On Fri, Apr 08, 2016 at 06:13:07PM -0400, Bob Camp 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. Thanks for the input, but alternating between dividing by two and three doesn't really sound like "analog frequency manipulation" to me. Maybe I'm completely wrong here, maybe I just need to see an analog circuit which does this. Best, Herbert > ==== > 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. > 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. _______________________________________________ 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.
