... | @@ -23,6 +23,48 @@ A: You mainly get two things: |
... | @@ -23,6 +23,48 @@ A: You mainly get two things: |
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A: Yes, it is documented in the [Note on using WR Switch in Grandmaster
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A: Yes, it is documented in the [Note on using WR Switch in Grandmaster
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mode](https://www.ohwr.org/project/white-rabbit/uploads/ae3282acd8f9f6c5a9067b061202277d/wr_external_reference.pdf).
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mode](https://www.ohwr.org/project/white-rabbit/uploads/ae3282acd8f9f6c5a9067b061202277d/wr_external_reference.pdf).
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### Q: We need to use 30m coaxial cable from the GPS to the WRS Grandmaster 10MHz/PPS signals and the cable will see strong temperature changes (15-35ºC).
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Temperature effects are typically very slow phenomena. The first
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question would be: do they care? Quite often people want to synchronize
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nodes among themselves, to a common reference, but they don't mind too
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much if that reference slips a bit with respect to for example GPS time.
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As you can see in
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"\[1\]":http://ipnpr.jpl.nasa.gov/progress_report/42-99/99E.PDF, the
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thermal coefficient of delay (TCD) expressed in (ppm/deg C) is itself a
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function of temperature. Taking a ballpark number of 100 ppm/degC for
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coax, we are talking about delay changes in the order of 0.3 ns. Would
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your client care if the reference slips slowly by 1 ns or so? Are they
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using this reference for something else than feeding it into the
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Grandmaster WR switch? If this phase slip is a problem, then you could
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explore coax cables not using a teflon dielectric
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"\[2\]":http://przyrbwn.icm.edu.pl/APP/PDF/119/a119z4p17.pdf or even
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active control of the temperature by enclosing the coax inside
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double-wall tubing and filling the space within the walls with
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temperature-controlled water
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"\[3\]":http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6297.pdf.
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Another possibility, if the temperature is kind of uniform along the
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cable length, is to characterize the temperature dependency of the very
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cable to be installed in a climatic chamber and then measuring
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temperature in the real system and applying phase shifts digitally.
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\[1\] http://ipnpr.jpl.nasa.gov/progress_report/42-99/99E.PDF
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\[2\] http://przyrbwn.icm.edu.pl/APP/PDF/119/a119z4p17.pdf
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\[3\] http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6297.pdf
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Note about \[2\]:
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- Management summary: LCF38-50J is most stable from the four cable
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types characterised. Take care of the teflon knee.
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The LCF38-50J is a rather thick (3/8") and possibly cumbersome
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(because rigid) cable type. If you want a fixed cable in your
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infrastructure it is recommendable though, since it is not
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excessively expensive. However, it only comes with N-connectors,
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fortunately those typically do have good phase stability.
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Authors: Javier Serrano/CERN, Tjeerd Pinkert/VU
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Amsterdam
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### Q: The last paragraph of the *Note on using WR Switch in Grandmaster mode* mentions HAL messages and RT log. I find HAL messages but what is meant by RT log and where to find it?
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### Q: The last paragraph of the *Note on using WR Switch in Grandmaster mode* mentions HAL messages and RT log. I find HAL messages but what is meant by RT log and where to find it?
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A: By the RT (i.e. realtime) log mentioned in the [Note on using WR
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A: By the RT (i.e. realtime) log mentioned in the [Note on using WR
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... | @@ -476,7 +518,7 @@ Additionally, wrs\_dump\_shmem will report ID's in fields calib.sfp.\* |
... | @@ -476,7 +518,7 @@ Additionally, wrs\_dump\_shmem will report ID's in fields calib.sfp.\* |
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-----
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-----
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12 January 2016
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21 January 2016
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