Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 65

The PC board via is perhaps the most common PC board component, and often the most overlooked. The effect of a via depends greatly upon how it is structured in the circuit. A single via to ground in the center of a transmission line appears as almost a pure inductance. However, a via between RF traces can have aspects of inductance and some parasitic capacitance (due to pads around the via) that can cancel, in part or all, the inductive effect. When a via is used in a mounting pad for a shunt element, such as a resistor used as a load, or a bypass capacitor, the mounting pad and via form a resonant structure such that the size of the mounting pad can increase the effective impedance of the via. Further, several vias are often used in parallel to ground devices, sometimes to lower their effective inductance and sometimes to provide greater heat sinking of an active device. Putting vias in parallel does lower their effective inductance, but not in a simple way. Rather than halving the inductance, mutual inductance between vias means that the value of effective inductance doesn't reduce as expected. For example, putting two 100 Ω resistors at the end of a line to ground, placed in parallel, may show much larger inductive effect the same two 100 Ω resistors place in a T pattern, where the ground vias are separated and the mutual inductance is less.