On the feasibility of using an active tuner for measuring microwave noise parameters

The advantages of using an active tuner to vary the source impedance in a microwave noise parameter measurement system are investigated. The additional noise figure measurement error introduced by the use of an active tuner (rather than a conventional passive tuner) is analyzed and shown to be negligible, compared to those errors common to both types of tuner, provided that the tuner excess noise output is kept within certain limits. A modification to the standard one-port active tuner design is described which significantly reduces the amount of excess noise produced by such a tuner, and a prototype of this design has been built and tested. Measurements performed over a limited range of impedances indicate that the tuner is capable of synthesizing any impedance on the Smith Chart while keeping well within the acceptable noise levels. The use of this active tuner would facilitate a wider constellation of source impedances to be used in microwave noise parameter measurement systems than has been previously possible with a passive tuner, thus increasing noise parameter measurement accuracy.