Attila Kinali

Hi For close in phase noise (< 10 Hz) the 10 MHz still wins over the 100 MHz after multiplication. ADEV of the 10 MHz (with or without frequency scale) will be better on the higher Q resonator. That will always be the low frequency overtone rather than the VHF crystal. Indeed, a large blank 5 MHz would beat the 10 MHz. It’s a good bet that if a 2.5 MHz cold weld SC with a 30 mm blank diameter existed, it would beat either one of them (Q would be much higher). Given the cost of coming up with that part …. not going to happen. Bob > On Dec 22, 2016, at 3:26 PM, Scott Stobbe <scott.j.stobbe@gmail.com> wrote: > > Well for the same Q a competing oscillator will still take a 20 dB phase > noise increase for every frequency decade you scale up to. If Q*f is > approximately constant, you take another 20 dB hit in phase noise from > degraded Q, totaling 40 dB/decade. Compared to 20 dB/decade plus the noise > introduced by the phase detector and loop-filter of the PLL. > https://en.wikipedia.org/wiki/Leeson's_equation > > On Thu, Dec 22, 2016 at 10:53 AM, Attila Kinali <attila@kinali.ch> wrote: > >> On Wed, 21 Dec 2016 18:59:20 -0800 >> Chris Albertson <albertson.chris@gmail.com> wrote: >> >>> Why to people always build 10MHz GPSDOs? If the use of the GPSDO is to >>> drive a microwave, why not build a MUCH higher frequency GPSDO. Is the >>> reason that 10MHz crystals just happen to be very good and there are not >>> good 100MHz ovenized crystals? Or for portable use could you not use the >>> 1PPS signal to discipline a microwave oscillator. >> >> Short answer: >> GPSDOs are mostly about high stability, not about low phase noise. >> The 10MHz just happend to be a good compromise on stability, phase noise >> and usefulnes. >> >> Long answer: >> A GPSDO has to exhibit good stability up to several 100 s to a few 1000 s. >> This dictates that the OCXO used has to have as high long term stability >> as possible. To get there you need an as thick crystal lab as possible. >> The lower the frequency and the higher the overtone, the better. >> Quartz resonators exhibit a nearly constant Q*f, so in first order >> approximation, there is no point in choosing a higher frequency >> crystal, as the Q will then decrease and thus increase the phase noise >> would have been the same as the increased phase noise of a frequency >> multiplier. Of course, frequency multiplication is not exactly perfect and >> the Q*f is not 100% flat. There is a sweet spot where Q*f is maximal >> between >> 5MHz and 10MHz. For historical reasons, 10MHz has been deemed the more >> useful >> value and that's the reason we have a lot of 10MHz OCXO. If you go for high >> stability oscillators, you will see a lot 5MHz OCXOs being used (for the >> increased stability). Of course nobody says that these are the only >> frequencies that can be used. For example, for specialized use cases you >> will find GPSDOs with "odd" frequencies (like the 30.72MHz/61.44MHz used >> for LTE). >> >> As others have already commented, when using GPSDOs as a frequency >> reference >> for an GHz link, one would use some high frequency oscillator in the lower >> 100MHz range (using a BAW quartz) or somewhere between 500MHz and 1000MHz >> (using an SAW quartz) as a low phase noise reference and upconvert this. >> Yes, it is possible to discipline such an oscillator directly using GPS, >> but for the sake of stability (see above), design reuse and ease of >> building/testing, using an 10MHz input is generally the better solution. >> This allows to use any device that can produce an 10MHz signal, like >> e.g. an Rb vapor cell standard. >> >> >> Attila Kinali >> >> -- >> It is upon moral qualities that a society is ultimately founded. All >> the prosperity and technological sophistication in the world is of no >> use without that foundation. >> -- Miss Matheson, The Diamond Age, Neil Stephenson >> _______________________________________________ >> 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.

                    Attila Kinali

The other interesting aspect, is that if the transceiver is mobile, even at a lazy pedestrian walking speed of 1 m/s, the resulting Doppler shift is 3 E-9 deltaF. On Thu, Dec 22, 2016 at 4:36 PM, Bob Camp <kb8tq@n1k.org> wrote: > Hi > > For close in phase noise (< 10 Hz) the 10 MHz still wins over the 100 MHz > after multiplication. > > ADEV of the 10 MHz (with or without frequency scale) will be better on the > higher Q resonator. > That will always be the low frequency overtone rather than the VHF crystal. > > Indeed, a large blank 5 MHz would beat the 10 MHz. It’s a good bet that if > a 2.5 MHz cold weld > SC with a 30 mm blank diameter existed, it would beat either one of them > (Q would be much higher). > Given the cost of coming up with that part …. not going to happen. > > Bob > > > On Dec 22, 2016, at 3:26 PM, Scott Stobbe <scott.j.stobbe@gmail.com> > wrote: > > > > Well for the same Q a competing oscillator will still take a 20 dB phase > > noise increase for every frequency decade you scale up to. If Q*f is > > approximately constant, you take another 20 dB hit in phase noise from > > degraded Q, totaling 40 dB/decade. Compared to 20 dB/decade plus the > noise > > introduced by the phase detector and loop-filter of the PLL. > > https://en.wikipedia.org/wiki/Leeson's_equation > > > > On Thu, Dec 22, 2016 at 10:53 AM, Attila Kinali <attila@kinali.ch> > wrote: > > > >> On Wed, 21 Dec 2016 18:59:20 -0800 > >> Chris Albertson <albertson.chris@gmail.com> wrote: > >> > >>> Why to people always build 10MHz GPSDOs? If the use of the GPSDO is > to > >>> drive a microwave, why not build a MUCH higher frequency GPSDO. Is > the > >>> reason that 10MHz crystals just happen to be very good and there are > not > >>> good 100MHz ovenized crystals? Or for portable use could you not use > the > >>> 1PPS signal to discipline a microwave oscillator. > >> > >> Short answer: > >> GPSDOs are mostly about high stability, not about low phase noise. > >> The 10MHz just happend to be a good compromise on stability, phase noise > >> and usefulnes. > >> > >> Long answer: > >> A GPSDO has to exhibit good stability up to several 100 s to a few 1000 > s. > >> This dictates that the OCXO used has to have as high long term stability > >> as possible. To get there you need an as thick crystal lab as possible. > >> The lower the frequency and the higher the overtone, the better. > >> Quartz resonators exhibit a nearly constant Q*f, so in first order > >> approximation, there is no point in choosing a higher frequency > >> crystal, as the Q will then decrease and thus increase the phase noise > >> would have been the same as the increased phase noise of a frequency > >> multiplier. Of course, frequency multiplication is not exactly perfect > and > >> the Q*f is not 100% flat. There is a sweet spot where Q*f is maximal > >> between > >> 5MHz and 10MHz. For historical reasons, 10MHz has been deemed the more > >> useful > >> value and that's the reason we have a lot of 10MHz OCXO. If you go for > high > >> stability oscillators, you will see a lot 5MHz OCXOs being used (for the > >> increased stability). Of course nobody says that these are the only > >> frequencies that can be used. For example, for specialized use cases you > >> will find GPSDOs with "odd" frequencies (like the 30.72MHz/61.44MHz used > >> for LTE). > >> > >> As others have already commented, when using GPSDOs as a frequency > >> reference > >> for an GHz link, one would use some high frequency oscillator in the > lower > >> 100MHz range (using a BAW quartz) or somewhere between 500MHz and > 1000MHz > >> (using an SAW quartz) as a low phase noise reference and upconvert this. > >> Yes, it is possible to discipline such an oscillator directly using GPS, > >> but for the sake of stability (see above), design reuse and ease of > >> building/testing, using an 10MHz input is generally the better solution. > >> This allows to use any device that can produce an 10MHz signal, like > >> e.g. an Rb vapor cell standard. > >> > >> > >> Attila Kinali > >> > >> -- > >> It is upon moral qualities that a society is ultimately founded. All > >> the prosperity and technological sophistication in the world is of no > >> use without that foundation. > >> -- Miss Matheson, The Diamond Age, Neil Stephenson > >> _______________________________________________ > >> 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. >

On 12/22/16 5:06 PM, Scott Stobbe wrote: > The other interesting aspect, is that if the transceiver is mobile, even at > a lazy pedestrian walking speed of 1 m/s, the resulting Doppler shift is 3 > E-9 deltaF. > I always visualize this as "how many wavelengths per second".. so if you're at 10 GHz, 3cm wavelength, 1 m/s is 33 Hz. to 1 sig fig, 1 m/s = 2 mi/hr if it's a radar (2 way path) the shift is doubled.