Microwave Tech - How to choose the PCB of high frequency device power divider and coupler

How to choose the PCB of high frequency device power divider and coupler

Power dividers and combiners are the most commonly used/common high-frequency devices, and the same is true for couplers such as directional couplers. These devices are used for power division, combination, and coupling of high-frequency energy from the antenna or the system, and the loss and leakage are small. The choice of PCB board is a key factor for these devices to achieve the expected performance. When designing and processing power splitters/combiners/couplers, it is helpful to understand how the properties of PCB materials affect the final performance of these devices. Limits include frequency range, working bandwidth, and power capacity.

Many different circuits are used to design power dividers (in turn, combiners) and couplers, and they have a variety of different forms. The power divider has simple dual-channel power points and complex N-channel power points, depending on the actual needs of the system. Many different directional couplers and other types of couplers have also been developed in recent years, including Wilkinson and resistive power splitters, Lange couplers and quadrature hybrid power-saving bridges. They have many different forms and size. Choosing the right PCB material in these circuit designs will help it achieve the best performance.

These different circuit types will compromise the structure and performance of the design, helping the designer choose the board for different applications. The Wilkinson dual power splitter uses a single input signal to provide dual output signals of equal amplitude and phase. It is actually a "lossless" circuit designed to provide a 3dB (or in other words) less than the original signal. Is half of the original signal) output signal (the output power of each port of the power divider decreases as the number of output ports increases). In contrast, the resistive dual power divider provides an output signal that is 6dB smaller than the original signal. The added impedance of each branch in the resistive power divider increases the loss, but also increases the isolation between the two signals.

Like many circuit designs, the dielectric constant (Dk) is generally the starting point for choosing different PCB materials, and the designers of power dividers/power combiners generally tend to use high dielectric constant (Dk) circuit materials because These materials can provide effective electromagnetic coupling in smaller-sized circuits than low-dielectric-constant materials. There is a problem with high dielectric constant circuits, that is, the dielectric constant in the circuit board is anisotropic, or the dielectric constant values of the circuit boards are different in the x, y, and z directions. When the dielectric constant changes greatly in the same direction, it is also difficult to obtain a transmission line with uniform impedance.

Maintaining the invariance of impedance is very important when realizing the characteristics of the power divider/combiner. The change of the dielectric constant (impedance) will cause the uneven distribution of electromagnetic energy and power. Fortunately, there are commercial PCB materials with superior isotropy that can be used in these circuits, such as TMM 10i circuit materials. These materials have a relatively high dielectric constant value of 9.8, and are maintained at a level of 9.8+/-0.245 in the three coordinate axis directions (measured at 10GHz). This can also be understood as that in the transmission lines of the power splitter/combiner and the coupler, uniform impedance characteristics can make the distribution of electromagnetic energy in the device constant and measurable. For higher dielectric constant PCB materials, the TMM 13i laminate has a dielectric constant of 12.85 and the variation in the three axes is within +/-0.35 (10GHz).

Of course, when designing power dividers/power combiners and couplers, constant dielectric constant and impedance characteristics are only one of the PCB material parameters that need to be considered. When designing a power splitter/combiner or coupler circuit, minimizing insertion loss is usually an important goal. Ideally, a dual Wilkinson power splitter can provide two output ports -3dB or Half of the input electromagnetic energy. In fact, each power splitter/combiner (and coupler) circuit will have a certain insertion loss, which usually depends on the frequency (when the frequency increases, the loss also increases), so for a power splitter/combiner In terms of design, the choice of PCB material needs to consider how to control, so that the insertion loss of the circuit is minimized.

In passive high-frequency devices such as power splitters/combiners or couplers, insertion loss is actually the sum of many losses, including dielectric loss, conductor loss, radiation loss and leakage loss. Some of these losses can be controlled by careful circuit design. They may also depend on the characteristics of the PCB material and can be minimized by choosing the PCB material reasonably. Impedance mismatch (ie, standing wave ratio loss) can cause loss, but it can be reduced by choosing a PCB material with a constant dielectric constant.

Minimizing loss is very critical in the design of high-power power splitters/combiners and couplers, because at high power losses will be converted into heat and dissipated in the device and PCB materials, and the heat will affect the dielectric of the material. Constant values (and impedance values) have an effect.

In short, when designing and processing high-frequency power splitters/combiners and couplers, the selection of PCB materials should be based on many different key material characteristics, including the value of dielectric constant, the continuity of the dielectric constant in the material, and environmental factors such as temperature, Reducing material loss includes dielectric loss, conductor loss, and power capacity. Choosing PCB materials for specific applications helps to design high-frequency power splitters/combiners or couplers for success.