The interconnection of the PCB board system includes chip-to-circuit board, interconnection within the PCB board, and three types of interconnection between the PCB and external devices.
In RF design, the electromagnetic characteristics at the interconnection point is one of the main problems faced in the PCB engineering design process. This article introduces the various techniques of the above three types of interconnection design, which involves device mounting methods, wiring isolation, and reducing leads Measures of inductance and so on.
There are signs that printed circuit board designs are becoming more frequent. As the data rate continues to increase, the bandwidth required for data transmission also pushes the upper limit of the signal frequency to 1GHz or even higher. Although this high-frequency signal technology is far beyond the range of millimeter wave technology (30GHz), it does involve RF and low-end microwave technology.RF engineering design methods must be able to handle the strong electromagnetic field effects that typically occur at higher frequency bands. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, cause unpleasant crosstalk (interference and total noise), and impair system performance. Return loss is mainly caused by impedance mismatch, and the effect on the signal is the same as that caused by additive noise and interference.
High return loss has two negative effects: 1. The signal is reflected back to the signal source, which will increase the system noise, making it more difficult for the receiver to distinguish 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, the harmonics generated when high-speed pulses rise will cause higher frequency signals to weaken the signal. Although the forward error correction technology can eliminate some negative effects, part of the bandwidth of the system is used to transmit redundant data, which results in a decrease in system performance. A better solution is to let the RF effect help rather than detract from signal integrity. It is recommended that the total return loss of the digital system at the highest frequency (usually the worst data point) is -25dB, which is equivalent to VSWR of 1.1.The goal of PCB design is to be smaller, faster, and less expensive. For RF PCBs, high-speed signals sometimes limit the miniaturization of PCB designs. At present, the main method to solve the crosstalk problem is to manage the ground plane, space between the wirings and reduce the stud capacitance. The main method to reduce return loss is to perform impedance matching. This method includes the effective management of insulation materials and the isolation of active signal lines and ground lines, especially when there is a gap between signal lines and ground that have a state transition.
Since the interconnection point is the weakest link in the circuit chain, in RF design, the electromagnetic properties at the interconnection point are the main problems faced by engineering design. Each interconnection point must be examined and existing problems solved. The interconnection of the circuit board system includes three types of interconnections, such as chip-to-circuit board, PCB-board interconnection, and signal input / output between the PCB and external devices.
Chip-to-PCB interconnect
Pentium IV and high-speed chips with a large number of input / output interconnection points have been introduced. As far as the chip itself is concerned, its performance is reliable, and the processing rate can already reach 1GHz. The most exciting part of the recent GHz Interconnect Seminar is that methods to deal with the growing number of I / Os and frequencies are well known. The most important problem of chip and PCB interconnection is that too high interconnection density will cause the basic structure of PCB material to become a factor limiting the increase in interconnection density. An innovative solution was proposed at the meeting, which uses a local wireless transmitter inside the chip to transmit data to adjacent circuit boards.Regardless of whether this solution is effective, the participants are very clear: As far as high-frequency applications are concerned, IC design technology is far ahead of PCB design technology.
