[time-nuts] Re: Best frequency to start for GHz synth ?

[Richard (Rick) Karlquist]

1.  Whatever the advantage of cryogenic operation, Keysight
could not consider that due to marketing reasons.  They
were happy to leave the lunatic fringe to our friends in
Western Australia.

2.  They were already limited by microphonics even at
room temperature, so cryo would be wasted.

3.  My understanding of the oven was that it was simply
a substitute for a Q reducing varactor diode tuning mechanism.
There was never an expectation that temperature compensation would
allow standalone operation of the sapphire.  Again, for marketing
reasons, it was always going to be locked to an OCXO at exactly
10 MHz.  That OCXO's EFC could be used to lock the OCXO to an
external reference with a very slow loop that would prevent
phase noise contamination.

Also of interest to time nuts:

Back when Keysight was part of Agilent, and before the sapphire
resonator, they had architectures using DRO's at about 8 GHz.
So any Sapphire replacement had to be in some sense a drop in
replacement.  Before that, there was a boondoggle project using
a 1 GHZ DRO with a resonator about the size of a hockey puck.
It kind of reminds me of the giant 1 MHz quartz resonators.
There was also a 1 GHZ Surface Transverse Wave Resonator
(STW) oscillator for a while, until the fab shut down.
It evolved from the 640 MHz SAW resonator used in early 5071's.
Again, the fab shut down and I designed it out.

They also wasted (IMHO) a lot of resources on a "opto-electronic"
(I think that was the term) oscillator around 15 or 20 years ago.
It depended on some spool of fiber optic cable.  I never thought
that was going to work, both because of basic principles and
because of the cable spool microphonics.

Rick N6RK

On 4/1/2021 9:06 AM, Chris Caudle wrote:
> On 2021-03-31 14:27, Richard (Rick) Karlquist wrote:
>> When I left Keysight in 2014, they were still trying to solve the
>> microphonic problem in their sapphire resonator oscillator.  Also,
>> it is still necessary to lock the oscillator to a 5 or 10 MHz
>> OCXO.  The oscillator is tuned by varying its oven temperature
>> set ppoint.
> 
> What kind of oven temperature range?  I thought sapphire oscillator was 
> pretty much synonymous with "cryogenic sapphire oscillator."Â  I found a 
> paper which described sapphire as a "low loss material with loss tangent 
> of 5×10^−6 at room temperature, 2×10^−8 at 77 K and 7×10^−10 at 4 K 
> giving Q-values of more than >10^7 at low temperatures."
> 
> That paper seemed to be describing some kind of temperature compensation 
> they  had developed to reduce the temperature sensitivity of 10ppm/K and 
> also move the turnover point from 77K to 92K.  92K isn't exactly what I 
> think of when I hear "oven" so presumably there is some mode that works 
> at around 300K that I didn't find discussed yet.
>