decibels relative to carrier (dBc)
dBc (decibels relative to carrier) is a measure of the strength of an instantaneous signal at radio frequency . The dBc increment is based on the decibel , a logarithm ic measure of relative signal strength. Suppose a signal has an unmodulated-carrier power of P watts and a modulation signal power of P watts at some specified instant in time. Then the instantaneous modulated signal strength in dBc, symbolized S dBc ,is:
S dBc = 10 log 10 P / P
Signal strength in dBc can also be calculated in terms of the effective signal voltage if the impedance remains constant. Suppose a signal has an unmodulated-carrier root-mean-square ( rms ) voltage of V volts and a modulated rms signal voltage of V volts at the same impedance. Then the instantaneous modulated signal strength in dBc is:
S dBc = 20 log 10 V / V
If the dBc figure is positive, then the instantaneous modulated signal strength is greater than the unmodulated-carrier signal strength. If the dBc figure is negative, then the instantaneous modulated signal strength is less than the unmodulated-carrier signal strength.