[time-nuts] time-nuts Digest, Vol 101, Issue 152
lists at rtty.us
Mon Dec 31 04:03:23 UTC 2012
They are using some pretty major fields in that paper. As they point out, high level tests likely do not extrapolate well to low level performance. Their data shows the impact diminishing quickly as the field drops.
If you accept their 1x10^-11 per gauss number as correct, your OCXO might see a 5x10^-12 change as you rotated it 90 degrees. Unless you have a very good OCXO, that's not a big impact.
In the case of a fan that's likely generating mili gauss of field (you don't get that much off of 50 ma at 12v), you could see 1x10^-13 level changes. Your OCXO's AVAR is likely well above that at any tau that the fan messing with.
Yes, I'm making the assumption that the fan motor is not in contact with the OCXO case. In any rational setup there will be at least a few inches between the fan and the OCXO. Not exactly far field at 20 Hz, but the field does drop off pretty fast.
On Dec 30, 2012, at 10:18 PM, Arthur Dent <golgarfrincham at yahoo.com> wrote:
> Bob Camp-
> "A quartz crystal in insensitive to magnetic field. This being Time Nut's that's
> not the whole story. Ferrite core inductors can indeed exhibit a bit of static
> mag field sensitivity. Your OCXO may or may not have some in it."
> "Precision crystals have clips made from nickel. The posts are likely Kovar.
> That's been true for a *long* time. Neither one is very magnetic. Both likely
> are hard enough to be even less magnetic than they might be."
> I've had a Manson Lab RD-180 running for about 35 years other than short
> down time to change batteries. I've always kept it oriented in the same
> direction once I initially adjusted it because I had read that the earth's
> magnetic field would cause a small frequency shift in the crystal frequency.
> I took this at gospel and never looked into the subject any further. After
> seeing this thread (that has now lost it's original title) I did a quick search
> and found plenty of references including this one that goes into detail about
> testing that was done to quantify this effect. The section "VI Conclusion" is
> a good summation.
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