Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 64

Inductors are perhaps the most complicated of the simple passive components. Because they are constructed of coils of very fine wire, sometimes multiple layers of coils, their parasitic elements are greatly affected by the details of their construction. Some inductors have the axis of the coil parallel to the PC board, and some are wound with the axis perpendicular. In both cases, the model for the inductor is essentially the same as the resistor, as shown Figure 1.39, but with the value of the series inductance equal to the DC value of the inductor, and the series resistance equal to the DC resistance. Inductors, because of the nature of their construction, have very large relative parasitic capacitances. In cases where an inductor is used for a bias element (relying on its impedance to be high at high frequencies) one often finds that the parasitic capacitance will become the main effect over the band of interest. Thus, in many cases the value of inductance used is carefully selected based on the overall effective inductance and sometimes utilizes the shunt capacitance to provide a high impedance at a particular frequency of interest. It may quite difficult to make a single inductor provide good RF performance over a wide band.

When inductors are used as elements in filters, the parasitic capacitance can often have significant effects for use in band‐pass filters, and the inductance must be evaluated for each use to find the effective value considering the parasitic capacitance.

A common figure of merit for inductors is the self‐resonant frequency (SRF), above which they act more like a capacitor (impedance goes lower with increasing frequency) than an inductor. The value of the SRF can be estimated in one way by looking at the length of the wire used in making the inductor. The SRF will be less than the frequency for which the wire is one‐quarter wavelength.