See the pages on my web-site (http://g4hup.com) - instructions for connecting external LO signal to SDR-IQ are given there - these came from RFSpace. WW2R has implemented this with a small switch on the back panel of the SDR so he can use internal or external LO when available.
Would be very interested to hear whether these same instructions apply to an SDR-14 - anyone prepared to open theirs up and have a look? 73, Dave, G4HUP ________________________________ From: WB6BNQ <wb6...@cox.net> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Sunday, 28 June, 2009 11:55:46 PM Subject: Re: [time-nuts] GPSDO project Hi Brad, I am a little confused about your intentions. There is no mention on the RFspace web site about a provision of being able to feed in an external reference signal to the SDR-IQ or the SDR-14 for that matter. If you study the Analog devices AD6620 component, you will see that it is a 3.3 volt * * * ONLY * * * process, which only adds to the problem. Additionally there are constraints on the rise and fall times of the timing signal. This is a horrifically complex component with intricate dependencies tied to the specific A/D used. This is not your daddy's VFO from yester-year. The fact that the internal DDS is a digital function means you will never, ever, get to a perfect cardinal point, except under very special conditions. In this case it will have an offset of 16 pico-seconds because that is the smallest resolution the DDS can do. While small it would require very special hardware for you to keep it locked to the GPS. As the internal clocking system runs more then just the DDS, other considerations come into play. The "clocks" built to run these chips are not designed for external control and studying the physical layout of the SDR-IQ, it does not appear that they provided any means to have external clocking ability. Besides the computer's sound card DSP is heavily involved with regards to the "frequency" calibration. So there are two independent clocks to keep synchronized that are not related to the "listening time," particularly if you record the digital bits for later listening or analysis. Fortunately, the software will allow for you to calibrate, to some degree, but it is not and you will not be able to approach what you expect to do with the Thunderbolt. Keep in mind that the best you could do with a Thunderbolt on a 1 second per second comparison basis is only 1 part in 10 to the minus ninth (1x10^-9). It gets worse if you consider shorter times like a continuous analog aspect that you would have for active listening. The noise of the GPS system would mask the 16 pico-second resolution of the SDR-IQ DDS. The real problem is temperature. You could improve that by providing a chamber for the SDR-IQ and raising, carefully, the temperature to just above your highest ambient level. After all that, I am not saying you couldn't/wouldn't/shouldn't consider it. Keep in mind that the SDR-IQ, or for that matter, the SDR-14 DDS only sets the center of the 190 KHz bandwidth swath that you are viewing at any one time. The real fine aspect is done in the computer and the clocks in those are all over the map, so to speak. In order to have a really tight calibration, with "Timenuts" quality, would involve some serious effort to build a clocking system for both the SDR-IQ and the computer. The final result is you would still be offset due to the digital processes in both the SDR-IQ and the computer, plus, perhaps, some latency in the software. As I get to the point of sending this email I see that Dave Powis, G4HUP, has gone to the trouble to lock the SDR-IQ internal oscillator to an external 10 MHz. If you look at all the information on his web site, you will see that it is not a trivial project. As I pointed out, this only stabilizes the "window" of the SDR-IQ still leaving the computer to tend with. My experience with the Thunderbolt shows that it has some temperature dependencies and the expected nominal noise of the GPS system in the short term regard. What most people do is have a high quality "house" standard and do long term comparisons to the GPS. A high quality "house" standard would, in effect, filter the short term noise of the GPS system. At any rate, this is my fantasy and I am sticking to it ! Bill....WB6BNQ Brad Dye wrote: > I have just purchased a Thunderbolt GPS receiver. I hope to use this > to discipline an oscillator on 66.66666 MHz so I can use it as a LO on > my Software Defined Receiver (SDR-IQ). > > Building such an oscillator is a little over my head so I thought I > would ask the group if anyone knows where I could buy the missing link > of this project. I have put a drawing of my project on the web showing > the specs of what I need. The oscillator is shown in the blue box. > > Any help or suggestions would be greatly appreciated. > > http://www.braddye.com/gps_do.html > > 73s > > Brad Dye K9IQY > ex KN9IQY, KN4BK, KM5NK, WB4JCF, ZP5TQ, WA4VXU, WA9RVL, /TI2, /9Y4, / > 6Y5, /KP4 > 52 years as a FCC licensed amateur radio operator > 36 years as a FCC licensed first class commercial radio operator > > _______________________________________________ > 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. _______________________________________________ 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. _______________________________________________ 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.