Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!eu.feeder.erje.net!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: Joerg Newsgroups: comp.arch.embedded,sci.electronics.design,comp.realtime Subject: Re: Comparing phase of physically distant signals Date: Sat, 03 Aug 2013 10:26:11 -0700 Organization: Consultant Lines: 88 Message-ID: References: <51fd247c$0$64328$c3e8da3$5e5e430d@news.astraweb.com> Reply-To: news@analogconsultants.com Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Trace: individual.net pW5XL/25mD3QDfs+ADq7bwtOI8yU3Y2vNcy3r0x010037NLzQR Cancel-Lock: sha1:vCdjrQ/Ar9eiqwY7XF46tJ8jrjc= User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070728 Thunderbird/2.0.0.6 Mnenhy/0.7.6.666 In-Reply-To: Xref: csiph.com comp.arch.embedded:12933 comp.realtime:263 John S wrote: > On 8/3/2013 12:05 PM, Joerg wrote: >> josephkk wrote: >>> On Sat, 03 Aug 2013 00:45:41 -0700, Don Y wrote: >>> >>>> Hi, >>>> >>>> [posted to S.E.D in the hope that a hardware analog might exist] >>>> >>>> I synchronize the "clocks" on physically distributed processors such >>>> that two or more different machines can have a very finely defined >>>> sense >>>> of "synchronized time" between themselves. >>>> >>>> During development, I would measure this time skew (among other >>>> factors) >>>> by locating these devices side-by-side on a workbench interconnected by >>>> "unquantified" cable. Then, measuring the time difference between to >>>> "pulse outputs" that I artificially generate on each board. >>>> >>>> So, I could introduce a disturbance to the system and watch to see how >>>> quickly -- and accurately -- the "clocks" (think FLL and PLL) come back >>>> into sync. >>>> >>>> How do I practically do this when the devices are *deployed* and >>>> physically distant (vs. "electrically distant" as in my test case)? >>>> >>>> Two ideas come to mind: >>>> 1) two equal length cables to connect the "pulse outputs" from their >>>> respective originating devices to the test gear. 2) two *radios* to do >>>> the same thing -- after accounting for different flight times >>>> >>>> [Though I wonder how hard it is to qualify two different radios to have >>>> the same delay, etc. Far easier to trim two long lengths of wire to >>>> the >>>> same length!] >>>> >>>> Of course, I would like to minimize the recurring cost of any solution >>>> as it is just present for system qualification (and troubleshooting) >>>> and >>>> offers no run-time advantage to the design. >>>> >>>> Thx, >>>> --don >>> >>> Sounds like a use case for NTP (network time protocol) which already >>> keeps millions of computers synchronized to a few milliseconds across >>> the >>> whole planet. >>> >> >> Don hasn't given us any specs yet but if this has to be in the >> sub-microsecond class then real terrestrial RF may be needed. Even old >> Loran could do that back in the 80's, over lots of miles: >> >> http://tycho.usno.navy.mil/ptti/1988/Vol%2020_13.pdf >> > > According to the paper you referenced, it was not sub-microsecond. > Under "Results" it says so. > Also, it discusses only the surface wave. Reflections may occur at other > frequencies and upset the path length. > It does fall apart when you get over 100 miles or really bad terrain. But I doubt Don needs that much. Loran is (or was) amazing. It literally saved the life of a guy at my university. He sailed his fathers large boat, alone, in the French Atlantic at the end of summer. Woe to whom who gets into a storm there. Long story short he got into a lull and then the mother of all storms rolled in. At night. He headed for a port but knew that it had a very narrow opening of just a couple hundred feet, with massive concrete and rock structures on either side. Structures that would have busted his boat into smittereens in that storm. He was not religious but turned to his navigation screen and prayed. The Loran coverage wasn't even that great. Many hours later his boat went right through the middle, he saw the two massive structures pass by on either side. He needed the bathroom, fast. -- Regards, Joerg http://www.analogconsultants.com/