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On 4/6/2009 5:27 PM, Graham / KE9H, in the thread entitled <i>[hpsdr]
GPS 1PPS board</i>, wrote:<br>
<blockquote cite="mid:49DA73D8.3040806@austin.rr.com" type="cite">The
Motorola UT+ (8 satellite tracking) was replaced by the
<br>
M12+ (12 satellite plus improved jitter) and is now replaced
<br>
by the iLotus M12M.(3 volt power, and more improved jitter.)
<br>
The Motorola design was sold to iLotus in Singapore. So new
<br>
12 channel GPS timing receivers with high accuracy timing
<br>
outputs are available, quoting 10 ns (1 sigma) jitter without
<br>
granularity (sawtooth) correction and 2 ns. jitter with the correction.
<br>
<br>
<a href="http://www.ilotus.com.sg">http://www.ilotus.com.sg
</a><br>
<br>
sold in US by
<br>
<br>
<a href="http://www.synergy-gps.com/">http://www.synergy-gps.com/
</a><br>
<br>
Trimble also makes a line of timing receivers called Trimble-Resolution
<br>
that have been out there for 5 years or so.
<br>
<br>
<a href="http://www.leapsecond.com/pages/res-t/">http://www.leapsecond.com/pages/res-t/</a>
<br>
<br>
<a class="moz-txt-link-freetext" href="http://www.trimble.com/timing/resolution-t.aspx?dtID=overview">http://www.trimble.com/timing/resolution-t.aspx?dtID=overview</a>
<br>
<br>
Development kits for the receivers in current production are available.
<br>
</blockquote>
Just to add some factoids to this discussion, I'd suggest that you look
at all the material that Rick Hambly (W2GPS) and I have posted on our <a
href="http://gpstime.com">http://gpstime.com</a> website. In
particular I draw your attention to<br>
<ul>
<li>My <font face="Trebuchet MS, Arial, Helvetica"><b><i><font
size="+1">Low-cost, High Accuracy GPS Timing </font></i></b></font>talk
from the ION 2000 conference. This pretty well documents the pre- and
post-S/A performance of the 6 & 8 channel Motorola Oncore receivers.</li>
<li>Rick's <font face="Trebuchet MS, Arial, Helvetica"><b><i><font
size="+1">Critical Evaluation of the Motorola M12+ GPS Timing Receiver
...</font></i></b></font> paper presented at the 2002 PTTI meeting
which discusses the performance and <i>accuracy</i> (as opposed to<i> </i>the<i>
precision</i>) of the Motorola M12+ timing receiver. Motorola dropped
out of the timing GPS receiver market and sold the design to iLotus who
now manufacture it in Singapore, sold in the US by Art Sepin @ Synergy
(URLs are all in Graham's message copied above).</li>
<li>The take a look at my <font face="Trebuchet MS, Arial, Helvetica"><b><i><font
size="+1">Timing for VLBI</font></i></b></font> series. I recommend
the 2007 version (there should be a 2009 update in a month or so). In
the 2007 paper, starting on Slide #20 and going thru #24, I describe
the reason that the old Oncores have ~100 nsec peak-to-peak sawtooth
"dither" while the newer M12+ shows about ¼ of the dither. In #23, you
see a "hanging bridge" when the receiver's internal xtal clock passes
thru a zero-beat with the GPS "true" clock.</li>
<li>In the M12+ (and in the earlier Oncores), the serial binary data
tells you the "sawtooth" error that<big> <b>will be</b></big> present
on the <big><b>next</b></big> 1PPS pulse with 1 nsec resolution. If
this correction is applied to an M12+ receiver, the sawtooth disappears
and the resultant 1PPS signal shows only ~1½ nsec jitter -- i.e. the
1pps pulse-to-pulse (corrected for dither) is a <i><b> frequency</b></i>
standard at a level ~1x10e-9 (i.e. 1 Hz @ 1 GHz). In my paper, on
slide #4, I introduce the Allan Deviation as a way to graphically view
the relative performance of several high-performance oscillators over
time scales ranging from 100 msec to a couple of weeks.</li>
<li>On slides #26 & 27, I describe the hardware circuit that Rick
has built into his CNS Clock 2 to provide a de-dithered 1pps signal on
a connector. It works. Slides #30-38 may also be of interest. Slides
from 39 onwards are quite specific to VLBI and should be ignored.</li>
<li>The <font face="Trebuchet MS, Arial, Helvetica"><b><i><font
size="+1">Improving the Performance of Low Cost GPS ... </font></i></b></font>paper
is pretty much the same as the 2007 VLBI paper, with one exception; on
slides #16-18 you will see us pondering why the M12+'s message telling
of the error on the next pulse, when fed into the programmable delay
line, results in only a ~90% correction to the M12+'s sawtooth. To make
a long story short, Rick replaced the Maxim delay line with a similar
part with 4x the resolution and changed the algorithm that generated
the programmed correction. We found that ALL Maxim part had the same
scale error. In essence, Maxim has a definition of 1 nsec about 10%
different from the rest of the world <span class="moz-smiley-s3"><span>
;-) </span></span>.</li>
</ul>
Concerning the Collins/Rockwell timing receivers with 10 kHz outputs:<br>
<ul>
<li>Yes, you can use the 10 kHz signal to program a GPSDO and it
works fairly. One design I did used a ~100 second time constant to lock
some pretty good xtals. I had to take special care in the 10 MHz =>
10 kHz divider chain. I found that a long ripple counter with cascaded
74HC390 dual 10x dividers (like several of the designs cited here)
exhibited large temperature driven variations in the phase of the 10
kHz signal -- upwards of 200 nsec on the 1PPS counter stage when used
in a typical lab environment! <br>
</li>
<li>The fix for the temperature-driven drift (which also took up a
lot less board space) was to use Tom VanBaak's deterministic PIC-based
"software" design. With one cheap PIC, he provides all the decade steps
from 1 MHz to 10 seconds with very little temperature problems. For
details, the design, including firmware code, is available at <a
href="http://www.leapsecond.com/pages/ppsdiv/">http://www.leapsecond.com/pages/ppsdiv/</a>.</li>
<li>The sawtooth dither on the 10 kHz signal appears like an
interpolated version of the dither on the Motorola. It's magnitude
between adjacent 10 kHz pulses is 1/10,000 of what it would be at 1PPS.
The 1PPS and 10 kHz signals are synchronous..</li>
<li>So the logical question to ask is "can we de-dither the signal
just like the Motorola?"; the answer is <b>NO</b>. After
Collins/Rockwell (CR) dropped their timing receiver was picked up by
Connexant. I met with some of the Connexant engineers and was able to
garner some idea of the internal architecture. Where the Motorola
design (see slide #20 in the VLBI Timing paper) uses a long counter
with the 1PPS output based on the integrated count, the CR (and its
derivatives) uses a programmable NCO. When the CPU in the receiver
determines the epoch of the next 1PPS tick and it sets the clock rate
to steer the clock to match GPS. In short, the Motorola <i>et al</i>
design matches the integrated time, the Collins/Rockwell <i>et al</i>
scheme changes the rate to minimize the epoch error. It's rather like
the difference between a full A/D converter (Moto) and a Sigma/Delta
A/D converter (the CR). The CR loses track of the integrated frequency.
The CR design has no integrator to inform the user of the predicted
error <span class="moz-smiley-s15"><span> :'( </span></span>.</li>
</ul>
I have never studied the innards of the Trimble Resolution-T so I can't
offer any comment on its performance. I did get one of the Trimble
GPSDOs via TAPR, and it's performance looks pretty good.<br>
<br>
Hope that provided some useful GPS Timing factoids. <br>
<br>
73 de Tom, K3IO (ex W3IWI) -- original developer of the TAC ("Totally
Accurate Clock") which yielded timing at levels better than 30 nsec
even when the DoD still enforced Selective Availability. But that's
another story!<br>
Hope that<br>
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