R.Lewis said:
The last thing I want to (ever) do is to invent a new wheel when it is not
necessary and I heartily concur with the sentiment of your post.
However I am having difficulty in finding the data for a core to suit my
task.
As you state an RM or Pot core would be the obvious first choice except
Philips do not do either in a 3B(n) material.
The materials that are available (in these shapes) do not seem to be
specified below 10kHz and I am looking at 200 to 700Hz.
Initial accuracy is not as important as long term stability.
I guess lack of 3B(x) parts is due to the historical age of this
material type. Current Ferroxcube recommended materials for LF
filtering (page 52 of their product selection guide) are 3D(x) and
3H(x) materials. Philips finally divested itself of it's ferrite
mfring arm (the self-same 'ferroxcube' facility) about 5 years ago.
I am current treading water by wading through a megatude (?) of alternative
manufacturers data in search of the holy grail. Any help you can offer would
be appreciated.
We still have no real idea of your application.
Temperature compensation of ferrite parts has traditionally been
achieved using polystyrene capacitive components in resonant or filter
applications. The two tempco's reacted to give an S-curve over a
fairly wide environmental range. This is 50 year old technology, using
70 year old materials, but it should still work if the part sizes are
practical and available, in your case.
Stability over time basically boils down to the careful construction
and packaging of the pot core. All those spacers, clamps and vibration
absorbing layers described in their selection guide, are intended to
make this a practical proposition. Doesn't look very simple, does it?
The polystyrene capacitors themselves are large, extremely expensive
in any size over 10nF and have an upper temperature limit (<<85degC)
and physical delicacy (they benefit from 'aging' and moisture control)
that restrict their use in many applications. Larger C values are also
typically offered only in low voltage (<63V) varieties.
They experienced a period in the 70's when many exotic and ungainly
sizes were offered in the surplus market, as the analog application
technologies were phased out. They are now quite rare, though some
fairly large parts (22nF) were still mfred by Philips at last count.
The polystyrene dielectric itself is avoided in other applications
where low-loss characteristics are still required, by substitution
with polypropylene or polysulphones, which have a different tempcos.
RL