Noise Parameters are comprised of four distinct characteristics, Fmin the minimum noise figure of the transistor, Γopt the optimum impedance at which Fmin occurs (in real and imaginary), and Rn the equivalent series resistance of the transistor.
In theory, any four controlled impedances Γs can be presented to the transistor and the corresponding noise figures F measured in order to satisfy equation
and solve four simultaneous equations for four unknowns.
Practically, the impedances selected for Γs should be in the range of Γopt and proper impedance selection becomes critical if the transistor is non-50Ω.
The Keysight N524x-series PNA-X is the only instrument of its kind to include an embedded electronic tuner capable of presenting impedances up to Γ=0.3 (VSWR~2:1). When measuring a near-50Ω transistor, the electronic tuner may be sufficient in presenting impedances to solve the unknown equations and determine noise parameters. For non-50Ω transistors, the embedded electronic tuner is bypassed and an external automated impedance tuner is used to present impedances closer to Γopt. Tuner model MT982BL01 is recommended for measurements between 0.8-18 GHz; tuner model MT983BL01 for measurements between 2-26 GHz; tuner model MT984AL01 for measurements between 8-50 GHz.
The PNA-X is also the only instrument to combine a vector receiver with a dedicated and optimized noise receiver. The sensitivity of the noise receiver is critical in determining the noise figure F by accurately measuring the noise power contribution of the transistor. The accuracy of the noise measurement is directly related to the second-stage noise figure of the noise receiver, the lower the better. While it is possible to directly use the noise receiver of the PNA-X, the second-stage noise figure can be reduced by 5-6 dB with the addition of an external noise receiver module (NRM) MT7553B03.