What is dBc?

dBc is the ratio of input signal power vs the carrier signal power. It is known as the decibels relative to the carrier. If the value of ratio is positive then the power of input signal is greater than the carrier signal and if the value is negative then the carrier signal has more power.

dBc is the unit used to measure the level of phase noise and harmonics/spurious signals with respect to carrier power level. For example, in the image below the Phase Noise is calculated to be - 70 dBc.