Joseph Gray wrote: > Another newbie question (sorry). I'm using an HP 5315A Universal > Counter and doing some basic experiments to teach myself. > > Here is the setup: 100KHz square wave as the "A" input to the 5315A. > Using a BNC-tee to connect a length of RG58A/U from "A" to the "B" > input and another tee with a 50 Ohm terminator. Using the TI Avg mode > and the gate time maxed at 13.4S, I read a TI of 22.5nS. > > What about the difference in propagation delay between the 2 counter inputs? This could easily amount to several ns and account for your results.
You need to measure this delay. Split the signal in 2 and feed it to both inputs with nominally equal length cables. Measure the difference then swap the 2 cables over and measure the difference. Average the 2 differences to get the interchannel differential delay. This fails if the counter cant measure negative delays and a more complex procedure is required. Another issue is if the slew rate of the signal at the trigger threshold is low then trigger threshold offsets between the 2 counter inputs will affect the effective differential delay between the 2 inputs. > Assuming I got the numbers correct, light takes 3.3356nS to travel 1M > in a vacuum. 22.5nS / 3.3356nS/M = 6.745M. If I divide the measured > length of the coax (3.825M) by 6.745M, I get a velocity factor of > 56.7%. This is quite a bit lower than the nominal 66% I expected. > > So, is the difference due to: > The BNC-tee connectors? > My use of a 100KHz square wave instead of a PPS? > It shouldn't matter as within the resolution of your instrument the cable is nondispersive. However you need to ensure that the reflection coefficient of each 5315 input is small. > The resolution of the 5315A (100nS single-shot, 1nS averaging)? > Some other failing of the 5315A? It was just out of cal in Feb 2009. > Something else entirely? > All of the above? > > Just the differential delay between the 5315 inputs. > The reason I'm using a 100KHz square wave is that I haven't yet modded > either of the Z3801A's to bring out a PPS signal where I could use it. > That is on my short list. The 100KHz from a Spectracom 8140 was the > closest I can get right now. Is this type of measurement useless with > anything other than a PPS signal? > > No, as long as cable dispersion is insignificant. > An interesting observation - when I remove the 50 Ohm terminator, I > get a much shorter TI. I assume this is due to reflections back down > the coax. > > Yes this will alter the signals seen by each input and due to the finite slope of the input signal and differences in trigger threshold between the 2 inputs the effective differential delay will change. > Joe Gray > KA5ZEC > > Bruce _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
