Packing Power Into Microwave PCBs
Power amplifier design at RF/microwave frequencies can be aided by a wise choice of active devices, such as discrete transistors or monolithic microwave integrated circuits (MMICs). But don’t overlook the importance of the printed-circuit-board (PCB) material when planning for a solid-state power amplifier (PA) circuit. The circuit material can help or hurt a PA design, and knowing what is important in a PCB material intended for a PA is the first step in selecting a circuit material that enhances the PA’s performance. Plain and simple, a suitable PA circuit material should support excellent RF/microwave performance, be consistent over time and temperature, and be capable of conducting heat away from a PA’s active devices.
Ideally, a circuit material for a solid-state PA should be a foundation for transmission lines that form the impedances needed to match to the input and output ports of those active devices and optimize the gain and power achieved for high-frequency signals into and out of those devices. A circuit material’s dielectric constant (Dk) is usually a good starting point for PA designers in search of a suitable PCB material for their PA design, not so much for a particular Dk value but for a material with minimal variations in Dk value, across the material and across a required temperature range. PA circuits generate power but they are not 100% efficient, so they generate heat as well, and those changing temperatures can impact a circuit material’s Dk value and consequently, the impedances of the matching circuits to and from a PA’s active devices.
For stable transmission-line impedances in PA circuits, PA designers have traditionally sought PCB materials not so much with a particular Dk value, but with a Dk value that is tightly controlled across the material. Commercial circuit materials can exhibit different ranges in their Dk variations, but a good target Dk deviation specification is ±1.5% or better at a desired Dk value. This consistency will help to achieve the consistent impedance values needed to extract the highest levels of output power from an active device or devices on a PCB-mounted PA.
Perhaps even more important than the Dk consistence, however, is the consistency of a PCB material’s thickness, or its substrate thickness tolerance. As with variations in Dk, variations in a PCB material’s thickness will result in variations in transmission-line impedance. This will mean inconsistent PA performance that is not as predicted by design calculations or by a commercial computer-aided-engineering (CAE) design program. Specifying a PCB material with consistent thickness tolerance will enable tight control of the impedance of the transmission lines and other circuit structures fabricated on the material, and consistent and predictable performance for a PA built on that circuit material. Although the thickness tolerances for commercial PCB materials varies widely across the industry, a value of ±10% or better can help maintain consistent impedance in a PA’s transmission lines and matching circuits.
The thickness of the PCB material, whether it is a relatively thin or a relatively thick circuit material, can also play a part in maintaining consistent impedance in a PA’s matching circuits. Variations in copper conductor width and thickness, for example, translate into variations in impedance. Those conductor width and thickness variations have more of an effect on impedance for thinner circuit materials than for thicker circuit materials. But thicker PCB materials can impact a PA design in other ways, since the thicker materials may suffer greater radiation loss than thinner circuit materials.
For both low-noise amplifiers (LNAs) and PAs, it helps to fabricate the circuits on PCB materials with low insertion loss. A PCB’s insertion loss can have a number of different loss components, such as conductor loss, dielectric loss, radiation loss, and leakage loss. Conductor losses, for example, are a larger part of the total PCB insertion loss in thinner circuits while dielectric losses are more of a dominant part of the total PCB insertion loss in thicker circuits.
The dielectric losses of thicker PCB materials can be minimized by selecting thicker circuit materials with lower dissipation factors.
Other circuit parameters, such as the surface roughness of the copper conductor, can contribute to higher losses from rougher surfaces. Copper surface roughness will have a greater impact on insertion loss for thinner circuit materials than for thicker circuit materials. Conductor losses from this source can be minimized by using a PCB material with a smoother copper conductor. The search for a low-loss circuit material for a PA (or an LNA) involves weighing the impacts of such parameters as conductor thickness and dielectric thickness on the different loss components and achieving a workable balance with acceptable loss performance.
Of course, given their tendencies to generate heat, solid-state PAs should be constructed on circuit materials with suitable thermal characteristics, including good thermal conductivity, coefficient of thermal expansion (CTE), and temperature coefficient of dielectric constant (TCDk). High thermal conductivity will support the flow of heat away from a PAs active devices and towards a heat sink or other heat-dissipating structure. Good TCDk will minimize variations in Dk (and transmission-line impedance) with temperature which can degrade a PA’s performance as it heat up at higher power levels.
Considering these various material characteristics, what is a commercial material that can meet the different requirements for a PA? RO4835™ circuit material from Rogers Corp. (www.rogerscorp.com) has a Dk of 3.48 in the z-axis (thickness) at 10 GHz. The Dk is maintained to a tolerance of ±0.05 across the material for consistent transmission-line impedance. RO4835 laminates maintain stable Dk with temperature, with a TCDk of +50 ppm/°C in the z-axis (thickness) over a wide range of processing temperatures from -100 to +250°C. The circuit material has high thermal conductivity (0.66 W/m/K) and can handle and help dissipate the heat produced by a solid-state PA’s active devices. RO4835 laminate has low dielectric loss, with dissipation factor of 0.0037 in the z axis at 10 GHz to minimize the generation of heat from active devices subject to high material loss. The material is RoHS compliant and compatible with standard FR-4 circuit manufacturing processes.
But this is just one example of a PCB material that is formulated for successful high-frequency PA applications. A short list of guidelines that can be applied when comparing different candidate materials would include finding a circuit material with tight substrate thickness tolerance, tight Dk tolerance, low insertion loss (low dissipation factor), high thermal conductivity, and low TCDk. Keeping a tight tolerance of the copper conductor plating thickness and a tight copper conductor width tolerance can help control conductor losses and conductor-based impedance variations, respectively, for better PA performance.
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