Noise measurements allow the determination of the four Noise Parameters of a device (transistor).
Noise Parameters: These are four numbers that fully describe the noise behaviour of an active or passive device (twoport) at a given frequency. For practical reasons we use the following quantities as Noise Parameters:
Minimum Noise Figure (Fmin): This is the smallest Noise Figure that the device can reach at a given frequency and bias, if it is optimally matched at the source.
Equivalent Noise Resistance (Rn): This is a number with the dimension Ohm that indicates how fast the Noise Figure increases when we mismatch the input (source).
Optimum Noise Reflection Factor (Γopt), is often used also as Optimum Admittance Yopt: Is the source admittance required for the DUT to perform Fmin ; (Yopt=Gopt+jBopt), 2 parameters.
The Noise Figure does not depend on the Load impedance presented to the device . It only depends on the Source Impedance. There exist a simple relation between the four Noise Parameters:
F(Ys) = Fmin + (Rn/Gs)*| Ys – Yopt|2
where Ys = Gs +jBs. This is the equation of a set of isometric circles on the Smith Chart (Noise Circles) for which the value of the Noise Figure is the Level on each Circle.
This Circle Representation is only possible because the Noise behaviour of transistors is a Small Signal Phenomenon.