#TRANSMISSION LINE RF TOOLBOX SERIES#

Notice that a 50Ω terminator is connected in series with the filter to preserve impedance match when measuring S 11 and S 22 parameters.įigure 3 : Moku:Lab configurations for S-parameter measurements Measurement Validation Our 2-port network will be a bandpass filter by Mini-Circuits (BBP-35A+) with a frequency range of 30-40MHz.įigure 3 shows the four arrangements of the Moku:Lab with respect to the 2-port filter (BPF) and RF coupler (Mini Circuits ZFDC-10-21). With the help of the directional coupler, Moku:Lab’s FRA is able to measure all four S-parameters of a 2-port system.įigure 2: Power flow inside a directional coupler under different circumstances Moku:Lab Set Up This means if we place a DUT at the input port of the RF coupler and drive the device from the output port, we can probe the reflected power by monitoring the CPL port. For signals traveling in the reverse direction, almost no power is coupled. For signals traveling from the input to output, a small portion of the power is coupled to the CPL port. It has an input (In), an output (Out), and a coupling (CPL) port.

The directional coupler we are going to use to measure S-parameters is a Mini-Circuits ZFDC-10-21. Directional couplerĪ directional coupler is an analog device that is designed to couple a certain amount of power transmitted a certain direction (but not the other direction). The matrix allows for a powerful and scalable linear tool that can be used to isolate and study certain port characteristics in an n-port network. In this format the rows and columns represent the number of ports present. These complex numbers arise from a mathematical representation known as a scattering matrix. On its own, Moku:Lab is capable of measuring S 12 or S 21 of a 2-port system, but not the S 11 and S 22. Moku:Lab’s FRA is capable of driving a DUT with a swept sine wave into a system’s input port and extracting the amplitude and phase response at a system’s output port. The first number is the output port (emerging) and the second number is the input port (applied) as depicted in this figure below.įor example, S 22 represents the reflected power (magnitude and phase) of the system from port 2 at a certain frequency. Notice that the four S-parameters for this 2-port network have subscripts relating the ports that are under consideration. Below is a figure representing a 2-port DUT network along with all the signal paths captured by S-parameters.įigure 1: S-parameter representation in a 2-port network

The beauty of S-parameters is that we can fully understand a DUT by just analyzing the transmitted and reflected signals as described by its S-parameters. This box can contain a multitude of system variables: resistors, filters, integrated circuits, or transmission lines, the details of which are hidden. S-parameter characterization of a Device Under Test (DUT) treats that DUT as a black box with one or more ports, where signals can both enter and exit any port. Since we care mainly about power gain or loss we will focus on the magnitude as a function of frequency. S-parameters are complex numbers, meaning they have both imaginary and real parts, thus can represent both magnitude and phase. We will visualize transmission line problems and impedance matching with Smith charts. We will be exploring this parameter in depth and showcasing its implementations when analyzing systems and filters at high frequencies using Moku:Lab’s Frequency Response Analyzer. In other words, it helps describe how RF energy propagates through a multi-port network. It is used to describe the reflection/transmission characteristics of a port network system. One useful parameter when designing at high frequencies is the S-parameter, or “Scattering parameter”. This reflection is suboptimal in RF design as it reduces transmission quality and efficiency. Any mismatch in impedance along the transmission lines of an RF system will result in signal reflection. This phenomenon of reflection is analogous to dealing with high frequency signals (hundreds of MHz or GHz) in the RF world. This time, when you shout your voice is reflected by the wall and echoes back at you. Next, imagine doing the same thing, but the hallway is truncated with a wall. The sound travels into the abyss with no discontinuity and eventually just fades into nothingness. Imagine shouting into a long, endless hallway. In this application note, Moku:Lab’s Frequency Response Analyzer is used in conjunction with an RF directional coupler for a complete S-parameter characterization of a two port network. Transmission and reflection signal information is vital when designing and validating RF components and systems.

Transmission line rf toolbox