[time-nuts] GPS orthodontics: sawteeth & hanging bridges

[Randy Warner]

Paul is right about the temp sensitivity. I am surprised he managed to
keep the receiver balancing on one foot for seven minutes, but if you
try hard enough, I guess anything is possible.

I thought everyone on this thread might be interested in what is causing
the hanging bridge, so here goes.

All Oncore receivers (M12+, UT+, GT+, M12M) are basically run by an
internally generated 1KHz clock that is "normally" asynchronous to the
1PPS epochs. If you could compare this 1KHz to the 1PPS on a scope you
would see the 1KHz slowly walking in comparison to the 1PPS. Under
typical conditions these "time slips" occur about once every 15 seconds
or so. Every time you see a "hanging bridge" you know that a time slip
has occurred. Naturally, as temp is changed the 1KHZ will change
frequency, changing the time between each bridge, but it's usually in
the 10-20 second range.

I have seen several receivers returned over the years that the customer
returned because of the sawtooth not working. Naturally, when the
receiver is fired up in my lab it works perfectly....... After reading
PHK's post I'll bet even money that the 1KHz became synchronous in the
affected receiver for whatever reason. 

Those of you who worked with the UT+ receiver may remember a problem
with the time slips in one version of firmware. Motorola updated the
firmware to do some minor housecleaning and accidentally messed up the
100PPS. The 100PPS worked normally until a time slip occurred at which
time the 100PPS pulse train was delayed by 1ms. It immediately corrected
itself on the next second, but having your 1PPS slip 1ms every 15
seconds or so was a real show-stopper. Luckily, few people use the
100PPS function, so it was not a critical problem. I sure got REAL TIRED
of reflashing those receivers however.....


Randy 
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____________________________

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Poul-Henning Kamp
Sent: Thursday, December 21, 2006 11:17 AM
To: Tom Van Baak; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] GPS orthodontics: sawteeth & hanging bridges
-theeffect of time averaging

In message <000801c7252f$90dad6a0$0b998843 at computer>, "Tom Van Baak"
writes:

>1 second raw samples: 10.40 ns
>then removing small linear frequency offset,
>1 second samples:  9.36 ns
>30 second averages: 9.62 ns
>300 second averages: 10.0 ns
>
>Did I do something wrong? PHK, what do you think about this?

Given what we know, the saw-tooth term has a box distribution from -A to
+B nanoseconds where A and B depends on the clock frequency used in the
receiver.

What we don't know is if A = B or for that matter if A and B are equal
to the period of the clockfrequency.

It is not inconceiveable that A might be 1 nanosecond larger than B due
to rounding and timing issues in the firmware and hardware and likewise
it is not inconceiveable that A and B are slightly larger than the clock
period for similar reasons.

We think we know that the negative sawtooth is a box distribution with
equal probability over the -A...+B interval, although the flanks may not
in fact be vertical, due to the mentioned rounding issues.

The theoretical average therefore must be the average of A and B
possibly with a minor error by the possibly nonvertical flanks.

The standard deviation must be approached with caution since it is a box
distribution and not a standard deviation.

So, based on what we think we know, the theoretical average probably is
zero, but could be a few nanoseconds on either side due to rounding and
timing.

The final thing we know is that no averaging time is long enough to
guarantee anything about the negative sawtooths actual average.

If you are unlucky, and your oncore is at a very stable temperature, the
clock generator may just stubbornly decide to run at an integer Hz for
an hour and if by pure bad luck the sawtooth value was 20ns at the start
of that, it will be 20 nsec all the way through.

In practice, the probability for this is quite low, but I have tried
once to provoke , and it is not only possible, it is disturbingly easy:

Wrap your oncore in a couple of blankets or similar and tweak the
temperature (I varied the supply voltage so the voltage regulator loss
changed the temperature.  It takes a couple of hours to get it just
right.  I saw a hanging bridge of slightly over seven minutes this way.

One interesting result of this is that the PRS10 will run better without
the 256 exponential filter if you feed it from an oncore, if your oncore
is in a good enclosure, but if you leave it out where the temperature
changes, the 256 exponential filter helps.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by
incompetence.

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