Thursday, June 21, 2018

Why 50 Ohms?

Ryan Foster, NuWaves Engineering, June 21, 2018

A common concern for radio frequency design engineers is providing sufficient margin in a module or system to survive the real-world application in which their work will eventually operate. Our engineering team is constantly dealing with the trade-offs of meeting the myriad of customer specifications while ensuring that our designs are robust enough to survive the unpredictable reality of fielding those designs. For instance, it is one thing to design an amplifier that operates in the perfectly controlled environment of a laboratory, but then connect that amplifier to an antenna which looks like a nice comfortable perch to a bird and you have a completely different story. One of the measuring sticks that engineers utilize to judge a system or modules robustness is to test is ability to survive a mis-matched load. Voltage Standing Wave Ratio (VSWR) is the most common method for quantifying the ability of a 50-ohm matched module to withstand the mis-matches that we all know it will experience when it is eventually fielded, with an output VSWR rating of 10:1 being common in the industry. But wait, where did that 50-ohms starting point come from?