Matching Circuit Design Made Easy

An essential task in the design of any wireless system is to ensure proper impedance matching between the different components in the RF chain. Impedance mismatch will degrade the transfer of energy between the amplifier and the antenna and thus reduce the efficiency, communication range and battery life of the wireless device.

Impedance matching may seem simple: every RF engineer knows how to match a given complex impedance to another one at a single frequency, using just two components (inductors or capacitors). However, in the real world, things get more complicated. First, as the wireless systems always have some operation bandwidth, the impedances should be matched over the whole frequency band. In broadband matching, more than two components need to be used and then the number of possible matching circuit topologies grows exponentially with the number of components. A second complication is that in real life, the inductors and capacitors always include parasitic resistances and reactances, which need to be considered in the optimization of the matching components, together with the effect of the circuit layout, i.e. the connecting lines and nearby metallization of the PCB near the matching components.

An important consequence of the presence of losses is that instead of trying to minimize the impedance mismatch, one should try to maximize the energy transfer through the RF chain. A trivial example is that a 50 Ohm resistor provides perfect impedance match to a 50 Ohm transmission line, but then all the power will be absorbed in this resistor, instead of being transmitted. While the optimization of the impedance match can be done using a network analyzer and the Smith chart, the optimization of the efficiency of the RF chain requires the use of simulation tools.

Now let me introduce our Optenni Lab RF Design Automation Platform, which is a software package helping RF engineers to design matching circuits and optimize the performance of the RF chain. Optenni Lab speeds up the design flow and automates many of the repetitive tasks in RF design.

The RF circuit design in Optenni Lab starts with the load impedance data, such as an antenna, which can be obtained directly from a network analyzer through a real-time link or from electromagnetic simulators, such as FEKO by Altair. Then the user enters the desired operation frequency ranges, maximum number of matching components to be used, and the inductor and capacitor series to be used.

Within a couple of seconds, Optenni Lab presents several optimized matching circuit topologies for the user. All the circuits are optimized for maximum power transfer, considering the resistive losses in the matching components and the parasitic reactances of the components and their packaging. The effect of the manufacturing tolerances can be easily analyzed using Monte Carlo analysis. It is also easy to check the maximum current and voltage over the components for a given input power level.

When implementing the matching circuit on the PCB, the layout effects can change dramatically the operation of the matching circuit. Optenni Lab can easily take this into account by considering either the layout as short segments of microstrip lines or using the electromagnetic simulation result of the layout part of the PCB. With these tools, matching circuits which match the simulated results can easily be built.

In the following design example, the matching circuit for the LTE Band 1 (1920-2170 MHz) is optimized with three components. The first result is using ideal lossless inductors and capacitors. This could be obtained manually by an experienced RF engineer.

In the second example, we have replaced the ideal components with the nearest available values from certain Murata inductor and capacitor series.

As can be seen above, the performance of the matching circuit has dramatically been changed by the parasitic reactances of the real inductors and capacitors, and the efficiency is only a few percent of the original simulation.

In contrast, below is an example of an optimized matching circuit from Optenni Lab, using the real inductor and capacitor models. Note that compared to the case of ideal components, the efficiency is somewhat reduced due to the losses in the inductors and capacitors. In addition, a better impedance match could be obtained using these component series, but this would naturally be at the expense of efficiency.

In this brief example I have discussed the basics of impedance matching. Optenni Lab can be applied to much more complicated RF matching applications too, such as matching multiple antennas at the same time and using tunable capacitors or switches to control the operation frequency of the matching circuit.

This guest post is written by Jussi Rahola, CEO of Optenni Ltd., developer of Optenni Lab. Optenni Lab is available through the Altair Partner Alliance.



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