----- Original Message ----- From: "Bruce Griffiths" <[EMAIL PROTECTED]> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Saturday, June 14, 2008 2:16 AM Subject: Re: [time-nuts] Non-impedance matched antenna cables
> phil wrote: >> I know you guys like to drill holes through hairs, but .... >> >> As I follow the discussions on merits of 50/75 ohm cable, cable length >> changing with ambient temperature, tuning cable to a fraction of >> wavelength, >> power supply noise etc. >> >> Does not the "weakest" link determine it's best accuracy? If so, the >> receiver/electronics and internal programming seems to be the weakest >> link >> and all these small nano/pico second variables discussed seems moot, at >> least with unit in question. >> >> If your receiver for example computes your location with a rather large >> error in lat/lon or altitude, that error I would think would be greater >> than >> the sum of all the "small" factors/errors being discussed. I have found, >> at >> least with the Thunderbolt receivers I have used, they are rather sloppy >> in >> it's location fix but even worse in it's altitude fix. >> >> > All GPS receivers have larger height errors than latitude and longitude > errors. >> As I understand it, each foot of distance is a little over a nanosecond >> in >> delay so would not position/altitude accuracy be the biggest variable, >> not >> to mention the proper calculation and offset of antenna cable >> attenuation/length. >> >> I would be curious how the older Thunderbolt units compares to a newer >> technology receiver/timebase in the "real world". >> >> Just a thought >> >> > Read the posted paper. > The error in determining the position using modulation on the GPS > carrier can be much larger than the cable delay variations. > The effect will also depend on the correlator type used. > > Also for those that have better receivers like the M12+T, MI2M T etc, > every last nanosecond of variation matters. > The more that is known about cable instabilities etc, the better chance > one has of actually realising the potential performance of a receiver. > > There is no point in using impedance transformers until the actual > impedance of the receiver and antenna are known. > > Bruce Bruce, I can appreciate the implication of all these small variables and the wanting to better "every last nanosecond of variation" My comment was, at least as I see it, only with a more expensive receiver and better timebase will these "smaller" variables be significantly relevant. Since the discussion revolved around the Thunderbolt receiver, I would think it's internal errors are far greater than the expansion of the antenna cable or cable impedance. Phil _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.