[time-nuts] Using GPSDO as a Refrence for Protable Amateur Radio Microwave Operations

Scott Stobbe scott.j.stobbe at gmail.com
Thu Dec 22 15:26:03 EST 2016

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.

On Thu, Dec 22, 2016 at 10:53 AM, Attila Kinali <attila at kinali.ch> wrote:

> On Wed, 21 Dec 2016 18:59:20 -0800
> Chris Albertson <albertson.chris at 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
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