RF Design: Electromagnetic Properties at Interconnect Points

The RF engineering design method must be able to handle the stronger electromagnetic field effects that usually occur at higher frequencies. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, causing annoying crosstalk (interference and total noise), and can impair system performance. The return loss is mainly caused by the impedance mismatch. The influence on the signal is the same as the additive noise and interference.

High return loss has two negative effects: 1. The signal is reflected back to the signal source will increase the system noise, making it more difficult for the receiver to separate the noise from the signal; 2. Any reflected signal will basically reduce the signal quality because the input signal The shape has changed.

Although the digital system only processes 1 and 0 signals and has very good fault tolerance, harmonics generated when high-speed pulses rise will cause the signal to be weaker at higher frequencies. Although forward error correction technology can eliminate some negative effects, part of the bandwidth of the system is used to transmit redundant data, which leads to a decrease in system performance. A better solution is to make the RF effect helpful rather than detrimental to signal integrity. It is recommended that the total return loss at the highest frequency of the digital system (usually a poor data point) be -25 dB, which is equivalent to a VSWR of 1.1.

The goals of PCB design are smaller, faster, and lower cost. For RF PCBs, high-speed signals sometimes limit the miniaturization of the PCB design. At present, the main method to solve the crosstalk problem is to perform ground layer management, make spacing between wirings and reduce the stud capacitance. The main way to reduce the return loss is to perform impedance matching. This method includes effective management of insulating materials and isolation of active signal lines and ground lines, especially between signal lines and ground where the state transitions.

Because the interconnection point is the weakest link in the circuit chain, in the RF design, the electromagnetic properties at the interconnection point are the main problems faced by the engineering design. It is necessary to examine each interconnection point and solve the existing problems. Board system interconnects include: chip-to-board, PCB-board interconnects, and three types of interconnects between the PCB and external devices.