[time-nuts] Using GPSDO as a Refrence for Protable Amateur Radio Microwave Operations
kb8tq at n1k.org
Thu Dec 22 16:36:56 EST 2016
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.
> 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.
> 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
>> 5MHz and 10MHz. For historical reasons, 10MHz has been deemed the more
>> 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
>> 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
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