Impedance is the combined effect of capacitance, inductance, and resistance on a signal. According to Ohm's law, voltage is the product of current and resistance at a given frequency. Impedance is the measure of resistance to electrical current flow when a voltage is moved across it. Impedance is measured in ohms and is the ratio of voltage to the flow of current. For a series circuit, the Reactance is
 

and Resistance is Rs then the impedance,
 

The impedance can also be expressed in terms of complex permeability as
 

In low frequencies, the inductance is prominent and at higher frequencies, the resistance. To have a high Q component, the resistance part must be low and inductive part must be high. At higher frequencies, these inductors act not as inductors but as resistors, dissipating the noise into heat and performs as a noise suppressor. Hence, in the selection of ferrites for EMI suppression, the impedance in the selected noise band is important.

Thus the impedance can be translated in to magnetic characteristics separating out the real and imaginary part. Another expression for impedance is
 

One advantage of this equation is that the impedance is proportional to the square of the number of winding (N) for a same product.


Resistivity of Ferrites depends on its chemistry. NiZn ferrites have a resistivity of more than 1 MW-m and MnZn ferrites is in the range of 1 to100W_-m. Due to the higher resistivity of NiZn ferrites, eddy current losses are at minimum making it useful for higher frequency applications. The DC resistivities are measured at room temperature (25°C). The resistivity reduces at higher frequencies as the grain boundaries get short-circuited due to electron hoping.


For a non-uniform core, a hypothetical torroid equivalent can be calculated using:-
 

And effective parameters are calculated from C₁ & C₂

A_e of ferrite is inversely proportional to saturation current and directly proportional to attenuation.
 

C₁ is also used to calculate inductance of a core configuration through

For a toroid,

Where r₁ = internal radius of toroid

r₂ = external radius of toroid

h = height of toroid

Performance of ferrites as EMI filters under DC bias conditions are considered as one of the significant parameters of ferrite quality. Very large lossy impedance is achieved in ferrites when it is operated under bias condition creating less than the saturating magnetic field. Once saturated, the permeability becomes one (equal to that of vacuum) and this condition is not good for any ferrite applications.The bias field reduces the impedance of ferrites and best impedance is achieved in zero DC bias conditions. Technically the bias conditions are important because there are many EMI suppression applications under the DC field. Hence, ferrites with higher saturation magnetization (B₅) is required to suit extreme bias conditions. ACME manufactures high B, NiZn ferrite materials for this purpose. Increase in the mass of ferrite components can also compensate this requirement.


Permeability increases with temperature to a maximum and drops suddenly after a certain temperature.This temperature is defined as Curie Temperature (Tc). Ferrites becomes paramagnetic after this temperature due to the complete disordering of magnetic moments caused by thermal energy.

Ic is normally controlled by the chemistry of ferrites. Thus each grade has its own Tc value and the higher the initial permeability, the lower the Ic value.

Saturation flux density also decreases with temperature. This is again due to the thermal disordering of magnetic moments in ferrites.AtTc,the flux density reaches zero.

The change in permeability with temperature is quantitatively expressed as Temperature coefficient of permeability (IC). Temperature factor of permeability is given by:
 

It is the frequency at which the distributed capacitance resonates with inductance. The inductor will act purely as a resist at this condition and provide maximum impedance.


Insertion Loss is expressed as:
 

Insertion loss is important when ferrites are used as EMI suppressors. In the RF range, ferrites shows very high impedance and due to this property, ferrites suppresses the RF passing through it converting it to heat.

E₀ is the voltage without ferrite and E is the voltage with ferrite inductor.