Directional couplers are used to detect and transmit signals without affecting the performance of the whole system.
Directional couplers can come in the form of microstrips, striplines, coax, or waveguides.
When two transmission lines are placed close to each other, the electromagnetic field interactions of each line cause power to be coupled between the two lines. This is the basic principle of microstrip and stripline directional couplers.
Directional couplers are crucial components in RF and microwave circuits, and during their selection, designers must carefully consider their costs, along with other parameters such as the ease of manufacturing and implementing them into RF circuits. Luckily, designers have many directional coupler designs to choose from.
Directional couplers can come in the form of microstrips, striplines, coax, or waveguides. Compared to other types of couplers, microstrip directional coupler designs provide a low cost and easily manufacturable option, making them one of the most commonly used couplers in RF and microwave circuits.
The Need for High Directivity in Microstrip Directional Coupler Design
Microstrip directional couplers are used to detect and transmit desired signals without impacting the performance of the whole system. Unfortunately, conventional microstrip directional couplers lack directivity and need some design modifications to mitigate this. However, before discussing how to improve their design, let’s first discuss the conventional microstrip directional coupler’s structure and modes.
The Structure of Microstrip Directional Couplers
Microstrip directional couplers are easy to integrate into RF circuits constructed on a single substrate. When two transmission lines are placed close to each other, the electromagnetic field interactions of each line cause power to be coupled between the two lines. This is the basic principle of microstrip and stripline directional couplers. When microstrip coupled lines are used to construct directional couplers, they do not support transverse electromagnetic (TEM) mode, and their mode of operation is approximated to quasi-TEM.
Usually, microstrip directional couplers are four-port systems consisting of two parallel signal lines, with the electric and magnetic fields of a signal on one line inducing currents and voltages on the other. Due to their physical structure, they can also be called microstrip parallel-coupled line directional couplers.
Even and Odd Modes in Microstrip Directional Couplers
According to the current flow in the coupled lines, there are two modes of propagation in quasi-TEM modes—odd and even modes, with characteristic impedances Z0o and Z0e, respectively. In odd and even modes of propagation, the phase velocities are different due to the different field configurations in the vicinity of the air-dielectric interface. The phase velocity of the even mode is greater than the odd mode phase velocity. In odd mode, the energy of electric and magnetic fields are concentrated between the strips (both in air and dielectric), whereas in even mode, it gets concentrated only in the dielectric region. The wavelengths of the odd and even modes are also different from each other.
The directivity of microstrip directional couplers is compromised due to the inhomogeneity in microstrip lines. The inhomogeneous nature of the dielectric substrate and air in microstrip lines results in unequal even and odd mode velocities, which results in poor directivity. As the coupling between strips decreases, the directivity of the coupler worsens.
It is difficult to achieve tight coupling due to impractical spacing between the coupled lines in conventional microstrip directional couplers. With the increasing value of dielectric permittivity, the directivity performance of the microstrip directional coupler decreases. The effective permittivity of the even mode is greater than the odd mode, and this induces unequal phase velocity. Luckily, microstrip directional coupler designs can be modified to achieve high directivity performance and tight coupling.
Microstrip Directional Coupler Design
There are several methods to equalize or compensate velocity inequality in even and odd modes of microstrip directional coupler designs. The dielectric overlay is one method, where the effective dielectric constant of the odd mode is increased to equalize the phase velocities. The wiggly line coupler and re-entrant mode couplers are recommended to provide high directivity performance and tight coupling. The capacitively and inductively compensated directional couplers exhibit equalized phase velocities.
In a capacitively compensated microstrip directional coupler, the capacitive compensation is used with parallel-coupled microstrip lines, and the additional capacitor is fabricated using a dielectric substrate. The lumped capacitor is responsible for reducing the difference between the phase velocities in a microstrip directional coupler’s odd and even modes. This design offers tight coupling between the coupled lines as well.
The effectiveness of microstrip directional coupler designs can be analyzed by measuring the directivity and tight coupling in dB and comparing it with a conventional microstrip coupler. Cadence's PCB Design and Analysis tools can support you in calculating the design parameters and phase velocities of microstrip directional couplers.