Hi Averaged over a long enough time (and without any hanging bridges) the frequency accuracy will be fine. The frequency accuracy of a 1 pps output on a GPS is “fine” on the same basis. Since 200 KHz is a “round division” off of any of the likely TCXO’s you will not have any jitter or spurs in the “static” case.
Bob > On Apr 9, 2016, at 10:07 AM, Bert Kehren via time-nuts <time-nuts@febo.com> > wrote: > > I do not know what U blox does but I know when we use 200 KHz out of the 1 > pps output on a $ 10 ublox 6 we consistently get better than 1 E-10 closer > to 1 E-11 out of the Morion have the data > Bert Kehren > > > In a message dated 4/9/2016 10:01:05 A.M. Eastern Daylight Time, > kb...@n1k.org writes: > > Hi > >> On Apr 8, 2016, at 9:39 PM, time...@metachaos.net wrote: > >> >> Hello Bob, >> >> Friday, April 8, 2016, 6:13:07 PM, you wrote: >> >>> Hi >> >>> If you start from a 24 MHz TCXO (different modules use different TCXO’ > s): >> >>> On an 8 MHz output, most of the time you divide by three. >> >>> On a 10 MHz output, you need to divide by 2.4. The net result is that > you >>> divide by 2 sometimes and 3 other times. >> >>> In the 10 MHz case, there is a *lot* of energy at 12 MHz and 8 MHz, > along with >>> the 10 MHz output. >> >>> In the 8 MHz case, most of the RF energy is at 8 MHz. >> >>> ==== >> >>> To correct the output by 1 ppm on the 8 MHz output, you need to either > drop or >>> add one pulse out of every million pulses. Effectively you divide the > 24 MHz by >>> 2 or by 4 when you do that. You get a bit of 12 MHz or a bit of 6 MHz > as a result. >> If you know you are doing a 24Mhz and a 10Mhz, why not divide the first > by 12 >> and the second by 5 and then phase lock the resulting 2Mhz? Or divide by > 24 >> and 10, respectively and lock the 1Mhz? That way, everything is exact. > > The bigger problem is that the 24 MHz is *not* exact. It is simply a free > running TCXO > that happens to be in a GPS module. It has a basic accuracy of +/- 1 ppm > or something > similar. It is no better or worse than any other TCXO you could buy. > > To make it accurate they have two choices: > > 1) Put a voltage control input on the TCXO and turn it into a TCVCXO, then > lock it up > with a loop. > > 2) Let the oscillator free run and “fix up” the output. > > For a variety of reasons, none of the small GPS modules go with option > number 1. They > all go with option number 2. The 24 Hz error on the (maybe) 24 MHz gets > taken out by dropping > 24 edges every second. That’s not a lot of edges, it’s not going to turn > the output into absolute > garbage you can see on a scope. It is plenty of nonsense to mess up a > radio or a piece of test gear. > > One easy way to look at it: You have ~1 ppm jitter on the output (in the > example of 1 ppm of error). A > phase locked GPSDO with only simple filtering of a 1 pps would get you > down to 0.01 ppm of jitter. > A sawtooth corrected 1 pps would get you to 0.01 ppm. A good filter would > get you to <0.00001 ppm. > Yes, I’m using a very hand waving definition of jitter here, but it does > illustrate the point. You could > look at the jitter on the pulse drop as 0.04 ppm. > > Bob > >> >> >> Mike >> >>> That can be filtered out with a RF filter. The same is true with a > (somewhat more >>> complex) filter on the 10 MHz output. >> >>> In addition to the “big” RF spurs, you get a low frequency component > to the output >>> modulation. You are “phase hitting” the output eight times a second. > That gives you >>> an 8 Hz sideband along with the further removed stuff. Since it’s not > simple / clean >>> phase modulation, there are more sidebands than just the few mentioned > above. >> >>> What messes things up even more is that you never are quite doing one > ppm. You are doing >>> corrections like 0.12356 ppm this second and 0.120201 ppm the next > second. >>> The pattern of pulse drop and add is not as simple as you might hope. > The low >>> frequency part of the jitter (and it will be there) is no different > than the noise on >>> a 1 pps output. You still need to do very long time constant (or very > narrow band) >>> filtering to take it out. >> >>> Bob >> >>>> On Apr 8, 2016, at 7:06 AM, Herbert Poetzl <herb...@13thfloor.at> > wrote: >>>> >>>> On Mon, Apr 04, 2016 at 06:07:54PM -0700, Alexander Pummer wrote: >>>>> and it is relative easy to make 10MHz from 8MHz with analog >>>>> frequency manipulation, which generates less jitter >>>> >>>> Could you elaborate on this a little if time permits? >>>> I'm more a 'digital person' but it sounds interesting. >>>> >>>> Thanks in advance, >>>> Herbert >>>> >>>>> 73 >>>> >>>>> On 4/4/2016 4:27 PM, Attila Kinali wrote: >>>>>> On Mon, 4 Apr 2016 17:56:29 -0400 >>>>>> Bob Camp <kb...@n1k.org> wrote: >>>> >>>>>>> The variable frequency output on the uBlox (and other) GPS >>>>>>> receivers has come up many times in the past. >>>> >>>>>>> If you dig into the archives you can find quite a bit of >>>>>>> data on the (lack of) performance of the high(er) frequency >>>>>>> outputs from the various GPS modules. They all depend on >>>>>>> cycle add / drop at the frequency of their free running TCXO. >>>>>>> Regardless of the output frequency, that will put a *lot* of >>>>>>> jitter into the output. >>>>>> That's why you should put the output frequency of the ublox modules >>>>>> to an integer divisor of 24MHz. Ie 8MHz works but not 10MHz. >>>> >>>>>> Attila Kinali >>>> >>>> >>>>> _______________________________________________ >>>>> 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. >> >> >> >> -- >> Best regards, >> Timenut mailto:time...@metachaos.net >> >> _______________________________________________ >> 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. _______________________________________________ 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.