Importance Of Mismatch Tolerance For Amplifiers Used In Susceptibility TestingSource: AR
RF amplifiers have a nominal output impedance of 50 Ω and ideally would only be used in applications where the load impedance is also 50 Ω. This ideal situation results in maximum power transfer from the amplifier to the load. 100% of the power is absorbed in the load with 0% power reflected back to the amplifier. Unfortunately, broadband RF amplifiers are used in "real life" applications that are characterized by load impedances other than 50 Ω. In fact, encountering a pure 50 Ω load is indeed rare. Not only is load mismatch common in most applications, but since load impedance and to a lesser extent amplifier output impedance vary with frequency, the extent of mismatch will also vary widely over the test frequency range. Susceptibly testing is just one such application where load mismatch can be extreme. This application note will focus on the often overlooked issue of mismatch in RF systems, the harmful effects of even a modest amount of mismatch and finally, how proper selection of the system amplifier can mitigate the ill effects of mismatch. Functioning as a key element in an EMC susceptibility system, the RF amplifier must be capable of dealing with extreme mismatches without compromising performance or reliability.
Mismatch -- The condition whereby the output impedance of the RF source differs from that of the load is said to be a "mismatch". The extent of mismatch can be characterized in terms of Voltage Standing Wave Ratio (VSWR). (See Annex A for VSWR formulas) In its simplest form, VSWR is seen as the ratio of the source output impedance (amplifier output) to the load impedance at a given frequency. For our purposes we will assume a nominal amplifier output impedance of 50 Ω. If the amplifier is driving an ideal load impedance of 50 Ω, the VSWR is 1:1 and there is no mismatch. This ideal condition results in maximum power transfer and zero power reflection. Real life applications are rarely characterized by 50 Ω oads and the resultant VSWR is greater than 1:1. In this typical situation, power is reflected from the load back into the source, or amplifier. The amplifier must be designed to routinely sink this reflected power without adversely affecting performance or reliability.
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Application Note: Importance Of Mismatch Tolerance For Amplifiers Used In Susceptibility Testing