Jim Hannon wrote:

> >
>  The parallel core flux gate design (my name for it)
> consists of two cores such as a rod or wire or strip of metglas. Each c=
ore
> has a excitation winding. The two coils are placed in parallel inside a=

> overall sense winding. The excitation currents in the two coils are
> arranged to produce cancelling flux in the sense coil. The signal from =
the
> sense winding is processed as usual.

Sure, the tapes of both the permalloy or Metglas should work fine in that=

configuration.  One reason I=92ve decided to concentrate on the inductive=

version is that the gain of a harmonic modulator style varies according t=
o the
Q of the tuned sense winding and the drive waveform.  There is a paper wh=
ich
discusses that when conditions are right (or wrong, it depends on your
viewpoint) the sense winding can be paramagnetically pumped and actually =
go
into oscillation, at which point the output has no relationship to the am=
bient
field.  The down side of the inductive version is that for various reason=
s it
probably will have an inherently higher noise floor due to the current fl=
owing
in the winding is common to the sense circuit as opposed to a flux gate w=
here
theoretically none of the noise of the drive winding should show up in th=
e
sense winding.   I think that current noise effect is low enough that I w=
ill
be limited by other effects first.  Anyway,. Back to the paramagnetic pum=
ping
effect -- unfortunately, this condition corresponds to higher gain, highe=
r Q
style units.  As a side point, this has been used in other areas that peo=
ple
may not be aware of.  There were some high primary to secondary isolation=

power supplies manufactured by Wanlass corporation, which depended on thi=
s
effect.  Literally the secondary was pumped by saturating its core in an
orthogonal fashion -- they used a pair of C-cores where one core is rotat=
ed 90
degrees to the other.  In years past, before transistors were available, =
the
bell ring generator for small business telephone systems was a
paramagnetically pumped sub-harmonic generator.  Some old books on mag-am=
ps
discuss the conditions necessary for unstable (read oscillating) mag-amp
circuits.  I=92ve built a small experimental variation where you take a s=
eries
combination of a small choke,  resonating capacitor and a 110 VAC light b=
ulb.
When adjusted correctly the bulb will go up and down in brightness in a f=
ew
second cycle indefinitely.  It=92s kind of neat.  No transistors or movin=
g
parts, just an unstable ferroresonance condition.  It also has an incarna=
tion
in pendulum bob drivers for clocks.
Regards,
Charles R. Patton



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