A Programmable Load for Noise Characterization

The characterization of low noise transistors in terms of the four noise parameters requires the measurement of the device noise figure under various loading conditions. Two approaches are used for the noise characterization of linear devices. One is to adjust the source impedance until the minimum noise figure is found, then to measure this input impedance (rL,,a,n)d. from a few more measurements to derive the noise parameter. The other is to measure the noise figure at different input impedances and then to perform a fitting procedure to find the desired four noise parameters. The first method requires a tuner which allows to present almost ail impedances. including that one were the minimum occurs. to the device under test. This is very difficult to achieve for high reflecting clevices like MESFETs and HEMTs in the lower frequency range.