[time-nuts] Thinking outside the box a super reference

[Attila Kinali]

On Thu, 3 Nov 2016 16:07:59 -0400
Bert Kehren via time-nuts <time-nuts at febo.com> wrote:

> Let me get to the real issue. There are not enough  HP5065A’s out there and 
> not affordable for all time nuts. Most are being kept  and are not for 
> sale. But if a combined effort by many time nuts it MAY be  possible to recreate 
> the guts of the HP5065A. The key word is MAY. 200 time nuts  be willing to 
> invest $ 5000 each may get us there. The market does not justify  such an 
> effort but time nuts keep bringing up discussions. All the other ideas  kicked 
> around in the past will cost more. There are some among us that know what  
> it cost and who can make the key elements like lamps, cells, filters etc. 
> Just a  thought outside the box and hopefully may turn in to a limited 
> constructive  dialog. We will continue on our path, which include FRK/M100, HP5065 
> and  Cs. 
> We would not be capable to contribute technically on the  physics package 
> but I would be willing to contribute financially and with  monitoring 
> equipment even if I would not be around when finished.


I challenge your cost estimate! :-)

My guestimate on the cost for a Rb vapor cell standard would be somwhere
in the region of 2000€ to 5000€, per unit. If you add some experimentation
it might become something like 10k€ for the first unit. How do I come to
this number? A vapor cell is quite cheap, 300-500€ can buy you an off
the shelf cell. They will not have the best buffer gas filling, but they
will be good enough for at least to get a reasonable stability.

>From here on, the analyis splits into two: dual resonance and coherent
population trapping standards.

For the dual resonance standard, a cavity is required. As the vapor cell
has a predetermined form, that cannot be changed (unless one goes for
full custom cells from the beginning), the cavity has to be adapted to the
cell. This means a cavity that is resonant at the 6.9GHz, yet fits the
cell has to be designed. My guess is, that this can be done by someone
with enough experience in microwave resonant cavities/filters and simulated
using tools like OpenEMS for verification. Producing them using aluminium
should be in the order of 200-1000€ plus cost of aluminium which I guess
to be less than 100€. Excitation of the cell can be done using either
the way the super-5065 with its filter, or tuning the laser diode. Both
way work and as far as i can tell are mostly a matter of taste with a
slightly increased complexity in the control loop for the tuned laser diode.
The detection electronics for the signal can be build for probably less
than 300€/board in batches of 10, definitely less than 1000€. Complexity
should be relatively easy to handle, as we have today access to the nice
UHF devices from Hittite, which handle all the 6.9GHz and bring it down
to easy to handle frequencies. Add a uC, some ADC and DAC and you are
basically done. For advanced features a small to medium sized FPGA (~20-50€)
can be added. If you want to be fancy, add an OCXO (like Abracon AOCJY4 or
Axtal AXIOM10HP/AXIM15) to the equation. What is missing is the magnetic
shielding, which is probably cheapest by using multiple layers of soft-iron.
Maybe using a composite of soft-iron and ferrite pads could improve things
at moderate costs. I don't think that mu-metal would be a good choice,
as it will in almost all cases need to be anealed which is not something
you can easily do (needs a furnace that goes to IIRC 800°C and can be
flooded with hydrogen)


For coherent population trapping, the cavity goes away. The excitation
is either done using a single laser that is modulated with 3.5GHz and
tuned like above, or by using two lasers that are locked to eachother
using an opto-electronic PLL (basically direct both beams at an avalance
photodiode, then use an ordinary PLL to control one of the laser).
For enhanced perfromance the two laser variant can use extenernal cavities
to narrow the laser linewidth from 10-100MHz of an "raw" laser diode
to 10-100kHz. Descriptions how to build such ECDL are available on the
net, including mechanical drawings. The price for the single diode
version is the same as the dual resonance approach, minus the cavity.
The double laser diodes add a slight cost for the OPLL (probably <100€).
An external cavity would probably be in the order of 200-500€ each.


All that said, if someone would want to tackle this project, I would
be willing to help with knowhow and electronic design. Unfortunately
due to limited finances I would not be able to invest much money.


			Attila Kinali
-- 
Malek's Law:
        Any simple idea will be worded in the most complicated way.