coupler !
1. When we say e.g -20dB coupler---what does it mean?
2. In a transmitter block after PA--usually there is a coupler--why ?
please....
Assuming a lossless device:
If you input 0dBm at the input port, then the power at the output through port is -0.0436 dBm ( 0 dBm - 20 dB). The output at the coupler port is -20 dBm.
It is a way to sample a small amount of signal power from the main signal. After the PA you could use it drive a power detector or drive a feedback network to linearize the PA.
Technically, I think you are wrong. If the output power from the main arm of the coupler is 0 dBm, then the coupled power is -20 dBm. The input power to the common connector would be a little higher than 0 dBm.
After PA is coupler because often there are two-stage amplifier. Two output signals from amplifiers is combine on coupler for increasing gain.
in coupler ther are two main parameters: coupling and directivity. i think that e.g means the coupling factor. you could see the Pozar book for detail information.
some times we use the directional coupler after the PA to get a sample of the output power and feed it to power detector , this is used to control the gain of the PA to increase the transmitter dynamic range
khouly
yes, directional coupler is used in repeaters after PA to take a part of signal to monitor the power level and to control it .