Hi Do a little research and You will find that a 3 db noise figure is as good as it gets with a receiver who’s front end is a perfect match to the transmission line ….
(Signal goes down 6 db when you match, noise drops by 3 db ….). Unless you are using some sort of ferrite isolator on the front end, a low noise device is not going to be one that matches the line. Bob > On Jun 6, 2020, at 8:51 AM, Dana Whitlow <k8yumdoo...@gmail.com> wrote: > > Basically true, Tom, but with some caveats: > > The GPS system was originally designed, as you say, to provide > positions and velocity > information to military users in the field. Errors of a few meters were of > little consequence. > > But now, we have the time-nuts, surveyors, etc, who are decidedly pushing > the envelope > of GPS accuracy. Some users want sub-nanosecond timing accuracy, surveyors > want > mm-level position accuracy, etc, and doing these things indeed requires > extreme attention > to detail. > > As I understand it, multipath components that are delayed by about a few > chips or more > basically appear as a wee bit of added noise and have negligible > consequence unless > they are comparable to the direct signal in power. However, MP components > whose > delay is less than about a chip time have the effect of tugging the > apparent temporal > location of the direct signal to some new value. Note that this is in the > range of practical > round trip cable delays in at least some installations. At Arecibo, for > example, the one-way > cable lengths from GPS antennas on the roof to their respective receivers > was about > 200 ft. Round trip was thus 400 ft physical, hence up to about 600 ft > depending on > what the cables were using for the dielectric. That's about 0.6 chip > length for the C/A > code GPS signal. > > Another factor which will influence the error introduced will be the RF > phase relationship > between the direct and delayed signal component. > > Gee, this stuff gets complicated ... > > Dana > > > On Sat, Jun 6, 2020 at 7:30 AM Tom Holmes <thol...@woh.rr.com> wrote: > >> Dana... >> The question that comes to mind is just how much effect a weak a long >> delayed reflection will have on overall system performance since it will >> only matter to SV’s with poor S/N. The modulation scene which allows all >> the SV's to transmit on the same frequency has to be pretty robust in the >> face of both widely varying signal strengths and multiple signals arriving >> at different times. It’s a similar scheme to CDMA cell phones, which >> operate in a much more difficult environment with regard to signal >> strengths, multi-path, and number of on channel signals. And those work >> amazingly well. >> >> Further, I am led to believe that once you have enough SV’s in view to >> get a good set of ‘readings’, ionospheric effects are the limiting factor >> until you go to a multi-band receiver. >> >> Yes, to wring the last ounce of performance out of GPS takes attention to >> the details, but don’t lose sight of how it was designed to work for users >> in less than optimum (military field operations) in the first place. >> From Tom Holmes, N8ZM >> >>> On Jun 6, 2020, at 7:14 AM, Dana Whitlow <k8yumdoo...@gmail.com> wrote: >>> >>> It's one thing to maintain lock in a multipath environment, quite >> another >>> thing >>> to get "full" accuracy of GPS measurements of PVT. >>> >>> An interesting difference between my scenario of poorly matched >> impedances >>> and "ordinary" multipath is this: In the poor matching scenario, all the >>> received >>> signals will be impaired identically, while in the ordinary multipath >>> scenario, >>> signals from different satellites will suffer different (and >> time-varying) >>> multipath >>> impairments. I'm not at all sure what effect this difference will have >> on >>> final >>> outcome, but my gut feel is that the case where all signals are impaired >>> identically >>> could lead to worse effects. >>> >>> Dana >>> >>> >>>> On Fri, Jun 5, 2020 at 8:43 PM Tom Holmes <thol...@woh.rr.com> wrote: >>>> >>>> Dana... >>>> >>>> I think that you are neglecting two important mitigating factors. >>>> >>>> 1. the cable loss at 1575MHz, even for a 25' run of RG-6, reduces those >>>> reflections quite a lot from one end to the other. It amounts to 2 >> - 3 >>>> dB in 25', depending on cable quality. >>>> >>>> 2. a 1.5:1 SWR is not a very big reflection to begin with, on the order >> of >>>> 20% of the incident power, about 7 dB. I am rounding a lot here just to >>>> keep the math easy...for me. >>>> >>>> By the time a reflection has made the round trip from the receiver back >> to >>>> the antenna and them back to the receiver, which is how the delay would >>>> have >>>> to manifest itself, it will be down at least 15 dB from its original >> self, >>>> and probably more. Given the coding of GPS signals which allows several >>>> satellites to share a common frequency band, that is not going to be >> much >>>> of >>>> a problem. And if only one end of the path actually is 75 ohms, then >> there >>>> won't be a delayed signal. >>>> >>>> Tom Holmes, N8ZM >>>> >>>> -----Original Message----- >>>> From: time-nuts <time-nuts-boun...@lists.febo.com> On Behalf Of Dana >>>> Whitlow >>>> Sent: Friday, June 05, 2020 9:01 PM >>>> To: Taka Kamiya <tkami...@yahoo.com>; Discussion of precise time and >>>> frequency measurement <time-nuts@lists.febo.com> >>>> Subject: Re: [time-nuts] ThunderBolt question >>>> >>>> I'd like to point out that mismatches at the ends of an antenna cable >> *can* >>>> cause trouble. When both ends are mismatched, each bit of detail in the >>>> signal >>>> gets partially reflected back and forth, each time delayed by the round >>>> trip propagation >>>> delay in the cable, and so you have something like multipath going on. >>>> Fortunately the >>>> successive reflections get weaker with time, generally quite rapidly. >>>> Since many >>>> GPS users seem very concerned about multipath resulting from poor >> antenna >>>> placement, >>>> I think this factor should be considered as well and not just get swept >>>> under the rug. >>>> >>>> The amplitude of the "multipath" resulting from cable mismatches >> depends on >>>> the product >>>> of the voltage reflection coefficients at the two ends of the cable. If >>>> either end is perfectly >>>> matched, then the quality of the match at the other end is not >> significant >>>> vis-a-vis apparent >>>> multipath problems and only affects transmission loss. >>>> >>>> But when there is a mismatch on both ends, then the length of the cable >>>> comes into play >>>> as well. A longer cable means more delay between successive >> reflections, >>>> which is just >>>> like multipath involving longer delays between the direct and the >> reflected >>>> signals. >>>> >>>> Cheers, >>>> >>>> Dana (K8YUM) >>>> >>>> On Fri, Jun 5, 2020 at 7:13 PM Taka Kamiya via time-nuts < >>>> time-nuts@lists.febo.com> wrote: >>>> >>>>> 50 ohm / 75 ohm question is really irrelevant in this kind of thing. >>>>> Trmble itself says in manual, not to be concerned with this apparent >>>>> mismatch. >>>>> In my particular case, I have a home lab standard and existing >> system. I >>>>> have an antenna and network of distribution amplifiers. They are all >> 50 >>>>> ohms and N connectors. Some ports have BNC adapters attached. I have >>>>> pretty much standardized everything to SMA, N, or BNC. >>>>> >>>>> I boxed a power supply, T-bolt, and buffer amp in a metal case. I >> bought >>>>> a short cable (RG58) that goes from F to BNC. On back of the case, I >>>> have >>>>> BNC to N adapter. I also have a few adapters that goes from F to BNC >> for >>>>> the test bench. It really doesn't matter what you use, as long as it >>>> makes >>>>> a solid connection. >>>>> >>>>> Advantage of F connectors and RG6 are, cheap, abundant, and low loss >> for >>>>> the size. Advantage of having house standard is, less adapters and >> less >>>>> headache..... >>>>> >>>>> --------------------------------------- >>>>> (Mr.) Taka Kamiya >>>>> KB4EMF / ex JF2DKG >>>>> >>>>> >>>>> On Friday, June 5, 2020, 7:22:33 PM EDT, Robert DiRosario < >>>>> ka3...@comcast.net> wrote: >>>>> >>>>> I have a Trimble ThunderBolt GPSDO that I just received. It has an F >>>>> connector for the antenna input, and BNC connectors for the 1 pps and >> 10 >>>>> MHz outputs. Is the receiver input impedance really 75 Ohms, or is it >> 50 >>>>> Ohms and they just used the F connector to distinguish it from the >>>>> others? What do people do, just use a 50 Ohm antenna? >>>>> >>>>> Thanks >>>>> >>>>> Robert DiRosario >>>>> >>>>> KA3ZYX >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>>> To unsubscribe, go to >>>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>>>> and follow the instructions there. >>>>> >>>>> _______________________________________________ >>>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>>> To unsubscribe, go to >>>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>>>> and follow the instructions there. >>>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>> To unsubscribe, go to >>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>>> and follow the instructions there. >>>> >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@lists.febo.com >>>> To unsubscribe, go to >>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>>> and follow the instructions there. >>>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.