RF Match Network Characterization and Importance of Quality Factor

mono match
The match unit or matching network is a key component in getting power from an RF power supply to a plasma. The output of the power supply has a typical impedance of 50 Ohm. To match this to a plasma or other dynamic load we need a matching network. The matching network plays two key roles

i) it presents a reactive impedance equal and opposite to the load so that the reactive components cancel and
ii) it matches the real resistance of the load to the 50 Ohms of the RF power supply.

The Octiv Mono VI probe and impedance meter is the ideal product for characterizing the matching network. The Octiv Mono is a precision RF wattmeter and directional power sensor used in a large number of laboratory applications. The Octiv Mono is immune to harmonics and operates to 1% true accuracy. It measures forward power, reflected power, absorbed power, VSWR, and complex impedance.

Figure 2: Schematic of the experimental setup used for RF matching network characterization using two Octiv Mono products.

The figure shows experimental setup used for RF matching network characterisation using 2 Octiv Mono sensors. There are three performance indicators for the matching network: the match impedance range, match efficiency including internal resistance, and match quality. The quality of the match network is not how well it is made or constructed. Instead, the quality, Q refers to the current amplification required to match the two resistances, Rload and Rsource. If the Load is 50 Ohms and the source were 50 Ohms then the Quality of the matching unit would be zero. If the load resistance was 10 Ohms then the Quality would be 2. In general for the common matching Network type;

Q = √ ( Rsource/ Rload – 1)

The quality factor is one of the most important quantities for an RF engineer that can be measured as it determines how much current amplification is required and as this current flows in the coil it also determines how much power is absorbed in the coil for a given coil resistance. Quality factor will depend on the range of impedance the matching network is designed to handle. The quality will directly determine the efficiency, that is how much power is lost in the matching unit, the closer the load is to 50 Ohm the lower the quality and the greater the efficiency. In the matching network analysed here the efficiency was 80% at Q=2 and 60% at Q = 4. Now we know exactly how much power we lose in matching a load. We also know how much the coil will heat and whether we need cooling or maybe silver plating the coil to increase the efficiency. There is a link to quality after all!

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