Hybrid coupler isolation limitation

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New for October2019: Hybrid couplers are often used in power combiners, and sometimes in reflection attenuators and phase shifters.  By "hybrid" in this case we mean a coupled-line coupler that provides close to 3dB, or equal, power split.

In the power combiner case, it is desirable to have the isolated port output down 20 dB from the input signal, this means you are wasting less than 1% of the input drive power in the isolation load, and less than 1% of the output power is getting dumped into the output isolated port. Referring to the ideal coupler schematic below, let's call port 1 (upper right) the input, for a power divider on the input of a pait of power amplifiers.  Then port 2 is the isolated port, port 3 is the through power and port 4 is the coupled port. This schematic allows the user to enter the coupling factor (-3.01 dB for a hybrid) and computes even and odd mode impedances (120.7 and 20.7 respectively for Z0=50 ohms) for any value of Z0.  Oops, need to fix the Ze equation, it starts with "50" instead of "Z0"... It also lets you detune the perfect coupler by introducing an error term to Zo.

Now, let's terminate the coupled and through ports with 100 ohm resistors, to represent a 2:1 VSWR at just these ports.  Often, the amplifiers you are trying to combine have poor return loss.... where does that returned power end up?

The reflected power goes out the isolated port.  Thus, for a 2:1 mismatch the isolation (which was perfect in our perfect example) is now 9.542 dB.  If you go to our VSWR calculator, you will find that the return loss of a 2:1 mismatch is also -9.542 dB.

What does this mean?  You should always strive for 20 dB isolation in a hybrid power combiner, but at the same time you will need to make sure that whatever you drop the circuit into has 20 dB return loss. If you design a thin-film hybrid and add RF connectors, the connectors may be the limitation on RF performance.

Coming soon: what happens to the power combiner on the output of an SSPA with regard to port match?

Author : Unknown Editor