Wikipedia
The Y-factor method is a widely used technique for measuring the gain and noise temperature of an amplifier. It is based on the Johnson-Nyquist noise of a resistor at two different, known temperatures.
Consider a microwave amplifier with a 50 ohm impedance with a 50 ohm resistor connected to the amplifier input. If the resistor is at a physical temperature T, then the Johnson-Nyquist noise power coupled to the amplifier input is P = kTB, where k is Boltzmann’s constant and B is the bandwidth. The noise power at the output of the amplifier (i.e. the noise power coupled to an impedance-matched load that is connected to the amplifier output) is P = Gk(T+T)B, where G is the amplifier power gain and T is the amplifier noise temperature. In the Y-factor technique, P is measured for two different, known values of T. P is then converted to an effective temperature T (in units of kelvin) by dividing by k and the measurement bandwidth B. The two values of T are then plotted as a function of T (also in units of kelvin), and a line is fit to these points (see figure). The slope of this line is equal to the amplifier power gain. The x-intercept of the line is equal to the negative of the amplifier noise temperature -T in kelvin. The amplifier noise temperature can also be determined from the y-intercept, which is equal to T multiplied by the gain.