High Frequency PCB Design Guidelines

Designers are constantly facing the challenge of improving the performance of electronic products. At the same time, designers need to consider the impact of high-speed signal in PCB design, because increasing clock frequency and reducing rise time may lead to signal integrity problems. This requires higher frequencies, and frequencies from 50MHz to 3GHz have become very common. The design of High Frequency PCB is very important to support practical application. When PCB processes High Frequency signal, the problem of signal transmission is prominent. The excellent high-speed board is a board that integrates various components and wiring, while avoiding signal integrity problems. The three main challenges we face on high frequency boards are signal integrity, EMI / EMC and dielectric loss.

As the core of high speed, PCB design is an interference problem. The faster the data rate, the more problems to protect the signal integrity. Most of these problems come from electromagnetic radiation. When it comes into contact with the circuit, the amount of radiation is relatively small. But electromagnetic interference (EMI) becomes a new concern when you run the device.

When to Pay Attention to Signal Integrity in High Frequency PCB Design?

Signal integrity: ideally, in a PCB, the signal should be transmitted from the signal source (TX) to the load (Rx) without interference / interference. But in reality, it won’t happen. The signal reaches the load with some losses (impedance mismatch, crosstalk, attenuation, reflection, switching problems). These terms are defined as signal integrity (SI) at high frequency. Signal integrity helps to predict and understand these key issues by providing practical solutions.

High speed PCB design requires that traces be visualized as transmission lines rather than simple wires. Determining the highest operating frequency in the design helps to determine the routing that should be considered as a transmission line. If the routing exceeds about 1 / 10 of the frequency wavelength, it can be regarded as a transmission line. These transmission lines require digital and analog analysis.

PCB substrate: the substrate material used during PCB construction can cause signal integrity problems. Each PCB substrate has different relative permittivity (ε R) values. It determines that the signal routing must be regarded as the length of the transmission line. Of course, in this case, designers need to pay attention to the threat of signal integrity.

Using the value of ε R, the designer can evaluate the velocity of signal flow (VP) and propagation delay (TPD). These parameters help to determine the length of a transmission line that should be treated as a routing line. The figure below describes how the insertion loss increases with the signal frequency. For FR-4 (glass epoxy) and high frequency Rogers ro4350b materials, the insertion loss (per inch) was measured. Higher insertion loss may lead to greater attenuation. Click here for more information about High Frequency PCB.

Three Schools of Thought, We Will Break Down the First Three Categories:

Frequency. The first thought is to classify high-speed designs according to the frequency of PCB and its ability to affect circuit performance. For some, this means frequencies above 50 MHz. Others classify it into several categories, namely low speed (< 25 MHz), medium speed (25-100 MHz), high speed (100-1000 MHz), and ultra-high speed or above for RF designers.

Traces. In another genre, you can use the physical size of the trace to determine if you are using a high-speed device. The rule states that once the routing exceeds one third of the rise time of the device switching speed, you have high speed.

Separation. The last point takes a general approach to look at your circuit design as a whole and raises the following question – does your system work in a unified way physically? Or do you have a bunch of sub circuits that are organized into a larger circuit, and everything works together? The latter brings you into the field of high-speed design.

Analysis of High Frequency PCB Design Ideas

• Always start your high-speed design process with planning
• Record every detail of the circuit board stack during the manufacturing process
• Floor plan – organize the board into logical parts
• Know how to use power and ground level
• Minimize the size of pad patterns
• Transmit signals for maximum shielding effectiveness
• Provide effective current return path
• Use 3W rule to minimize coupling between routing
• Minimizing planar coupling using 20h rule
• Complete general routing criteria