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. 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 <[email protected]> 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 <[email protected]> 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 -- [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. > _______________________________________________ > 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. -- Best regards, Timenut mailto:[email protected] _______________________________________________ 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.
