一個耦合器的問題

有一個問題大家一塊思考下。
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

選擇C啊，15dB，一個做題技巧是，這種dB的計算一般只有加減，沒有乘除運算。
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.有點意思吧