一个耦合器的问题

有一个问题大家一块思考下。
An ideal directional coupler has a directivity of 25 dB and an isolation of 40 dB. What is its coupling value?
a) 65 dB
b) 40 dB
c) 15 dB

选择C啊，15dB，一个做题技巧是，这种dB的计算一般只有加减，没有乘除运算。
A directional coupler is characterized by five main parameters as follows:
1.  Frequency band of operation.
2.  Power coupling expressed as dB down from the input power level.
3.  Isolation of the coupled port from the output port (essentially coupling factor from the output port to the coupled port)
4.  Directivity, which  is mathematically the difference between the magnitudes of the isolation and the coupling. If the coupler in this case had 0 dBm signals applied to both the input and output ports, the coupled port would see -15 dBm from the input port and -40 dB from the output port, hence, an isolation of 25 dB.
Coupling = 40 dB - 25 dB = 15 dB.有点意思吧