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

Fri Jul 28 02:42:55 UTC 2006

To get sure your Tektronix triggers correctly, use the
"Manual trigger" mode. This way, you won't seee any
trace on the screen without a trigger source (in auto
mode, the scope will show a trace regardless it's
triggered or not). Also, many Tek scopes have a "trig
view" mode which i find very useful to view the actual
trigger source. It's especially useful to adjust the
trig threshold. On digital signals, stay away from the
HI and LOW rails (noise). One other thing, get sure
that the trig source input is DC coupled. AC coupling
(especially if the PPS pulse is wide) will show RC
time constant curves, which you do not want. Finally,
get sure your scope trigs on the UTC edge of the PPS
pulse (which is usually the LOW to HIGH transition).
The return transition is absolutely NOT standardized
to nothing and is useless.

Also, i do not know if your scope is digital (memory)
or analog, but it becomes VERY hard to precisely time 
pulse time differences with high resolution (µs/cm) on
an analog scope with a 1PPS signal (the trace becomes
extremely faint. You'll have to shut off all
surrounding lights to be able to view the trace).

Hope that it will help.

73 de Normand Martel VE2UM.
Montreal, Qc. Canada

--- Phaysal Khan <mortal_f at yahoo.com> wrote:

> 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
> 
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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 some funky 
> version of the spec. This happened a while ago when
> some sites upgraded 
> to an early version of the CDMA2000 spec, for
> instance.
> 
> b) It's synched to a CDMA base station that's just
> completely lost its 
> clock. Last time this happened, it was a BS that was
> configured with 
> the wrong # of leap seconds (a few years ago, before
> the last one), but 
> this 0.7 second offset is just weird.
> 
> Some settings and debug info from the Ct, which
> probably won't make much 
> sense unless you've got one of these things
> yourself.
> 
> 
> spstat shows:
> 
> LKD PRIB 57 212 35130 5.0 0.787
> 
> 
=== message truncated ===

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