[time-nuts] Better than Quartz?
richard at karlquist.com
Sun Apr 27 20:21:58 EDT 2008
Any decent crystal oscillator circuit will have a loaded Q that is
a large percentage of the unloaded Q. The bridge circuit in the
E1938A does not enhanced the loaded Q, compared to, for example,
the 10811. It doesn't seriously decrease the loaded Q either, but
in any event, Q is not the raison d'tre of the bridge circuit.
What the bridge circuit does is make the oscillator circuit (as
distinct from the crystal) very stable. If you don't have a bridge
(ie any oscillator other than the E1938A), then the loaded Q of
the crystal will affect how much the oscillator circuit can pull
the crystal. In this sense it could affect stability. But if
the circuit is sufficiently ovenized, and run on stable power
supplies, etc., then the stability of the oscillator will be determined by
the intrinsic stability of the crystal.
There is no direct correlation between the Q of the crystal
and its intrinsic stability. It is fairly easy to make a
"junk" crystal with a Q of many 100,000's at 10 MHz. This
Q is at least within an order of magnitude of the theoretical
QF limit of quartz. Yet the stability is many orders of magnitude
removed from a precision crystal.
AFAIK, the loaded Q of exotic crystals like BVA is not significantly
higher than other good crystals, since they are already close
to the theoretical QF limit.
There was an interesting paper given at FCS about 10 years ago
about the quartz shortage during WWII. There were govt projects
to search for a replacement material. It sounded like a good
effort was made, but in the end nothing else even came close
Rick Karlquist N6RK
Murray Greenman wrote:
> That's is essentially the point - it is the Q of the resonator (assuming
> environmental effects are controlled) which ultimately determines the
> stability. Very high performance oscillators (the HP E1938A is an
> example) use a bridge circuit to enhance the loaded Q of the crystal.
> Other designs (the QAS and BVA) use a complex structure of crystal which
> places the electrodes outside the oscillating structure in order to
> enhance the loaded Q.
> Murray ZL1BPU
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