Inductance is the property in an electric circuit which resists a change in current.

This resistance to current change is the result of the energy stored within the magne-tic field of a coil. A coil of wire will have inductance.

An inductor may be viewed as an AC resistor since in most cases it is used to im-pede the flow of alternating current.

A common term used with inductors is impedance. Impedance in a circuit is the total resistance to flow of an alternating current as a result of resistance and reactance.Impedance therefore is simply AC resistance.

The amount of impedance changes in proportion to the freguency (That is the higher the frequency the higher the impedance).

Each inductor has a high frequency limit above which its impedence will no longer increase in proportion to the frequency. This frequency limit is known as the self re-sonant frequency(SRF).

The SRF is the point above which the inductor will not have a function in a desired manner.
Normally in the production of an inductor the SRF is maintained as its highest achievable level thus broadening its usable frequency range. For this reason the SRF is normally specified as a inimum frequency.

Two other parameters commonly associated with inductors are DC resistance and Q. DC resistance is simply the resistance of the copper wire with which the inductor is wound. For most application DC resistance is viewed as an undesirable factor and it is minimized "DCR" is normally specified as a maxmun.

The "Q" of an inductor could also be called the quality rating of an part. The hi-gher the "Q" the better in performance of the inductor. Since high "Q" (in almost all cases) is a desirable parameter. it is expressed as a minimum. Factors which affect the magnitude of "Q" are DCR, core loss and leakage reactances.

When ordering or referring to inductors, the following parameters should be consi-dered:

Inductance : The desired inductance as measured by the unit system of henrys(H).

Tolerance : The percent variation of henry from the specified inductance.

Q : Afigure specifying the minimum performance level of the component.

Test Frequency L & Q : specifies in Hertz the frequency of which tests for L (induc-tance) and Q were measured. The test frequency are as specified in MIL-C-15305.

Self-Resonant Frequency Minimum : Specifies in Hertz the frequency above which the inductor will cease to operate in a desirable manner.

DC Resistance Maximum : Specifies in ohms the maximum amount of DC resistance possessed by the inductor (an undesirable quality - the lower value the better.)

Rate DC Current : Specifies in milliamperes the amount of direct current which can be adequately handled by the inductor