Decibel converter

The impedance is the bridge between the two halves of the page: dBm, dBW and dBuW describe power, the other four describe voltage, and neither makes sense against the other until an impedance is assumed. 50 ohms is RF practice, 600 ohms is the old broadcast line standard and the reason dBu is anchored at 0.7746 V. Stage gains in dB add because multiplying linear gains means adding their logarithms; levels themselves never add that way, so two 0 dBm sources mixed together are not 2 dBm.