RF 101: Reflections and S-parameters

Radio Frequency (RF) engineering is a specialized field of electrical engineering focused on devices and systems that operate in the radio frequency spectrum. Design of such systems are often difficult without sufficient experience in the field. The RF 101 series by SignalCraft RF will address the basics needed to understand RF systems and to educate the reader in analysis of such systems.

An RF signal typically refers to any signal with frequency ranging from 3 kHz to 300 GHz. This is a huge range of frequencies and is an overall ambiguous definition in my opinion. A more useful definition of RF signals is any signal that is transmitted via electromagnetic radiation. While all signals contain an electromagnetic radiative component, RF signals and systems rely on this property as their primary function.

For example, take a radar system: An RF emitter generates and transmits an electromagnetic wave to be bounced off of a target. This reflection is then measured at the receiver to determine important characteristics about the target. Radar, and other RF systems, are commonplace in our lives today, and their analysis is essential to understand to advance technology.

An RF signal at its core is just like any other electrical signal: it possesses some voltage and some current, related by the impedance of the medium. The main difference is that the wavelength of an RF signal becomes significant when compared to the physical dimensions of the medium.

The implications of this difference are massive. When a non RF, low frequency, signal is imposed on a medium, its long wavelength compared to the medium travels to the load in a time insignificant compared to the period. This makes the signal appear as if it instantaneously appears at the load. This leads to our ideal circuit analysis.

In reality, when a signal is introduced to a medium, it propagates through the medium in accordance to the mediums speed of propagation, or Vp. The signal is composed of voltage and current waves, which are related by the impedance of the medium per unit length. When the signal arrives at the load, a portion is reflected at the junction and a portion is transmitted. The reflected signal then travels back to the source in much the same manner as how it arrived. When it reaches the source, a portion will be reflected and a portion will be transmitted to the source, and then cycle continuous. This medium is what we would call a transmission line.

The reflection present at the load is quantified by the loads reflection coefficient. Simply put, it is the power transmitted to the load over the power reflected back to the source. We can express this in log terms to get the loads S-parameter. The S parameter characterizing the reflections at the output of a system is the called the output return loss.

S-parameters are common ways for RF engineers to talk about this reflection occurring. We call each termination where a junction occurs, and thus a reflection can occur, a port. We can characterize a linear system entirely through its S-parameters of its ports. Take for example the simplest 2 port system: The transmission line from earlier. a 2 port system will have 4 S parameters: S11, the input return loss, S21, the attenuation through a connection, S22, the output return loss, and S12, or the isolation.

S-parameters are ratio of powers. So why do we not just express things as voltages and currents? RF engineers express things in terms of power as opposed to voltage and current because often the voltage and current are misnomers when looking at a system. These individual waves are still present, but they are often transformed over a system and it becomes confusing to look at.

For example, At the source, you land an oscilloscope probe at the output of your RF DAC you may see a 1 Vpp wave. But at the end of a transmission line, you see 1.5 Vpp. Remember that a transmission line is just a conductor. How can you gain voltage with just a wire? The answer: the current wave has decreased to make up for this increase in voltage. The power wave remains constant, or attenuates. We choose to express signals in terms of power and impedance for this reason. We will go over this topic in more detail in future blogs.

At SignalCraft RF, we utilize all of these concepts to perfect a solution to your RF problem, taking the guesswork out of RF design. Contact us at contact@signalcraftrf.com for more information or with any questions.