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Low-noise amplifiers (LNAs) are essential to many RF/microwave wireless systems, typically employed in receivers to boost low-level signals without adding noise. Although LNA designers and specifiers often think of these amplifiers in terms of their active devices, the choice of printed-circuit-board (PCB) material can play a large role in the ultimate performance achieved from an LNA, with the PCB design typically focused on providing good impedance-matching networks for the active devices, low loss from the active devices to the antenna, and minimal electromagnetic interference (EMI).

A suitable PCB material can also aid an LNA design by minimizing the effects of heat on such amplifier performance parameters as noise figure and gain. All in all, PCB selection can contribute quite a bit to the final performance levels possible from an LNA design.

A number of different PCB material properties should be considered for any LNA “design candidate,” including dielectric constant (Dk or εr), temperature coefficient of Dk, dissipation factor (Df), thermal conductivity, and even substrate thickness tolerance. For example, to achieve the tight impedance matching often needed to maintain low amplifier noise figures, a PCB material’s Dk should be tightly controlled across the material. These impedance-matching networks—and an LNA’s noise figure—can also be affected by a PCB material’s temperature coefficient of dielectric constant (TCDk).

In addition, impedance-matching circuits are impacted by variations in substrate thickness, so tighter substrate thickness tolerances are recommended for LNA designs. Low circuit-material Df and smooth PCB copper surfaces also can help minimize loss from a feed line to an LNA, so as to help maintain low noise figure. Since an LNA’s noise floor can rise with temperature, high PCB thermal conductivity helps to minimize these noise-figure-elevating temperature rises at higher signal levels.

Circuit laminates designed for high-frequency applications usually feature tight control of Dk across the material, but materials can differ in terms of their levels of Dk tolerance. Although most materials provide Dk tolerance within ±10%, materials with tighter Dk tolerance are also available. Ironically, although Dk control and tolerance is often directly related to achieving the impedance-matching networks needed for LNAs, it is often the variations in substrate thickness that have greater impact on achieving tight impedance-matching networks. While variations in thick circuit substrates can have less of an effect on impedance-matching networks, thinner substrates are typically used for minimizing noise in LNA circuits, and the thickness tolerance of these thinner substrates is important for achieving tight impedance matching networks.

Other circuit-material tolerances that can affect impedance-matching circuitry include conductor width tolerances, copper thickness tolerances, and issues associated with circuit fabrication. The weight of such tolerances depends on the particular LNA circuit design. The copper thickness tolerance, for example, has more influence on coupled circuit features such as coplanar circuits while the effects of conductor width on a circuit are related to the substrate thickness: thinner circuits will exhibit more change in impedance for a change in conductor width than thicker circuits.

Comparing impedance values

As the table shows, a number of variables can affect the characteristic impedance of a PCB. Comparing the variables in the table reveals that substrate thickness has the most significant effect on the circuit impedance. For many circuit laminates, a thickness variation of ±10% is a realistic value, although high-frequency laminates are available with tighter thickness tolerances.

Another PCB material parameter that can affect the impedance-matching networks needed in LNA designs is the Dk tolerance. The table shows materials with Dk values within 10% of their nominal values, although high-frequency laminates are available with tighter Dk tolerances. Some circuit laminates are available with Dk controlled within ±0.05 of the Dk value which, for a circuit material with Dk of 3.5, translates to a variation within about ±1.4%. Using a material with such tight Dk tolerance minimizes the effect of this material parameter on LNA performance by enabling tight impedance-matching networks

As the table shows, two items specific to PCB fabrication can affect impedance matching and LNA performance. Copper thickness variations of 1 to 2 mils are not unusual, although circuit fabrication with tighter specifications are available. Also, variations in conductor width can also affect impedance matching, and a difference of 1 mil in conductor width is not unusual for different PCBs.

The top and bottom parts of the table compare microstrip circuits fabricated on 20-mil-thick (top) and 10-mil-thick (bottom) substrate materials. The differences in circuit-material thickness are affected differently by the variables, such as copper thickness and conductor width. Basically, the thinner circuits are more sensitive to conductor effects and changes in copper thickness and conductor width.

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