RF and Amplifier PCB Materials

PCB materials for amplifiers need to have low dissipation factors and loss tangents. Common FR-4 material, for example, has a non-uniform dielectric constant and lower loss tangent, so it is not suitable for high-frequency applications.

Another important consideration is the stability of the materials over time. Power-amplifier circuits can be subject to harsh environments, such as heat and humidity. PCB materials such as FEP and ceramic-filled PTFE have lower lamination and re-melt temperatures and can withstand thermal stresses.

Transmitter Lines

A PCB track is devoid of capacitance and inductance at low frequencies, but as the frequency increases, these factors begin to impact its performance. For instance, if the impedance of a trace is not well-matched with its load, the extra energy reflected back towards the transmitter can cause overshoot and undershoot in the signal. To address this, traces must be terminated with resistors.

A transmission line can be defined as a pair of conductors separated by a dielectric layer with one current return path on each side. The resulting characteristic impedance of a stripline is dependent on its width, thickness, and the type of dielectric.

Other important parameters of a transmission line include its resistance, which depends on its cross-sectional RF and Amplifier PCB area, and its capacitance, which is dependent on the distance between the conductors. In addition, the transmission line’s loss is a function of both its length and the frequency, which depends on both the loss coefficient of the dielectric and the speed of the alternating current in the conductors.

When designing a transmission line, the designer must take into account the effect of temperature, as this can change the characteristics of the transmission line. Additionally, the PCB should be designed to minimize the effects of signal bends, as radial bends are less discontinuous than right-angle ones. The designer must also consider how the transmission lines are routed across the board, as the impedance may change when the line moves from one layer to another.

Impedance Control

As PCBs get smaller and faster, signal traces must be able to travel without being distorted. This is a major concern because if the impedance of a trace changes as it travels, it can cause the energy to be reflected back to the source, which can result in EMI. This is a big issue because it can ruin signal quality, which could lead to performance problems in the circuit board and shorten its lifespan.

The impedance of a PCB is determined by the dielectric materials, copper thickness and trace width. There are calculators available to help determine the right size of a trace in order to achieve the desired impedance. However, it is important to remember that the outer laminated layers of a PCB have a different dielectric constant than the core. For this reason, it is essential to use a laminate that is as uniform in dielectric constant as possible.

In addition, when routing a PCB, it is best to avoid using sharp bends in the traces. Instead, radial bends should be used whenever possible. This will minimize the characteristic impedance change as a signal moves through the bend. The best choice for a bend radius is three times the trace width. This will also help to minimize any polarity issues that can occur when bending a differential signal pair.

Inductance Control

RF amplifiers run high speed digital signals that carry large amounts of data. These lines need to be impedance matched in order to minimize power losses. Impedance mismatch caused by parasitic inductance and capacitance will cause signal reflections that can increase timing jitter and bit error rates. Using the right layout rules and design software can help you avoid these problems.

Keeping traces short and the distance between them as large as possible can reduce the parasitic inductance. Also, routing the traces through different layers can help to decrease parasitic capacitance. The use of the right dielectric materials is important as well; higher permittivity dielectrics produce more stray capacitance than lower ones.

The placement of components and wires, component separation, guard rings, power planes and ground planes, shielding between output and input, and other techniques can help to prevent EMI. Shielding also helps to prevent EMI by enclosing the circuits and cables in a conductive material that will block RF waves.

Another technique for reducing parasitic inductance is to RF and Amplifier PCB Supplier insert via holes with an extremely tight hole wall-to-hole wall spacing. This can reduce the inductance by half, compared to a standard PCB stackup.

Stackup

The layer stackup is a vital part of the PCB that determines its Electromagnetic Compatibility (EMC) performance. Choosing an optimal multilayer stack up minimizes electromagnetic radiation, and stops circuits from being interfered with by external noise sources. It also improves signal integrity and reduces impedance mismatch.

RF signals are generally more sensitive to crosstalk than low-level digital and analog signals. The PCB stack up and layout should be designed with this in mind to ensure that high-frequency analog, low-frequency analog, and digital signals do not crosstalk with each other. This can be done by using multiple layers in the board, by placing a separate power plane for each signal type, or by routing them through a buried layer to prevent crosstalk.

A 2-layer RF PCB provides cost savings and can operate at a higher frequency than a 4-layer design, but requires careful planning of component and signal placement to achieve optimum performance. This is because the RF layer has to contain both component and signal routing, so you’ll need to carefully plan the routing to avoid interference.

Ideally, the RF layer will be adjacent to a ground plane, and both of these should be linked with stitching vias to provide the shortest path for return currents. If the ground plane is not continuous, this will increase the return current’s path length and could cause decoupling problems.