Noise figure in PSPICE
Where does 8.28e-19 come from?


Thermal noise of a 50 ohm resistor in a 1 Hz bandwidth at 27 degC is 9.1e-10 volts

To convert this to an equivalent power it needs squaring and this produces a number of 8.28e-19.

Thermal noise calculator.

The formula also reduces to 20 log\$_{10}(\frac{V_{NOISE}}{9.1e-10}\$) i.e. it compares actual RMS noise against 1Hz-limited voltage noise from a 50 ohm resistor.