Determining the OIP3 and Bias Network Resonances of Phased-Arrays Using Far-Field Techniques

The output 3
^rd
-order intercept point (OIP3) is a figure of merit to evaluate the linearity of an amplifier or a system of amplifier blocks. Also, the OIP3 is a useful term to estimate the 3
^rd
-order intermodulation (IM3) components and the adjacent channel power ratio (ACPR) of a transmitter for complex modulation measurements. A phased array operating in a transmit mode combines the output of several power amplifiers in free space, and the far-field contains an average and a scaled version of the linear and nonlinear components of the individual power amplifiers. In this paper, a method to determine the OIP3 of phased-array amplifiers using a far-field measurement is derived and verified by experiment. The far-field OIP3 is then measured using a 5G 28 GHz 32-element phased array employing 2x2 beamformer chips, and the far-field derived OIP3 agrees well with measurements done on a single amplifier. This technique therefore allows the user to determine the (average) OIP3 of the power amplifiers used in an array even if they do not have access to the individual beamformer chip. An additional benefit is the ability of characterizing the PCB bias network resonances in a phased-array environment.