Lead wire inductance effect

In practice, the limit at high frequencies is controlled by the inductance in the circuit, (0L. The influence of this on the impedance (in contrast to that of the capacitance) increases with an increase in frequency. The difficulty is that the inductance that becomes significant when the frequency exceeds, say, 104 cps, is often more an irrelevant inductance, not one caused by the electrode process. Thus, it may arise because of some contribution from the wire connections to the cell and their interaction with the surroundings. Hence, very short leads to the cell should be used. It is possible to build circuits that compensate for the inductance effects, but usually the practice is to keep the frequency within the 10 kilocycle/s upper range, so as to make coL negligible. 

Until now, only circuits containing resistances and capacitances have been discussed. Inductive effects in electrical circuits appear when alternative electrical current flow creates a magnetic field interacting with the flowing current of course, in a strait wire the inductance is very small, but in looped wires or a coil it becomes larger. The inductive effects always lead to positive imaginary impedances, as will be shown in what follows. Let us first consider the circuit in Fig. 2.40, which contains inductance L in series with resistance Rq and a nested coimection of two (RQ circuits, i.e., LRo(Ci(Ri(R2C2))). The complex plane and Bode plots for this circuit without inductance were presented in Fig. 2.39. 

It is made clear that the so-called inductive characteristic of a grounding impedance is caused, in many cases, by the inductance of a current lead wire used in the measurement. The wave propagation characteristic on the electrode is determined by the soil permittivity in a VHF region, but is determined by in a lower-frequency region, where p is the soil resistivity and f is the frequency. Although the characteristic impedance of an electrode is proportional to In where r is the electrode radius, h the buried depth, and x the length, the effect of is more pronounced. 

It is well-known that a transient response in a circuit is significantly influenced by the impedance of a lead wire used for grounding, connecting circuits, and measurements as explained in Section 7.4.2. In Reference 47, it is said that the grounding of the metallic sheath of a control cable may not be effective at all during a high-frequency transient because of the grounding lead inductance. 

The above observations agree with those explained in Reference 47 that sheath grounding does not become effective for a transient due to the inductance of a lead wire. 

Traces, pads, and vias often act as inductors, capacitors, and coupling elements in the actual circuit. Their shapes may have a material effect on overall circuit performance. For example, the lead inductance and capacitance in a transistor collector circuit wire may act as the resonant components for an RF amplifier or it may degrade performance if it is unwanted. Figure 13.1 shows the impedance of traces as a function of their capacitance. 

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