14.1 Introduction

For decades high frequency circuits were developed using unbalanced (non-symmetrical) structures. Typical line systems, representing this kind of structure, are coaxial or coplanar line systems. Each unbalanced system consists of a signal line and a ground. The measurable signal is referenced to the ground.

Balanced (or symmetrical) structures are not used that often. A typical balanced structure is a parallel line system (Lecher line), a Low Voltage Differential Signal Line (LVDS-line) or balanced amplifiers and filters. Typically such a structure consists of two lines (simply said, a 'plus' and a 'minus') and a signal can be measured between these two lines. In practice, these structures create some additional phenomena compared to unbalanced systems which must be analysed in detail.

In an unbalanced system only the non-symmetrical TEM mode is present and it can be compared to the so-called common mode, which we will discuss later. In a coaxial system the inner conductor is the signal line and the outer conductor represents the ground. In addition this ground functions as a shield. Unbalanced line systems are normally connected to unbalanced circuits. Under the condition of power matching the measured voltage U ₁ against ground is U ₀₁/2.

We can conclude that such an unbalanced system offers very high noise immunity, it generates less radiation, the integration density is high and the losses are acceptable.

If such a line is connected to, for example, a non-shielded circuit, a signal generated by an interferer (like ground...