[time-nuts] What’s the BEST crystal?

[Attila Kinali]

On Sun, 1 Mar 2020 07:39:43 -0800
jimlux <jimlux at earthlink.net> wrote:

> How does a CSO compare with a Mercury Ion clock - the latter does fit in 
> a satellite and is intended to replace the USO kind of function.

They use/used CSO's as local oscillators for the Hg ion clock.
Their first papers ([1] and [2]) used to use a super conducting
cavity Rb maser that generates 2.5GHz. In later papers,
(e.g. [3] and [4]) they silently switched this to a CSO.

The big problem with the Hg ion clock is the low photon efficiency
it has. Ie, even though they have a lot of ions to probe, only a
few react with the light comming in. Thus the SNR is very low. To
get good performance they need to incrase the Ramsey interrogation
time , which works well thanks due to the long life time of the
hyperfine splitting in Hg and the low buffer gas pressure. With
an VCO (ie OCXO) we are talking about ~10-20s interrogation time,
with a CSO we are talking about 100s. Using this and a CSO as local
oscillator, they even beat an active hydrogen maser with their
best performing clock.

As synthesizer for the 40GHz they are using a DRO for
the space missions if I understood the previous papers
correctly (easier space qualification, apparently)
and an SRD based system for ground, which gives almost
an order of magnitude better performance due to lower
noise (c.f. Dick Effect)

			Attila Kinali


[1] "Hg+ Trapped Ion Standard with the Superconducting Cavity Maser
Oscillator", by Perstage, Tjoelker, Dick and Maleki, 1993
https://doi.org/10.1109/19.278549

[2] "A mercury ion frequency standard engineering prototype for the NASA
deep space network", by Tjoelker, et al., 1996
https://doi.org/10.1109/FREQ.1996.560296

[3] "Mercury Atomic Frequency Standards for Space Based Navigation
and Timekeeping", by Tjoelker et al. 2011
http://time.kinali.ch/ptti/2011papers/Paper37.pdf

[4] "Mercury Ion Clock for a NASA Technology Demonstration Mission",
by Tjoelker et al, 2016
https://doi.org/10.1109/TUFFC.2016.2543738

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