RF and Microwave Layout encompasses the Design of Analog Based Circuits in the range of Hundreds of Megahertz (MHz) to Many Gigahertz (GHz). In general, RF actually in the 500 MHz – 2 GHz Band Design Above 100 MHz considered RF). Microvia PCB is considered for frequency above 2 GHZ.
At radio frequencies, the wavelength of the RF signal can become comparable to the physical dimensions of the network and transmission lines can be a considerable fraction of the network.Lumped element models in such cases can no longer be applied, and it becomes important to consider the distributed models to account for the magnitude and phase shift of the signal over the length of the transmission lines. If not, reflections occur and can cause significant loss (return loss) in the power transfer between various stages in a design. Impedance matching is critical for maximum power transfer. Additionally, electromagnetic radiation and capacitive coupling among the elements causes unintentional losses that may also significantly alter the performance of the circuit and must be considered while laying out the design.
The maximum power theorem states that maximum power is transferred when the internal
resistance of the source equals the resistance of the load. When extended to alternating current circuits it states that to obtain maximum power transfer the load impedance must be complex conjugate of the source impedance. To maximize power transfer for optimum performance it is important that impedance matching is considered carefully in radio frequency designs.PCB traces at radio frequencies have to be designed carefully, accounting for their distributed character as the physical length of the traces becomes a considerable fraction of the signal wavelength. The impedance of a PCB trace at RF frequencies depends on the thickness of the trace, its height above the ground plane, and the dielectric constant and loss tangent of PCB dielectric material.
RF PCB designs are usually designed on 2- or 4 layer printed circuit boards. Two-layer PCB boards (Figure 1) are designed to have component and signal routing on the top layer, while the bottom layer is designed to be predominantly ground, providing the shortest path for return currents. The ground plane should be continuous; if it is divided, especially under the RF path, it can increase the return current’s path length and can significantly affect the desired performance. A double sided pcb design provides cost savings compared to a four layer PCB design and can provide comparable performance to a four layer design, but requires careful signal routing and component placement. These designs are generally limited to thicknesses of 0.8-1.0mm, as using a greater thickness micro-strip line causes the corresponding widths for common impedances (e.g., 50 ohms) to be too large for practical designs.
2nd Layer: Ground Plane
3rd Layer: Power Plane
4th Layer: Ground Plane and Signal Routing
Note that in any four-layer RF PCB design a ground plane must always be below the top
component and signal plane. The increased thickness (~60mils) in 4-layer PCBs also
provides mechanical strength to the PCB as compared to a 2-layer design.
