Page 293 - Analog and Digital Filter Design

290 Analog and Digital Filter Design

and has two halves that separate it to allow the bobbin to be inserted. Half of
the core has a hole for an adjuster to pass through, and the other half has a
threaded brass rod fixed at its center to allow an adjuster to be screwed onto it.
When the pot-core is assembled, the two halves of the ferrite core are held
together by two spring steel clips.

Pot-core inductors are very popular. This form of inductor is suitable for values
of a few micro henries up to about one henry. Pot-cores come in standard sizes,
from RM4 to RM14. This notation is related to the physical size required when
the inductor is mounted on a printed circuit board: an RM4 requires a 0.4 x
0.4 inch board area (1Omm’); an RM8 requires an 0.8 x 0.8 inch board area
(20mm’), and so on. Typically, the inductor’s Q for this type of construction is
from 100 to 500, dependent upon the particular ferrite used, the frequency, and
the inductance value.

The advantage of the RM type of pot-cores is that they can be made to any
value. Each core type has an AL value determined by the manufacturer. This
is the inductance in nano-henries that will be produced for a single turn of
wire. Remembering that inductance is proportional to the number of turns
squared, the number of turns required is given by the simple formula: N turns
= dL (nH)/AL. L is the required inductance in nano henries and AL is the core’s
inductance factor (nano henries per turn).

The pot-core’s AL value is related to the permeability of the ferrite material used.
Different ferrite materials are used depending on the frequency at which the
inductor is operating. A particular AL value is obtained by removing some of
the ferrite material from the center of the core, thus creating an air-gap. The air-
gap has a lower permeability, so the AL value is reduced by increasing the gap.
A typical core gap is 100,um, although it may be larger or smaller depending on
the ferrite permeability and the required A, value.

Adjustment of the pot-core’s AL value is made possible by screwing a small
ferrite slug, which is in a threaded plastic molding, onto a threaded brass rod
that is fixed in the center of one half of the ferrite core. The adjuster can be set
to allow some of the magnetic flux to bypass the air-gap and hence increase the
permeability of the core. Positioning of the ferrite slug is achieved by screwing
it down a brass thread, fixed into the bottom half of the core. After adjustment
a small blob of melted wax can be used as a temporary seal. When melted wax
is applied to the pot-core slug it will warm the ferrite, which will change its
inductance temporarily.

As an aside, the presence of an air-gap in inductor cores makes them suitable
for making transformers with a high saturation level. This is particularly valu-
able in applications where the transformer windings have to carry direct current,
as well as the AC signal. This could perhaps be an application where a remote