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Microstrip narrow edge coupler simulation design case study

Coupler no. 5: Microstrip narrow-edge couplers for non-uniform media. Three minutes of debris and a drop in the bucket.

Having mentioned above that the unequal phase velocities of the odd and even modes in a non-uniform medium lead to poor coupler directionality, let us now look at specific simulation examples.

Although the near-end coupling is all 20dB, the far-end isolation is very different.

2.5GHz frequency point, with 46dB of distal isolation and 26dB of directionality for the ribbon dual line.

2.5GHz frequency point, with buried microstrip twin-wire reaching 33dB of distal isolation and 13dB of directionality

2.5GHz, bare microstrip dual-line distal isolation of only 25dB, directionality of only 5dB.

We often talk about microstrip couplers, precisely the aforementioned bare microstrip dual-line couplers, with the worst directivity of 5dB.

The electromagnetic field of the ribbon bi-wire lies only within a homogeneous medium.

the electromagnetic field of the buried and exposed microstrip bi-wires is partly within the medium and partly in the air, so that the medium is said to be inhomogeneous.

In comparison, the buried type has a slightly better dielectric homogeneity than the bare type.

Simulation design of microstrip narrow edge couplers

Design the coupler in the following steps.

  1. first calculate the odd-mode impedance and even-mode impedance according to the voltage coupling degree k0 formula below.
  1. then calculate the line width and line spacing in POLARSI9000 software (omitted).

or in HFSS to establish a symmetric surface half model of the microstrip bilinear, adjust the line width and line spacing, several superposition can be fitted to get the line width and line spacing in line with the odd-mode impedance Z0o and even-mode impedance Z0e.

The boundary conditions for the symmetry surfaces of the odd-mode and even-mode truths are set as follows.

As mentioned above, an invisible mirror exists directly in the middle of the parallel bilinear coupler along the axial direction (direction of signal propagation).

If the odd mode is simulated, the symmetric mirror is set to PerfectE.

if simulating an even mode, set the symmetry mirror to PerfectH.

Set the cross section of the half of the model with the microstrip bilinear symmetric mirror, the whole to Waveport, with a normalised impedance of 50 ohms.

3, set the rest of the boundary conditions and sweep range, the simulation is complete, you can observe the TDR simulation results: the

You can also follow the ADS optimization method in “003_Differential Mode ImpedanceCommon Mode ImpedanceOdd Mode ImpedanceEven Mode ImpedanceWhat the hell is that? The ADS optimisation method in “003_Differential Impedance Common Mode Impedance Even Mode Impedance” is used to simulate the differential and common mode impedances. The odd-mode impedance and even-mode impedance can then be calculated indirectly.

Then observe the odd-mode phase and even-mode phase.

With the same model, the same physical length of 1200 mil, and the same frequency point of 1.5365 GHz, the odd-mode phase of the bare microstrip bilinear is 90 degrees, while the even-mode phase is 99 degrees.

It is easy to see that the odd-mode phase speed and the even-mode phase speed are not equal according to the equation V = f * λ. The odd-mode phase speed is faster.

Vo= (1.53651000000000)[(360/89.97)(12000.0254*0.001)] = 187392544 m/s

Ve= (1.53651000000000)[(360/98.89)(12000.0254*0.001)] = 170489505 m/s

Far from it.

  1. Preliminary modelling of line width and line spacing according to the TDR simulation.

Number the ports according to the convention of “1 input, 2 pass-through, 3 near-coupling, 4 far-coupling”.

  1. Check the indicators for full port impedance matching, coupling, remote isolation, phase, etc.

Summary

Based on the above simulation results, it is possible to summarize the characteristics of the bare microstrip two-wire coupler as follows.

Ø Microstrip narrow-edge coupler capable of full-port lossless matching; the twin lines in the coupling area are suitably thin to suit the impedance matching of each port.

Ø The central frequency (minimum) is determined by the length of the coupling zone being equal to the λ/4 guided wave wavelength.

Ø maximum coupling at the centre frequency (lowest) and its odd multiple frequency point for λ/4 guided wave wavelengths.

Ø the shortest electrical length is the λ/4 guided wave wavelength.

Ø the coupling degree is determined by the parallel bilinear gap.

Ø The phase difference between the coupling end and the through end is 90 degrees, independent of frequency.

Ø Microstrip narrow-edge couplers are suitable for weak couplers, where it is difficult to achieve a coupling degree stronger than 10 dB.

Ø The directionality index at the central frequency point can only be about 5 dB.

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