[time-nuts] Why discipline Rubidium oscillator?

Gregory Maxwell greg at xiph.org
Mon Nov 20 15:39:17 EST 2017

On Mon, Nov 20, 2017 at 8:28 PM, Jerry Hancock <jerry at hanler.com> wrote:
> Bob, I was referring to the rubidium standard of 6834682610.904 Hz.  For some reason I thought it was closer to 9Ghz.
> I assume then rubidium standards oscillate (if that is the correct term) somewhere around that number but not exact or is it in the detection where things fall down?

I think you are confused by the difference between primary and
secondary standards.

A typical rb gas cell is a secondary standard.  Its exact frequency is
distorted by a number of factors like gas pressure, interaction with
the cell walls, and ambient magnetic fields which cannot be canceled
by the design of the standard.  This is why it is useful to discipline
a telecom rb against GPS, disciplining can be accomplished through
control of a biasing magnetic field.

Something like a cesium beam standard is able to internally cancel
most of these biases "under standard conditions".  A drift free
frequency source can also be constructed using rubidium, such as
rubidium fountains just as a secondary standard could be constructed
using cs-- like cs gas cell standards (such as the sa.45 CSAC).

[Hopefully I haven't mangled things].

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