LHC Timing system

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LHC Timing system

Post by Tau » Tue Sep 11, 2018 9:27 am

I was reading an old document about the design of the LHC, and found it quite interesting:
http://www.hephy.at/project/cms/trigger ... LECC02.pdf
This article, even though it contains a lot of technical details, was absolutely worth the read.
To give you an idea, are some interestion quotes I found:
  • At 7 TeV, the bunch clock frequency
    will be about 40.07897 MHz

    "about" seriously? You are specifying it to 7 digits!
  • The 1917 Sopwith Camel (Fig. 1) illustrates a common
    attribute of timing and synchronisation systems

    Great introdution for such an article.
  • Phase stability measurements have been made for a period
    of several weeks over the 28.6 km fibre loop from Building 4
    through SR1 and the PCR up to SR4 and back again. They
    indicate that during a typical LHC run it should not be
    necessary to make adjustments of more than one or two of the
    TTCrx timing receiver fine deskew steps of 100 ps.

    There is so much information in here: they do take their testing seriously; timing adjustments are needed and are in steps of 1e-10 s; and they assume that correction is part of normal LHC operation.
  • The TTCrx delivers a raw 40.079 MHz bunch clock with
    an rms jitter of about 80 ps, which is rather large for the
    primary timing reference for an LHC experiment.

    Personally, I find 8e-11 s accurate enough for my daily needs, but I am glad they fixed this (downto 7e-12 s, to be exact).
  • (About latency) In the case of the TTCex transmitter, for example,
    the delay is 68 ns plus the propagation delay of 4.9 ns/m in
    the fibre.

    In plain English: if you cut off your cable 2mm too short, your timing is completely off due to the speed of light in the cable!
  • Cost is an important issue for the TTC optoelectronic
    receivers since a total of well over 10,000 will be required.

    Again, two nuggets in one sentence: yes, they do care about cost, and second: there seem to be quite a few points were accurate timing is important.
So, doing a bit of literature search, even in old LHC documents, shows you a lot on how complicated all this is. This makes me respect the design even more.
- Tau

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