Didier Juges wrote: > Dr Bruce Griffiths wrote: > >> Christopher Hoover wrote: >> >> >>>> Most (except for Trimble,..) GPS receivers and antennas >>>> are designed to use 50 ohm cable. >>>> Trimble Bullet GPS antennas have a 50 ohm output impedance. >>>> Trimble literature however is ambiguous in that in the >>>> Resolution T receiver datasheets talk about using RG59 >>>> to connect to the antenna. >>>> >>>> >>>> >>> FWIW, the Thunderbolt manual says this on page 3-5: >>> >>> -------- >>> Note - RG-59 is a 75 ohm coaxial cable. The ThunderBolt and >>> the Bullet antenna are compatible with either 50-ohm or >>> 75-ohm cable. Compared to most 50 ohm cable, 75 ohm cable >>> provides superior transmissibility for the 1.5 GHz GPS >>> signal and a better quality cable for the price. Mismatched >>> impedance is not a problem. >>> -------- >>> -------- >>> Note - The input impedance of the ThunderBolt RF input & >>> its antenna is 50 ohms. >>> -------- >>> >>> >>> I would also add that BNC and N connectors come in both 50Z and 75Z flavors. >>> In fact 75Z BNC connectors are pretty common, being used in professional >>> video applications. (Check surplus BNC patch cords carefully.) On the >>> other hand, 75Z N connectors are much less common, but were used in CATV >>> plants, IIRC. >>> >>> -ch >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list >>> time-nuts@febo.com >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >> Impedance mismatch can be more problematic when connecting the PPS >> output of a GPS receiver to a time interval counter. >> RCA to BNC cables can catch one out if one doesn't notice that these use >> 75 ohm coax. >> >> 93 ohm RG62 cables with BNC connectors are not unknown, they were used >> in some nuclear instrumentation. >> I have a few of these lying around. >> >> Bruce >> >> > I noticed that when my counter insisted the PPS output of the > Thunderbolt was 5 Hz, even though it looked fine on the scope at > 200mS/div, until I set the scope's sweep fast enough (20nS/div or so) > and then I saw the reflections in the ~6 foot cable. In my case, it was > not an issue of 50 vs 75 ohm cable impedance, simply I had to terminate > the cable by setting the input impedance of the 5370A to 50 ohm instead > of the default 1 Mohm. I tried both 50 and 75 ohm terminations > (externally) and it did not make much difference to the counter, even > though the signal was definitely cleaner with 50 ohm (that cable was 50 > ohm). > > > Didier KO4BB > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Didier
I found, as expected when using a 75 ohm cable to connect a 220 ohm GPS PPS output to an HP5370A terminated in 50 ohms, the threshold setting is a bit more critical than when using a 50 ohm cable. Its advisable to trigger on the incident wave and not on a subsequent reflection. This is less of a problem if the PPS source has an output impedance closer to the cable impedance. Bruce _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
