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 <att...@kinali.ch> wrote: > On Wed, 21 Dec 2016 18:59:20 -0800 > Chris Albertson <albertson.ch...@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.
