关系:P(W) = VRMS2 / Z0;dBm = 10·log10(P/1mW)
dBu = 20·log10(VRMS/0.775),dBV = 20·log10(VRMS/1V)
波形:正弦 Vrms=Vpeak/√2;方波 Vrms=Vpeak;三角 Vrms=Vpeak/√3