From: "Joseph Gray" <[EMAIL PROTECTED]> Subject: [time-nuts] Agilent appnote Date: Mon, 25 Jul 2005 01:46:35 -0600 Message-ID: <[EMAIL PROTECTED]>
Joseph, > Does anyone have a copy of Agilent appnote 174-10 "Measuring the electrical > length (delay) of cables"? A search of the Agilent site doesn't find it. > > Thanks. Sadly, I don't have that App-note, however... May I propose that you take a sufficiently low frequency (100 kHz may suffice) squarewave, split it so that you trigger START (directly, put the splitter on the start BNC) and then the other output of the splitter goes to the cable which connects to the STOP channel. Measure and preferably integrate over some 100 measurements at least. That is at least what I would assume that 174-10 says: The 100 kHz squarewave has a cycle of 10 us, so it should suffice for a cable length of about 10000 [ns] * 2/3 * 0.3 [m/ns] = 2000 [m] = 2 km. A 1 MHz would handle cables up to 200 m, a 5 MHz up to 40 m and then naturally a 10 MHz up to 20 m, so it all depends on your cable length. If one run with a too high frequency, one has the electrical length modulo cycletime, but if you handle that ambiguity it will work just as well. There is a number of error-sources in the method (different delays down the legs of the power-splitter, different trigger positions, different waveshapes to trigger due to slope differences etc.) so if you *really* care you need to take compensation actions (measuring rise-times and compensate difference for instance). The resolution for this measurement is limited by the counters time resolution. However, if you are using the same counter for measuring the cable as you do for the actual measurements, the time-resolution is relatively on par with each other (depending on the detailed measurement setup) so it should be fairly OK. If you have access to a high speed TDR, then measure first the time to the adapter (just leave it open), then hook on the cable and leave end open and measure the time to the now delayed reflection. Half the time difference is the electrical length of the cable. Unfortunatly high speed TDRs isn't lying around as they should do, it is a good tool. Cheers, Magnus _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
