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

Poul-Henning Kamp phk at phk.freebsd.dk
Fri Nov 4 19:04:22 EDT 2016

In message <59dc074a-3a09-6315-29d4-6877c3bf7510 at rubidium.dyndns.org>, Magnus Danielson write

>> With respect to precision machining, that space has changed a lot
>> over the last five years, with precision CNC machines, factory
>> or home-built, dropping dramatically in price.
>You need to tune it regardless.

First:  Yes, but if you pick a sensible vibration mode for your
microwave resonance, that can be done with an screw-in endcap.

Second:  No, I would actually not need to tune it.

Historically resonance cavities were used so that step/avalance
diode multipliers had enough power to excite them.  Today we have
semiconductors which work at those frequencies.

Later people kept the resonance, because it works well with low
power budgets in telecoms/milspec applications.

But the resonanance leads to all sorts of trouble, including frequency
pulling, temperature sensitivities etc.

We're neither space nor power constrained, we'd probably be
perfectly happy if the end result is 4U and 100W, so resonance
is not mandatory.

Third:  A lot of the "everybody knows" about which atoms can be
used for active vs. passive atomic standards comes from the
state of the art electronics about 30 years ago.

Using laser-pumping and modern semiconductors, it might actually
be possible to detect the 6.8GHz photons from the Rb.

They won't be coherent photons, like in a Hydrogen maser, but we
don't need them to be, in fact that just causes the same exact
problems as the tuned cavity anyway, as long as we can measure
the frequency well enough.

(No, I havn't done the math on this, my wife has banned me from
starting any new projects until our house is finished.)

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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