Hi Said et al., (replying to this instead of your original because I initially forgot to turn on mailing-list delivery---been lurking for a while and only reading online!)
I don't have firsthand experience but my sense is that phase noise in the 10-100 Hz region is not a huge issue for the radio, i.e., it is not a performance requirement to be immune to a strong adjacent signal that close to the desired signal. I definitely don't want to sacrifice any PN by the time we are getting beyond a few hundred Hz. If I knew where the PN plots of the NEO and the VCXO crossed, wouldn't that be a good starting point for the loop-filter BW? It would be nice if ublox published this :) To Bob's question, yes, at room temperature the VCXO moves 300 ppb per degree, so I would have thought a loop BW in the 1-10 second range would be too slow to follow temperature variations...? Regardless of the limits of analog design, a digital implementation is attractive because then I could play with the loop filter in a working system. I have space in my FPGA. Is it as simple as running both clocks into the device and implementing dividers + PFD + digital filter there? Anyone have a reference? My google searches only turn up descriptions of the on-chip PLLs, which I think cannot be used without also using the on-chip VCO. I have read that FPGA IOs introduce considerable jitter; is that less of a concern when the loop BW is very narrow and you are averaging over thousands of cycles? The greatest common factor between my clocks is 640 kHz so I was thinking to run the PFD at that frequency. Regards, Mark On Sun, 28 Sep 2014 11:23:32 +0200 Stéphane Rey <[email protected]> wrote: > " With that VCXO you want to have a 5s to 10s or more loop time > constant (0.1Hz BW) which typically can only be done in the digital > domain.." > > Hi Said, > Could you point us on something describing that ? What kind of > digital processing do you think about ? > > Cheers > Stephane > > -----Message d'origine----- > De : time-nuts [mailto:[email protected]] De la part de Said > Jackson via time-nuts Envoyé : dimanche 28 septembre 2014 07:50 > À : Discussion of precise time and frequency measurement > Cc : [email protected] > Objet : Re: [time-nuts] GPS-disciplining an ordinary VCXO? > > Mark, > > In the analog domain you can probably do a PLL with a 1Hz loop BW. > Using a PLL chip like ADF 4002 or similar. This means all the nasty > noise from the NEO will taint your PN up to 20Hz or more, very > significantly close-in. If you don't care about noise (jitter) below > 100Hz then this is fine. If you do as it will dominate your ADC > jitter then you can't use an analog PLL. > > With that VCXO you want to have a 5s to 10s or more loop time > constant (0.1Hz BW) which typically can only be done in the digital > domain.. This allows you to use the excellent 1Hz to 100Hz PN of that > VCXO without tainting it by the noisy NEO. > > An even better setup would be to lock a very low noise 5 or 10MHz > ocxo to the GPS with >100s time constant, then use the analog PLL > with wider bandwidth (say 30Hz) to reduce the VCXO PN close-in even > further by using the ocxo to supress the vcxo PN. > > Welcome to our world, if you look at the archives there are 10++ > years of discussions about exactly doing this... > > Bye, > Said > > > > Sent from my iPad > > On Sep 27, 2014, at 21:01, "Mark A. Haun" <[email protected]> wrote: > > > In my quest to learn Verilog and get my hands dirty with > > software-defined radio, I'm currently designing a direct-sampling > > shortwave receiver. This uses an 80-MSPS ADC, which requires a > > low-phase-noise oscillator, e.g. Crystek CVHD-950 or Abracon > > ABLNO. It would be nice to have some provision for locking this > > oscillator to an external reference, hence my question: > > > > All of the amateur GPSDO designs I've seen are disciplining an > > OCXO. I understand this is easier because the excellent short-term > > accuracy of the OCXO means the feedback can run slower, so even a 1 > > PPS signal can be used. > > > > I am wondering what sort of performance could be achieved by > > disciplining my VCXO directly with a good GPS module. I have a > > NEO-7N (Ublox) with configurable timepulse up to 10 MHz. Someone > > mentioned that this is derived from 48 MHz, so jitter is reduced if > > you pick an integer divisor. That is fine, but I don't have a feel > > for what other irregularities may be present in the timepulse > > output, and how they would affect the performance. I also don't > > know how to go about designing a PLL loop filter. I understand the > > goal is to marry the long-term GPS stability with the short-term > > VCXO stability but all I have is a phase-noise plot for the VCXO. > > How do you know where to split the difference? > > > > It is not essential to the larger project, but what I am ideally > > going for is 1 ppb frequency match between two ends of a radio > > link, and 1 ppb stability over data symbol times. That is, carrier > > stability of ~ 1/10 cycle at 10 MHz over one-second symbols. > > (Channel coherence imposes this limit.) I know the experts here > > can tell me whether this is impossible, totally doable, or > > somewhere in between! > > > > Thanks, > > > > Mark > > _______________________________________________ > > 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. > > > > --- > Ce courrier électronique ne contient aucun virus ou logiciel > malveillant parce que la protection avast! Antivirus est active. > http://www.avast.com > > _______________________________________________ > 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.
