Component Behavior in a RF Spectrum
Wire in an RF circuit can take many forms. Wire wound resistors, inductors, and axial-and radial-leaded capactors all use a wire of some size and length either in their leads, or in the actual body of the component or both. The behavior of a wire depends to a large extent on the wire's diameter and length.
A conductor, at low frequencies, utilizes its entire cross-sectional area as a transport medium for charge carriers. As the frequency is increased, and increased magnetic field at the center of the conductor presents an impedance to the charge carriers, this decreasing the current density at the center of the conductor and increasing the current density around its perimeter. This increased current density near the edge of the conductor is known as skin effect. It occurs in all conductors including resistor leads, capacitor leads and inductor leads.
Straight wire inductance might seem trivial. But any opposition to chang is called self-inductance and we call anything that possesses this quality an inductor.
Resistors are used everywhere in circuits, as transistor bias networks, pads, and signal combiners. However, very rarely is there any thought given to how a resistor actually behaves once we depart from the world of direct current (DC). In some instances, such as in transistor biasing networks, the resistor will stillperform its DC circuit function, but it may also disrupt the circuits RF operating point. In the equivalent circuit of a resistor at radio frequencies R is the resistor value itself, L is the led inductance, and C is a combinaton of the parasitic capacitances which varies from resistor to resistor depending on the structure. Carbon composition resistors are notoriously poor high-frequency performers. Wire wound resistors have problems at radio frequencies too.
The usage of a capacitor is primarily dependent upon the characteristics of it dielectric. The dielectric's characteristics also determine the voltage levels and the temperature extremes at which the device may be used. Thus, any losses or imperfections in the dielectric have an enormous effect on the circuit operation.
There are no perfect components and inductors are certainly no exception. As a matter of fact, inductors are probably the component most prone to very drastic changes over frequency.
A toroid, very simply, is a ring or doughnut-shaped magnetic material that is widely used to wind RF inductors and transformers. The Q of a toroidal inductor is typically high because the toroid can be made with an extremely high permeability. The net result of adding a high permeability core to an inductor is the gaining of the capability to wind a given inductance with fewer turns than what would be required for an air-core inductor. On a personal basis, thousands of satisfied customers have utilized my VIRTUAL LIBRARY. A library that is current, developed and reliable. THE PAY-OFF CAN BE ENORMOUS!
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