > Running the latest firmware from the website:
> > 
> > Praecis Ct FW 6010-0001-000 v 2.18 - May 25 2004
> 16:09:30 Praecis FPGA 
> > 6020-0001-000 v 09
> > 
> > The node is located in Salt Lake City, UT, at the
> University of Utah.
> > 
> > My guess, based on the other times I've seen
> things like this happen, is 
> > either:
> > 
> > a) It's synched to a CDMA base station that's
> running 
=== message truncated ===

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