[time-nuts] Antique Rb Standard - Thanks, Pictures, Parts Request, Question

ed breya eb at telight.com
Sun Apr 29 20:19:08 UTC 2012


If the Rb cell "drifted" enough (maybe if it was filled with water) 
to de-tune the mechanical cavity resonator, it wouldn't work at all. 
Only a small amount of the RF power in the cavity is coupled into the 
Rb gas. The bandwidth of the Rb resonance is a fraction of a Hz (Q in 
the millions), while the bandwidth of the cavity is probably 20-50 
MHz, (Q may be up to 100).

The multiplier will multiply frequency by integers at each stage, but 
each stage is tuned to maximize its response at the desired harmonic. 
You have a chain of these, each needing to be peaked to get the net 
result - enough power at the right frequency to drive the SRD. Then 
the SRD has to be optimally biased to maximize its power output at 
the desired harmonic. Then the cavity has to prefer that particular 
(98th or whichever) spur - but it isn't an extremely narrow filter - 
it can't be since it's just a single section. It has to be good 
enough to "reasonably" attenuate the nearby spurs spaced at around 69 
MHz away, and to be the tuned load for the SRD to maximize the power 
at the desired frequency.

So, the guts need to provide the cascade of discrete frequencies that 
ultimately lands one near the Rb resonance in order to detect it, but 
each stage and each part has to be right for it to work. The Rb 
resonance is by far the most stable of all of the parts of the system 
- even after 40 years or any number of years.

Ed




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