The Importance of S-Parameter Insertion Loss in the Design and Development of Microwave Circuits

Key Takeaways

• Insertion loss describes the ratio between input power and transmitted power.

• When a circuit or component is modeled as a two-port network, Insertion Loss= -20 logS21.

• The insertion loss in terms of the S-parameter is expressed in dB. When the S-parameter insertion loss measured is equal to 3dB, it shows a 50% loss in signal strength and makes the component useless.

Communication systems are never free from attenuation and loss

Communication systems are never free from attenuation and loss. However, the performance of front-end circuits, transmission lines, and components plays an important role in minimizing attenuation. 

S-parameter insertion loss is a term that is closely associated with RF and microwave signal transmission. Generally, RF and microwave signals get absorbed and reflected in components and transmission lines, leading to increased energy loss. Insertion loss describes the ratio between input power and transmitted power. In this article, we will take a closer look at insertion loss and its role in designing and developing microwave circuits. 

S-Parameter Insertion Loss

In RF and microwave circuits, signals are transmitted over transmission lines to various components. The input signal power, also called incident power, is lost in a variety of ways—conductor losses, dielectric losses, and radiation losses. Apart from these losses, some of the incident energy gets reflected in the input. The signal power that reaches the output side, called transmitted power, is always less than the incident power.

The ratio of incident signal power to the transmitted signal power (expressed in dB) is called insertion loss:

Consider a transmission line or a component as a two-port network. There is always a difference between the input signal and output signals, measured at the input port and output port, respectively. A zero difference indicates a lossless line or component, which is mostly surreal. The quantification of signal loss when traveling in and out of a line or component is essential for designing a good circuit. The insertion loss expresses the loss in signal when it travels from the input port to the output port. When a transmission line is modeled as a two-port network and  the matrix given below represents its S-parameter matrix, then the insertion loss can be given as equation 2 (below):

Insertion loss in terms of the S-parameter is expressed in dB. When the S-parameter insertion loss measured is equal to 3dB, it shows a 50% loss in signal strength and makes the component useless. Signal loss analysis becomes easier with the S-matrix. The S-matrix representation of the insertion loss is helpful, as a large circuit can be analyzed by cascading individual S-matrices. 

Ideal S12 Values

Ideal values of S21)

S-parameter matrix values are complex and are dependent on frequency. Circuit components—such as resistors, capacitors, and inductors—are reciprocal and amplifiers, filters, and phase shifters are non-reciprocal. For a reciprocal system:

Therefore, the insertion loss equation (2) can be given in either S12 or S21. Since we have seen the importance of the S12 value in determining insertion loss, let’s examine the same for some ideal 2-port reciprocal components. The table above shows the ideal values of S21.

The Role of Insertion Loss in the Design and Development of Microwave Circuits

For the proper operation of RF and microwave systems, the output needs adequate power. The various losses, impedance mismatches, line terminations, and reflections influence the transmitted power or output power. Any variation in transmitted power reflects on S-parameter insertion loss.

In the design and development of microwave circuits, such as transmission lines, amplifiers, filters, or attenuators, insertion loss plays an important role. The effect of inserting a circuit or component in a microwave system can be studied using S-parameter insertion loss values. For example, in the case of a transmission line, its main purpose is to carry input power to the output. If the S-parameter insertion loss values calculated from the S-parameter matrix greatly deviate from the ideal value, then the purpose of the transmission line is not fulfilled.

To design high-speed electronic systems, S-parameter insertion loss values are critical. Cadence’s software offers a suite of design and analysis tools for measuring insertion loss.
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