SBOS710D October 2014 – February 2018 LMH5401
Noise figure (NF) is a helpful measurement in an RF system design. The basis of this calculation is to define how much thermal noise the system (or even on the component) adds to this input signal. All systems are assumed to have a starting thermal noise power of –174 dBm/√Hz at room temperature calculated from P(dBW) = 10 × log (kTB), where T is temperature in Kelvin (290k), B is bandwidth in Hertz (1 Hz), and k is Boltzmann's constant 1.38 × 10–23 (J / K). Whenever an element is placed in a system, additional noise is added beyond the thermal noise floor. The noise factor (NF) helps calculate the noise figure and is the ratio between the input SNR and the output SNR. Input SNR includes the noise contribution from the resistive part of the source impedance, ZS. NF is relative to the source impedance used in the measurement or calculation because capacitors and inductors are known to be noiseless. Equation 9 calculates NF:
For thermal noise calculations with different source resistances, Equation 11 can be used to calculate the NF change with a new source resistance. For example, Equation 9 uses a source resistance of 50 Ω. By using a source of 100 Ω, the new noise figure calculation (Equation 11) yields an NF of 6.6 dB, which provides a 3-dB improvement that results from the increase in Zs.