Philips Semiconductors S-Parameter Definitions General S-PARAMETER DEFINITIONS Measurement The S-parameter symbols in this section are based on IEC publication 747-7. The return losses are measured with a network analyzer after calibration, where the influence of the test jig is eliminated. The necessary termination of the other port with Zo is done automatically by the network analyzer. S-parameters (return losses or reflection coefficients) of a module can be defined as the S11 and the S22 of a two-port network (see Fig.1). a1 b2 S11 S 22 b1 a2 D.U.T. Fig.1 The network analyser must have a directivity of at least 40 dB to obtain an accuracy of 0.5 dB when measuring return loss figures of 20 dB. A full two-port correction method can be used to improve the accuracy. MLB335 Two-port network with reflection coefficients S11 and S22. b 1 = S 11 a 1 + S 12 a 2 (1) b 2 = S 21 a 1 + S 22 a 2 (2) 1 a 1 = ------------------ ( V 1 + Z o i 1 ) = signal into port 1 2 Zo (3) where: 1 a 2 = ------------------ ( V 2 + Z o i 2 ) = signal into port 2 2 Zo 1 b 1 = ------------------ ( V 1 + Z o i 1 ) = signal out port 1 2 Zo (4) 1 b 2 = ------------------ ( V 1 + Z o i 2 ) = signal out port 2 2 Zo From (1) and (2) formulae for the return losses can be derived: b S 11 = -----1- a 2 = 0 (5) a1 b S 22 = -----2- a 1 = 0 a2 (6) In (5), a2 = 0 means output port terminated with Zo (derived from formula (4)). In (6), a1 = 0 means input port terminated with Zo (derived from formula (3)). 1999 May 17 1