Peter, I should mention that all you need to avoid amplitude-modulating your 10MHz signal by the 1PPS DC return current and pumping massive current through your coax and termination resistor is to add a 41 to 47 Ohms series resistor to your 1PPS source as close to the driver as possible... It is really that simple.
Bye, Said Sent from my iPad On Sep 14, 2014, at 9:48, "Peter Reilley" <[email protected]> wrote: > I don't want to repeat an old discussion. You certainly can > use unterminated coax as long as you are aware of the ringing > issues and the possibility of false triggering. > > I am using a terminator on the scope end of the cable. I can > see ringing similar to what is shown on the leapsecond web site > when I don't terminate. With termination the waveforms are > also similar so I am comfortable that my cables and terminators > are working as they should. My scope does not have a built-in > terminator. > > Is there a good way of searching the archives? > > My scope is a Rigol DS2202 200 MHz scope that samples at 1 GSP > in two channel mode. I understand the problems that you can run > into as you approach the sampling limits of digital scopes. > > Pete. > > > -----Original Message----- > From: time-nuts [mailto:[email protected]] On Behalf Of Said > Jackson via time-nuts > Sent: Sunday, September 14, 2014 11:36 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Correcting jitter on the 1 PPS > signalfromaGPSreceiver. > > Peter, > > You don't need nor do you want a 50 ohms end-termination on a > series-terminated 50 ohms coax cable. > > This has been discussed here extensively before, please check the archives. > Your last sentence is not correct. > > Also, you are running into your scope's BW limit if you are measuring a 4ns > risetime with a 200MHz scope.. > > Bye, > Said > > Sent From iPhone > >> On Sep 14, 2014, at 6:19, "Peter Reilley" <[email protected]> wrote: >> >> My rise time is about 4 nS. I am measuring that with my 200 MHz >> scope. I am only using 50 Ohm termination, anything else is not >> valid when using 50 Ohm coax. >> >> Pete. >> >> >> -----Original Message----- >> From: time-nuts [mailto:[email protected]] On Behalf Of Tom >> Van Baak >> Sent: Saturday, September 13, 2014 10:34 PM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] Correcting jitter on the 1 PPS signal >> fromaGPSreceiver. >> >>> The cables are not exactly the same lengths. Differences in length >>> will result in a fixed offset. I am not concerned about such fixed >>> errors, only jitter. >>> >>> I am comparing the rising edges which is what the spec defines as the >>> reference edge. >>> >>> Pete. >> >> Pete, >> >> Correct, the survey position is determined only by the phase center of >> the antenna, not by cable length. And cable length mismatches should >> make no difference in your jitter measurements. >> >> But one thing to check is how sharp the 1PPS rising edge is -- right >> at the input to your TI counter. I use a BNC tee with one leg open >> allowing a 'scope check (set to 1M input). If your risetime is a >> couple of ns like mine is, then all is well. Slow risetime can be a huge > source of timing jitter. >> Check both 50R and 1M at the counter input. Use DC, not AC coupling. >> Use fixed trigger, never auto-trigger. Pick a trigger level that >> matches the maximum slope. >> >> Some examples of good/bad GPS 1PPS risetimes: >> http://leapsecond.com/pages/gpsdo-rise/ >> >> /tvb >> >> >> _______________________________________________ >> 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. >> >> _______________________________________________ >> 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. > _______________________________________________ > 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. > _______________________________________________ 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.
