Noisecom uses very accurate measurement devices - in this case a power meter. But the substitution method eliminates the effects of any non-linearity in the measuring device so the result is even more accurate.
How does it work?
The First step is setting the signal and the noise to the same power level at the power meter input.
Then the noise power is then offset by the desired ratio.
Effects such as thermal drift becomes negligible since the noise and the power are measured within a very short time frame.