Otto Julius Zobel - Innovations - Impedance Matching

Impedance Matching

A common theme throughout Zobel's work is the issue of impedance matching. The obvious approach to filter design is to design directly for the attenuation characteristics desired. With modern computing power, a brute force approach is possible and easy, simply incrementally adjusting each component while recalculating in an iterative process until the desired response is achieved. However, Zobel developed a more indirect line of attack. He realized very early on that mismatched impedances inevitably meant reflections, and reflections meant a loss of signal. Improving the impedance match, conversely, would automatically improve a filter's pass-band response.

This impedance matching approach not only led to better filters but the techniques developed could be used to construct circuits whose sole purpose was to match together two disparate impedances. Zobel continued to invent impedance matching networks throughout his career. During World War II he moved on to waveguide filters for use in the newly developed radar technology. Little was published during the war for obvious reasons but towards the end with Bell Labs in the 1950s, Zobel designs for sections to match physically different waveguide sizes appear. However, the circuit noted above which still bears Zobel's name today, the constant-resistance network, can be viewed as an impedance matching circuit and remains Zobel's finest achievement in this regard.