What's the differences between couple and isolator?

Could you tell me what's the difference for couple and isolator , How about the input impedance and output impedance?

Hi xinxin

A coupler is a 3 (or 4) port device. Signal source is connected to port 1 and flows nearly undamped to port 2. A small amount of the energy (depending on the coupler, e.g. -20dB) is coupled to the third port. Coupler are directional devices. You can measure forward and reflected power (this is the reason why some couplers have 4 ports).

An isolator is a circulator with a terminated third port. Energy can only flow from port 1 to port 2, but not from port 2 to port 1. It can be used to protect an amplifier if you connect an unmatched antenna.

Each port on both devices normally have 50Ohm impedance.

Bye

Thanks , on the other hand , how about is attenuator? is it has only 2 ports?

The reason of your question is probably one of the four parameters of the coupler:
- Directivity or Isolation
- VSWR
- Coupling coefficient
- Insertion loss
Directivity is a measure of how well a coupler can isolate two signals. Directivity, therefore, sets the limits on how accurately a coupler can perform a specific measurement. Ideally, one would like to measure the magnitude of the reflected signal alone.
However, because of directivity, the reflected signal is combined with a small, undesired portion of the incident signal. It is important to emphasize that the magnitude of this reverse-coupled signal depends strictly on the directivity of the coupler.

regards