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 12:44:02 -0700 Organization: Consultant Lines: 111 Message-ID: References: Reply-To: news@analogconsultants.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Trace: individual.net UUvlCKkAjvdAXhLpMyesDQ6pQGOa7RjD7DLl77PYfFbdIXcBMn Cancel-Lock: sha1:4+SjjycR0812AkUmuCa9qCCMkr8= 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:12948 comp.realtime:278 Don Y wrote: > Hi Joerg, > > On 8/3/2013 8:17 AM, Joerg wrote: >> Don Y wrote: >>> 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. >> >> Installers will hate this. > > Exactly. Imagine point A and point B are on different floors > in different buildings, etc. "Hey, Tony, head out to the truck > and fetch the REALLY REALLY REALLY LONG cable set..." > > (Then, *deploying* those cables -- even if only for an hour or so) > No to mention the Motrin they will need for the back pain from schlepping that cable drum up three flights of stairs. >>> 2) two *radios* to do the same thing -- after accounting >>> for different flight times >> >> Installers will love this. But why different flight times? Where is this >> stuff going to be located? Down a borehole? > > If you are measuring at a third point (without knowing its > distance/ToF from each of the other two points -- cuz you > have no reference you can rely upon!), then you need to > know the difference "in time" from the "reference output" > on each device, *through* the respective radio, through the > "air" and into the "receiver" -- before it can be applied > to some bit of test kit. > If it absolutely has to be done from a separate console at a third point you can have it measure the ToF automatically in both directions. Or triangulate. AFAIU you don't need a reference, you can just pick on of the units as reference. >>> [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!] >> >> Aside from using GPS, WWV or some other reliable transmitter you could >> have a very stable oscillator on each module. Such as a TCXO. Then you >> have a transceiver on each. You perform a loopback echo locally, via an >> RF switch. That gives you the latency of each radio (should be pretty >> much the same for each module). Now only the path adds in which should >> normally be reciprocal. > > [I don't want this to be a recurring cost. So, the radio is > a "bag" attached solely for testing/calibration/troubleshooting] > Then it's even easier because you can have calibrated radios of higher quality. Why not ditch the third point and have the installer do it while next to one unit? > You still don't know what the difference in the propagation > (through air) from each transceiver. > With calibrated radios that's a piece of cake to find out. Pulse-echo. Or let the whole link oscillate. > I.e., if the distance to device 1 is A and the distance to device 2 > is B, then there will be a difference between the signals arriving > that corresponds to the difference between A and B. Even after you > compensate for any differences in the radios themselves. > > [The same is true with a single radio at device 1 "received" at > device 2 -- how much time did the RF signal take to get to device > 2 cuz it's arrival will occur after device 2's notion of the > device 1 event with which it is intended to correlate.] > > That's the appeal between "two equal lengths of wire" -- the > delay through each can be made "identical". > But radio can measure them. How much precision do you need? [...] -- Regards, Joerg http://www.analogconsultants.com/