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

Attila Kinali attila at kinali.ch
Thu Dec 22 10:53:44 EST 2016

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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