[Introduction] This article will introduce various techniques for the design of the three types of interconnection of the circuit board system, namely, the chip-to-circuit board, the interconnects within the PCB board, and the interconnects between the PCB and external devices, including device installation, wiring isolation, and measures to reduce the lead inductance, to help designers minimize the RF effect in the PCB interconnection design.
The interconnection of circuit board system includes three types of interconnection: chip to circuit board, PCB internal interconnection and PCB and external devices. In RF design, the electromagnetic characteristics at the interconnection point is one of the main problems faced by engineering design. This paper introduces various techniques of the above three types of interconnection design, including device installation methods, wiring isolation and measures to reduce lead inductance.
At present, there are signs that the frequency of printed circuit board design is increasing. With the continuous growth of data rate, the bandwidth required for data transmission also makes the upper limit of signal frequency reach 1GHz or even higher. Although this high-frequency signal technology is far beyond the range of millimeter wave technology (30GHz), it does also involve RF and low-end microwave technology.
The RF engineering design method must be able to deal with the strong electromagnetic field effect that usually occurs at the higher frequency band. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, resulting in annoying crosstalk (interference and total noise) and damaging system performance. Back-loss is mainly caused by impedance mismatch, which has the same effect on the signal as additive noise and interference.
High return loss has two negative effects:
1. The signal reflected back to the signal source 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 shape of the input signal has changed.
Although the digital system only processes 1 and 0 signals and has very good fault-tolerance, the harmonics generated when the high-speed pulse rises will cause the higher the frequency, the weaker 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, resulting in the reduction of system performance. A better solution is to let the RF effect help rather than damage the integrity of the signal. It is recommended that the total return loss at the highest frequency of the digital system (usually the poor data point) is - 25dB, equivalent to a VSWR of 1.1.
The goal of PCB design is to be smaller, faster and cheaper. For RFPCB, high-speed signal sometimes limits the miniaturization of PCB design. At present, the main method to solve the crosstalk problem is to manage the ground plane, space the wires and reduce the lead inductance. The main method to reduce return loss is impedance matching. This method includes effective management of insulating materials and isolation of active signal line and ground wire, especially the separation between signal line and ground wire with state jump.
Because interconnection points are the weakest link in the circuit chain, in RF design, the electromagnetic properties at the interconnection points are the main problems faced by engineering design. It is necessary to investigate each interconnection point and solve the existing problems. The interconnection of circuit board system includes chip to circuit board, PCB internal interconnection and signal input/output between PCB and external devices.
1、 Interconnection between chip and PCB
Pentium IV and high-speed chips containing 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 has reached 1GHz. At the recent GHz interconnection seminar, the most exciting thing is that the method to deal with the growing number and frequency of I/O has become widely known. The main problem of interconnection between chips and PCBs is that too high interconnection density will cause the basic structure of PCB materials to become a factor limiting the growth of interconnection density. An innovative solution was put forward at the meeting, that is, using the local wireless transmitter inside the chip to transmit data to the adjacent circuit board. Whether this scheme is effective or not, the participants are very clear: in terms of high-frequency applications, IC design technology has been far ahead of PCB design technology.
2、 PCB interconnection
The skills and methods for designing high-frequency PCB are as follows:
1. 45 ° angle shall be adopted at the corner of transmission line to reduce return loss (Fig. 1);
3、 PCB and external device interconnection
Now we can think that we have solved all the signal management problems on the board and on the interconnection of various discrete components. So how to solve the signal input/output problem from the circuit board to the wire connecting the remote device? Trompeter Electronics, the innovator of coaxial cable technology, is committed to solving this problem and has made some important progress (Figure 3). In addition, look at the electromagnetic field shown in Figure 4 below. In this case, we manage the conversion from microstrip to coaxial cable. In coaxial cable, the ground wire layer is annular interleaved and evenly spaced. In the microstrip, the ground plane is below the active line. This introduces some edge effects that need to be understood, predicted and considered during design. Of course, this mismatch will also lead to back loss. It must be minimized to avoid noise and signal interference.
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