[time-nuts] The 5MHz Sweet Spot

Richard (Rick) Karlquist richard at karlquist.com
Sat Nov 2 02:56:35 UTC 2013


On 11/1/2013 7:12 PM, Tom Knox wrote:
> A while ago I mentioned 5MHz oscillators were used in most metrology applications compared to the more commonly available 10MHz because 5MHz was a sweet spot for quartz. At the time I didn't know why. I finally had a chance to ask the person I learned this from why. The main reason is simply physical size. The larger crystal lattice allows many manufacturing advantages that allow for a higher Q. He also explained I was wrong in an earlier statement, metal/quartz migration on quartz oscillator was not a major problem even after decades, but could become more of a factor if driven hard. That does not mean the deposition and lead bonding has no negative effect. The BVA solves this by capacitive coupling the quartz rather then direct metal deposition.
>
> Thomas Knox
>

A lot of issues conflated together here.

1.  There is a theoretical QF product for quartz.  Being at 5 MHz 
basically doubles your Q, all other things being equal.

2. Having a higher Q reduces the contribution of the sustaining
amplifier, but only within the 3 dB bandwidth.  With the Q being
in the millions, this is only a few Hz.

3.  In general, the sustaining amplifier is not a player in
a well designed quartz oscillator in the first place.

4.  Q probably has a negative correlation with flicker noise,
meaning higher Q is associated with lower flicker noise.
However, the correlation is not strong.  There is no theory
that says that Q puts a bound on flicker noise.

5.  So that leaves us with the larger physical size.  Perhaps
it allows higher Q, but again it is unclear how this is connected
with flicker noise.

6.  You didn't mention the theory that more total atoms of quartz
provides averaging flicker noise over a large population.

7.  You didn't mention the notion that larger physical size permits
higher drive level.  Since the Q is also large, perhaps it doesn't.
Also, a higher drive level is probably only going to help with
far out noise.

8.  Many, or maybe most, 5 MHz resonators are made with undersized
blanks which are enabled by energy trapping.  So we don't have a
simple scaling of all 3 dimensions.  What is the effect of this
"cheating"?

If someone can shed additional light on this, please jump in
and educate us.

Rick Karlquist N6RK



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