Feature 7: Stray Coupling Analysis

Stray Coupling is unwanted coupling effects, leaking energy between non-adjacent resonators, which may cause either the unbalanced notch level or extra unwanted transmission zeros. The most common stray coupling effect happens to the CQ (cascade quadruplet) topology which has two symmetrical transmission zeros:

Starting from the design and simulation stage, the user needs to avoid the unwanted stray coupling either from the 3D coupling structure or feasible selected topology in order to achieve the design target. The serious unwanted stray coupling may fail IL ripple and sometimes may cause uneven distributed Q factor which may affect the power handling capability.

In order to catch up with the stray coupling effect under the “Computer Aided Tuning” Function, the user needs to modify the topology to take into account the topology modification before the capture. Failure to do this may lead to the undesired filter response.

Step 1: Synthesize the matrix based on target topology.

Step 2: Reset the topology before the matrix extraction

Once user starts to do the matrix extraction, the topology needs to be reset to take account into the stray coupling effect. For example, the CQ topology may have 2 possibilities, M13 or M24(Select the topology check box without clicking “Calculate All”)

The user needs to decide which assumptions match with the designed coupling schemes. No matter which possibility, the algorithm will only take this assumption as the auxiliary parameter to assist the capture process.

Comparing with both extracted coupling matrices, one can see the differences between these two assumed stray coupling schemes are less than 3%. If the user is not sure which one is suitable for the 3D model, randomly select one and stick with this assumption until completing the whole design.

This example will select M13 as the stray coupling and do the matrix extraction until the RL is lower than -23dB:

  1. Step 1: Do the synthesis work and obtain coupling matrix without stray coupling M13
  2. Step 2: Start to do the matrix extraction and select the M13 as assumed stray coupling (Please note that only changing the topology is required, DO NOT CLICK “CALCULATED ALL” )
  3. Step 3: Import the S2P data an do the matrix extraction;

After simulating it 10 times, the RL can be lower than 24dB. The stray coupling cannot be eliminated; however, the user can modify the structure to minimize the notch level to meet the design target: