I have an interest in making RF filters with low distortion (for
example, as preselection filters for receivers). I've found that very
often I need to use air-core coils to get the low distortion I want,
but it would be convenient if I could use powdered iron or ferrite
core toroids to get higher Q at low frequencies without having to make
the coils physically big. I've always assumed that powdered iron
would give me the best results, but someone suggested I try low-
permeability ferrite. I just did some experiments with that. I wound
three toroids, all 0.37 inch OD cores; one is on mix 2 powdered iron,
and the other two are on type 68 and type 67 ferrite. I put each
coil in a simple 1MHz high pass filter: two 2.2nF caps in series,
with the coil connected between them to ground. I tested for harmonic
distortion with a 1MHz input, and for the 2.42MHz intermod product
with 0.62MHz and 1.52Mhz inputs.

With about +16dBm coming out of the test filter at 1MHz, I measured
second harmonic at about -95dBc for the T37-2, -90dBc for the FT37-67
and -57 for the FT37-68; the third was at -74dBc, -61dBc and -51dBc
for the three coils in the same order.

The 2.42MHz intermod, with each tone at +18.5dBm out of the combiner,
measured -71dBc, -58dBc and -38dBc for the coils, again in the same
order.

My conclusion from this is that at least these two ferrites aren't
going to help me in my search for a better low-distortion core
material, but at the same time, it was interesting to note that the
higher permeability -67 performed better by far than the lower
permeability -68. It gives me some hope that if the ferrite materials
guys went looking for a formulation that would be specifically as low-
distortion as they could get, they might find something pretty good.

Before someone asks, no, I didn't run a T37-6 core coil along with
these. I expect it would be a little better than the T37-2. And I've
measured a few coils wound on (unknown formulation) powdered iron rods
that show significantly lower distortion than those on toroid cores;
but of course those have a large "air gap" and relatively high
external field.

Cheers,
Tom