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Selected Reprints on February 1973 S - Parameters HEWLETT ihp] PACKARD application note 95A WHAT ARE "S" PARAMETERS? "S" parameters are measured so easily that obtaining accurate "S" parameters are reflection and transmission coefficients, phase information is no longer a problem. Measurements like elec- familiar concepts to RF and microwave designers. Transmission co- trical length or dielectric coefficient can be determined readily from efficients are commonly called gains or attenuations; reflection co- the phase of a transmission coefficient. Phase is the difference be- efficients are directly related to VSWR's and impedances. tween only knowing a VSWR and knowing the exact impedance. Conceptually they are like "h," "y," or "z" parameters because VSWR's have been useful in calculating mismatch uncertainty, but they describe the inputs and outputs of a black box. The inputs and when components are characterized with "s" parameters there is no outputs are in terms of power for "s" parameters, while they are mismatch uncertainty. The mismatch error can be precisely calcu- voltages and currents for "h," "y," and "z" parameters. Using the lated. convention that "a" is a signal into a port and "b" is a signal out of a port, the figure below will help to explain "s" parameters. Easy To Measure Two-port "s" parameters are easy to measure at high frequencies TEST DEVICE because the device under test is terminated in the characteristic impedance of the measuring system. The characteristic impedance termination has the following advantages: 1 S,, S 22 ] S|2 1. The termination is accurate at high frequencies . . . it is possible to build an accurate characteristic impedance load. "Open" or "short" terminations are required to determine "h," "y," or "z" parameters, but lead inductance and capacitance make these termi- In this figure, "a" and "b" are the square roots of po