Why 50 Ohms is the Standard in RF Design and the Role of Alumina in High-Performance RF Circuits

Why 50 Ohms Impedance is the Standard in RF Design?

As with many things in life, there is more then one explanation.

A common belief is that the choice of 50 ohms as the standard impedance in RF and microwave systems is the result an engineering trade-off that which dates back to the early days of television and radio communications in the late 1920s when coaxial cable was designed in the Bell laboratories.

Engineers had to balance two key factors: minimizing signal losses and maximizing power handling capability.

– For a coaxial cable, the impedance that minimizes losses is around 77 ohms, (which is why TV coaxial cables often use 75 ohms).

– On the other hand, the impedance that maximizes power handling capability is around 30 ohms.

Source

Why Alumina is a Good Material for RF Circuit Designs?

Ceramic materials, especially alumina (Al₂O₃), are widely used in RF circuit designs due to their excellent properties. Here’s why alumina is a preferred choice:

1. High Dielectric Constant (Dk): Alumina has a high Dk value, which allows for narrower trace widths to achieve the same impedance. This is beneficial for compact and efficient PCB designs.

2. Low Loss Properties: Alumina exhibits low dielectric loss, which is crucial for maintaining signal integrity in high-frequency applications. Lower dielectric loss ensures that the signal strength remains strong over longer distances, reducing the chances of signal degradation.

3. Thermal Characteristics: Alumina has excellent thermal conductivity, which helps in efficiently dissipating heat generated by high-power RF components. This prevents overheating and ensures stable performance.

Photo: Alumina RF circuit

Photo: grounded coplanar waveguide design on Al2O3 96%

Compare trace width between PCB Laminates

Let’s compare the trace widths required to achieve 50 ohms impedance for different materials with a 35μm copper trace:

• Alumina 96% (Dk = 9.2) =>Adjusted trace width: 0.91 mm

• FR-4 (Dk = 4.5) => Adjusted trace width: 2.74 mm

• Rogers Materials (Dk = 2.0) =>Adjusted trace width: 4.23 mm

From the number above, it is clear that for miniaturized designs, Alumina and other ceramics with similar dielectric constants are very well suited. Especially thank to the combination with their low loss and thermal conductivity characteristics. If you have more space and signal delay is a key issue, then Rogers material might be the better option. Typically ceramic RF circuits are used as submounts and packages in a RF design followed by a Rogers fan out board.

Conclusion

To answer the initial question, 50Ohms impedance is simply a good compromise and has therefore been the standard for many decades now.

And to reach this standard in dense and miniaturized RF designs Alumina is an excellent material due to its high dielectric constant, low loss properties, and superior thermal characteristics.

It is particularly suitable for applications that require a small footprint, efficient heat dissipation and high mechanical strength, such as high-power RF components and harsh environments.