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

[Scott Stobbe]

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 at 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 at 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 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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