## Electric-Field Coupling |

Electric field coupling (also called capacitive coupling) occurs when energy is coupled from one circuit to another through an electric field. As we shall see, this is most likely to happen when the impedance of the source circuit is high. Consider the two circuits sharing a common return plane shown in Fig. 1. If the return plane had zero resistance, the common impedance coupling would be zero. However, it is also possible for coupling to occur between the two circuits due to the electric field lines that start on one signal wire and terminate on the other. For example if one of the signal voltages is +1 volt and the other is 0 volts, then the potential difference between the two signal wires results in electric field lines that start on the +1-volt wire and terminate on the 0-volt wire. Schematically, this can be represented by a capacitor between the two signal wires. Of course, there are other electric field lines that start on the +1-volt wire and terminate on the 0-volt plane. This can be represented by a capacitance between the wire and the plane. A schematic representation of the two circuits in Fig. 1 that includes the electric field coupling capacitances is shown in Fig. 2. Fig. 1: Two circuits above a signal return plane.
Fig. 2: Schematic representation of the circuits in Fig. 1 including capacitive coupling paths.
In this case, the capacitance between the wires, (1) If we try to find the exact solution, the procedure for analyzing this circuit with 9 elements can be time consuming. However, if we redraw the circuit and take advantage of the relative size of some of the impedances, we can greatly simplify the analysis.
First, let’s redraw the circuit in Fig. 2 as shown in Fig. 3. By putting Circuit 1 on the left side of the schematic and Circuit 2 on the right side, the important coupling,
Fig. 3: More intuitive schematic representation of the circuits in Fig. 1.
To calculate the crosstalk in Circuit 2 due to the signals in Circuit 1, we set
Fig. 4: An even simpler representation of the circuits in Fig. 1. Now the circuit is relatively easy to solve. The crosstalk can be expressed as, (2) ## Example 5-1: Calculating the crosstalk between two 150-ohm circuits
We start by determining the capacitances (3) The capacitance between the two wires is approximately, (4)
The impedance of (5) It is helpful to note how changing the various circuit parameters would have changed the coupling in this case. For example, doubling the frequency would have doubled the crosstalk (i.e. at 100 MHz, the calculated crosstalk would be -34 dB). For the weak coupling case, the electric field coupling is proportional to the frequency. Doubling the source resistance of the victim circuit would also have doubled the crosstalk in this example. Note that the parallel combination of the source and load resistances was almost equal to the source resistance. In this example, doubling the load resistance would have little effect on the crosstalk, since it is the parallel combination of the source and load resistances in the victim circuit that is important.
The other important parameter in this example is the mutual capacitance, |