Power dividers/combiners may be among the most popular and most used of high-frequency components. And couplers, such as directional couplers, are not far behind. These components help to divide, combine, and direct high-frequency energy from antennas and within systems with minimal loss and leakage, and the choice of printed-circuit-board (PCB) materials for these components can be a strong factor in approaching the ideal performance levels expected from each component. When designing and fabricating power dividers/combiners and couplers, it can be helpful to better understand how different PCB material properties relate to the final performance possible with these components, to help set limits on a number of different performance parameters, such as frequency coverage, operating bandwidth, and power-handling capability.
A wide range of circuits have been developed as power dividers (which serve as power combiners when used in reverse) and couplers, and they are available in many forms. Power dividers can be as simple as two-way dividers or as complex as N-way power dividers, with N a fairly large number as required by a system’s design. Many different directional and other coupler configurations have been developed over the years, including Wilkinson and resistive power dividers and Lange and quadrature hybrid couplers, in many different shapes and sizes. Matching a PCB material to any one of these circuit designs can help in the quest for achieving optimum performance.
These various circuit types offer tradeoffs in construction and performance, to help designers match them to different applications. A Wilkinson two-way power divider, which is designed to provide two output signals with equal amplitude and phase from a single input signal, is essentially a “lossless” circuit, designed to provide a pair of output signals that are each 3-dB less (or one-half the power level) than the input signal (with power dividers having more output ports suffering more loss per output port as a function of the number of outputs). In contrast, a resistive two-way power divider may provide a pair of output signals that are each 6 dB less than the power level of the input signal. The additional resistance in the signal path, while it adds loss, also adds isolation between the two signal paths.
The previous ROG blog offered guidance on selecting PCB materials for high-frequency circuits with coupled features, and much of that advice can be applied to any search for circuit materials for power dividers/combiners and couplers. As with many circuit designs, the dielectric constant (Dk) is often a starting point when surveying different PCB materials, and designers of power dividers/combiners and couplers generally tend towards using circuit materials with higher Dk values, since those materials support efficient coupling of electromagnetic (EM) energy using smaller circuit features than materials with lower Dk values. A problem with higher-Dk circuit materials, as the earlier blog explained, is the tendency towards anisotropic Dk characteristics across a circuit board, or having different Dk values in the x, y, and z axes of the circuit board material. Wide variations in Dk, also within one axis of the material, can make it difficult to achieve transmission lines with consistent impedance.
Maintaining consistent impedance is critical to achieving high performance in power dividers/combiners and couplers, where variations in Dk (and impedance) can result in uneven distributions of EM energy and uneven power distributions. Fortunately, as noted in the previous blog for circuits with coupled features, commercial PCB materials with excellent isotropic behavior are available for these types of applications, such as the TMM® 10i circuit materials from Rogers Corp. (www.rogerscorp.com). These materials exhibit a relatively high Dk value of 9.8 and the value is consistent within +/-0.245 of 9.8 for all three axes of the circuit material (as measured at 10 GHz). This translates into consistent impedance for the transmission lines of power dividers/combiners and couplers, resulting in consistent and predictably distribution of EM energy in these component. For a PCB material with even higher Dk value, TMM 13i laminate has a Dk value of 12.85 which remains within +/-0.35 of that value (at 10 GHz) for all three axes. In fact, for those interested in reviewing the chief material parameters to consider when impedance matching is of prime importance (for low VSWR performance), an earlier blog highlighted those key material materials, using the RO3010™ and RO3035™ circuit materials from Rogers Corp. as examples:
Of course, consistent Dk and impedance is only one PCB material parameter to consider when designing power dividers/combiners and couplers. Minimizing insertion loss is usually an important goal when designing any power divider/combiner or coupler circuit. Ideally, a two-way Wilkinson power divider would provide two output ports each 3-dB or one-half the power level of the EM energy applied to the input port. In reality, every power divider/combiner circuit (and coupler) will suffer some amount of insertion loss, usually depending upon frequency (with loss increasing as frequency increases), and a consideration for a PCB material for a power divider/combiner is how to manage or, hopefully, minimize, the insertion loss of the circuit.
The insertion loss in a passive high-frequency component, such as a power divider/combiner or coupler, is actually the sum of a number of separate losses, including dielectric loss, conductor loss, radiation loss, and leakage loss. While some of these losses can be controlled through careful circuit design, they may also be dependent upon the capabilities of the PCB material and can be minimized through thoughtful selection of PCB material. Leakage loss is minimized in PCB materials from Rogers Corp., for example, since the materials are designed with high volume resistivity to provide high isolation with low leakage loss for fabricated transmission lines. Loss can also result from impedance mismatches (VSWR losses), which can be minimized by the choice of PCB materials with consistent Dk characteristics.
Minimizing loss is of particular importance in power combiners/dividers and couplers that are intended to handle higher power levels, since losses at higher power levels turn to heat that must be dissipated by the component and its PCB material, and that heat can have an effect on the Dk value (and impedance) of the material.
In short, when designing and fabricating high-frequency power dividers/combiners and couplers, the choice of PCB material should be based on a number of different key material properties, including Dk value, consistency of Dk across the material and with environmental factors such as temperature, minimal material losses, including dielectric and conductor losses, and power-handling capability. Starting with PCB material that is matched to the application can help ensure the success of a high-frequency power divider/combiner or coupler design.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.