Hi > On Apr 25, 2021, at 10:13 AM, Lux, Jim <[email protected]> wrote: > > On 4/25/21 6:40 AM, Bob kb8tq wrote: >> Hi >> >> >>> On Apr 25, 2021, at 9:31 AM, Lux, Jim <[email protected]> wrote: >>> >>> On 4/25/21 6:02 AM, Bob kb8tq wrote: >>>> Hi >>>> >>>> The thing that I find useful about a GPS simulator is it’s ability to >>>> calibrate the >>>> time delay through a GPS based system. In the case of a GPSDO, there may be >>>> things beyond the simple receiver delay that get into the mix. Getting the >>>> entire >>>> offset “picture” all at once is nice thing. Yes, that’s a Time Nutty way >>>> to look at it….. >>>> >>>> So far, I have not seen anybody extending this sort of calibration to the >>>> low cost >>>> SDR based devices. Without digging into the specific device, I’m not sure >>>> how >>>> well a “generic” calibration would do. Indeed, it might work quite well. >>>> Without >>>> actually doing it … no way to tell. >>>> >>>> So if anybody knows of the results of such an effort, I suspect it would >>>> be of >>>> interest to folks here on the list. >>>> >>>> Bob >>> >>> A double difference kind of relative measurement might be useful - compare >>> two (or three) GNSS receivers. Then the absolute timing of the test source >>> isn't as important. >> Well …. it is and it isn’t. If you are trying to get “UTC in the basement” >> (or even >> GPS time) to a couple nanoseconds, then you do need to know absolute delays >> of a number of things. Is this a bit crazy? Of course it is :) >> >> Bob >> > Good point.. > > For many SDRs, it's tricky to get the output synchronized to anything - a lot > were designed as an RF ADC/DAC for software SDR (like gnuradio). The software > SDRs are sort of a pipeline of software, with not much attention to absolute > timing, just that the samples come out in the same order and rate as samples > go in, but with a non-deterministic delay. Partly a side effect of using > things like USB or IP sockets as an interface. And, to a certain extent, > running under a non-real time OS (where real time determinism is "difficult > programming" - although clearly doable, since playing back a movie requires > synchronizing the audio and video streams ). > > If your goal is "write software 802.11" you don't need good timing - the > protocol is half duplex in any case, and a millisecond here or there makes no > difference. > > A SDR that has a FPGA component to drive the DACs might work pretty well, if > you can figure a way to get a sync signal into it. One tricky thing is > getting the chips lined up with the carrier - most inexpensive SDRs use some > sort of upconverter from baseband I/Q, and even if the I/Q runs off the same > clock as the PLL generating the carrier, getting it synced is hard. > > The best bet might be a "clock the bits out and pick an appropriate harmonic > with a bandpass filter". If the FPGA clock is running at a suitable > multiple of 1.023 MHz, maybe this would work? Some JPL receivers use 38.656 > MHz as a sample rate, which puts the GPS signal at something like 3/4 of the > sample rate. > I'd have to work it backwards and see if you could generate a harmonic that's > at 1575... >
I think that coming out of a “one bit DAC” on the FPGA is the most likely way to generate the signal. Run that at carrier / N and away you go. I don’t think you want to get N to terribly high. Having all those images running around may not be a good idea. Something in the 5 to 10 range seems about right. Is there enough “harmonic” to make it work or do you need some sort of SRD-ish thing to womp it up? ….. My guess is that there still will be a bunch of little delay ambiguities That pile up on you. The bigger issue is: do you want to make this a “lifetime project” ? It’s only one of many things that need attention if you want to get to the final goal. Spend enough time on each little step and this all becomes a “many decades” sort of adventure. In this case I’ll let Said worry about the details and just use his gizmo :) Bob > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
