locus

A locus curve is a graphical representation for the frequency-dependent impedance. This form of representation can be used wherever the impedance behavior of two- or four-poles, cables, filters or oscillating circuits is influenced by impedances and ohmic resistances.

The coordinate system of locus curves consists of the real resistance (real part), which is plotted in the horizontal, and the impedance, the imaginary part, which is plotted in the vertical. The frequencyparameter is plotted on the locus curve.

If the quadripole consists of an ohmic resistor, then the locus curve is a vertical line that starts at the real resistance value on the horizontal and runs vertically, since the resistance value does not change even with increasing frequencies.

Locus curves for an ohmic resistor (blue) and an apparent resistor (green)

The situation is different for an inductor. It has a low resistance value at direct current, which increases with higher frequencies. If, on the other hand, we consider the imaginary conductance of an RL series circuit, then the imaginary conductance of the ohmic resistor with 1/R corresponds to its conductance. With increasing frequency, the influence of the inductance increases until the conductance is zero at an infinitely high frequency. A semicircle is formed as a locus, which starts at the conductance of the ohmic resistor and ends at zero, the conductance of the inductance.