How To Reduce The RF Effect In The PCB

The interconnection of the board system includes: chip-to-board, interconnection within the board, and three types of interconnections between the PCB and external devices. In RF design, the electromagnetic characteristics at the interconnection point is one of the main problems facing engineering design. This paper introduces the various techniques of the above three types of interconnection design, including device installation methods, wiring isolation and measures to reduce lead inductance. and many more.

There are now signs that printed circuit board designs are getting more and more frequently. As the rate continues to increase, the bandwidth required for transmission also causes the upper limit of the signal frequency to reach 1 GHz or even higher. Although this high-frequency signal technology far exceeds the millimeter wave technology range (30 GHz), it does involve RF and low-end microwave technology.

RF engineering methods must be able to handle the strong electromagnetic field effects typically produced at higher frequency bands. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, causing annoying crosstalk (interference and total noise) and can compromise system performance. Return loss is mainly caused by impedance mismatch, which has the same effect on the signal as additive noise and interference.

There are two negative effects of high return loss:

1. Signal reflection back to the signal source will increase system noise, making it more difficult for the receiver to distinguish noise from signal;

2. Any reflected signal will basically degrade the signal quality because the shape of the input signal changes.

Although the digital system only processes the 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 forward error correction techniques can eliminate some of the negative effects, part of the bandwidth of the system is used for transmission redundancy, resulting in reduced system performance. A better solution is to let the RF effect help rather than detract from the integrity of the signal. It is recommended that the total return loss at the highest frequency (usually the worst point) of the digital system is -25 dB, which is equivalent to a VSWR of 1.1.

The goal of PCB design is to be smaller, faster, and less costly. For RFPCB, high-speed signals sometimes limit the miniaturization of PCB designs. Currently, the main method to solve the crosstalk problem is to perform ground layer management, spacing between wiring and reducing lead inductance (studcapacitance). The main method to reduce the return loss is to perform impedance matching. This method involves efficient management of the insulating material and isolation of the active signal and ground lines, especially between the signal lines where the state transitions and ground.

Since 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. The interconnection of the board system includes three types of interconnections, such as chip-to-board, interconnection within the PCB, and signal input/output between the PCB and external devices.

First, the interconnection between the chip and the PCB

Pentium IV and high-speed chips with a large number of input/output interconnection points are available. As far as the chip itself is concerned, its performance is reliable and the processing rate has been able to reach 1 GHz. At the recent GHz Interconnect Symposium, the most exciting thing is that the way to deal with the growing number and frequency of I/Os is well known. The main problem with chip-to-PCB interconnects is that too high interconnect densities can cause the basic structure of the PCB material to be a factor limiting the increase in interconnect density. An innovative solution was proposed at the conference, where local wireless transmitters inside the chip were used to transmit to adjacent boards.

Regardless of whether this solution is effective or not, the participants are very clear: in terms of high frequency applications, IC design technology is far ahead of PCB design technology.

Second, the interconnection within the PCB

The techniques and methods for high frequency PCB design are as follows:

1. The corner of the transmission line should adopt a 45° angle to reduce the return loss;

2. It is necessary to use a high-performance insulated circuit board whose insulation constant value is strictly controlled according to the level. This method facilitates efficient management of the electromagnetic field between the insulating material and adjacent wiring.

3. There is a tapped inductor in the protruding lead, and the use of leaded components should be avoided. In high frequency environments, it is best to use surface mount components.

4. For signal via, avoid using a via processing on the sensitive board because this process can cause lead inductance at the via. If a via on a 20-layer board is used to connect layers 1 to 3, the lead inductance can affect 4 to 19 layers.

5. Provide a rich grounding layer. These ground planes are connected using molded holes to prevent the effects of 3-dimensional electromagnetic fields on the board.

6, to choose non-electrolytic nickel or immersion gold plating process, do not use HASL method for plating. This plated surface provides a better skin effect for high frequency currents (Figure 2). In addition, this high solderable coating requires fewer leads and helps reduce environmental pollution.

7. The solder mask prevents the flow of solder paste. However, due to the uncertainty of thickness and the unknown nature of the insulation properties, covering the entire surface of the board with the solder resist material will result in a large change in the electromagnetic energy in the microstrip design. A solder dam is generally used as a solder mask.

8, to improve the PCB design specifications for high-precision etching. Consider the total line width error of +/- 0, 0007 inches, manage the undercut and cross section of the wiring shape, and specify the wiring sidewall plating conditions. Overall management of the wiring (wire) geometry and coating surface is important to address the skin effect issues associated with microwave frequencies and to implement these specifications. If you are not familiar with these methods, consult an experienced design engineer who has worked on military microwave circuit board design. You can also discuss with them the range of prices you can afford. For example, a copper-backed coplanar microstrip design is more economical than a stripline design, so you can discuss them for better advice. Good engineers may not be accustomed to considering cost issues, but their recommendations are also quite helpful. It is now a long-term job to train young engineers who are unfamiliar with RF effects and lack experience in dealing with RF effects.

In addition, other solutions can be used, such as improved computer type, with RF effect processing capabilities.

Third, the PCB and external devices are interconnected

It can now be assumed that we have solved all signal management issues on the board and on the interconnection of discrete components. So how do you solve the signal input/output problem from the board to the remote device leads? Trompeter Electronics, an innovator in coaxial cable technology, is working to solve this problem and has made some significant progress (Figure 3). Also, look at the electromagnetic field given in Figure 4. In this case, we manage the conversion between the microstrip and the coaxial cable. In coaxial cable, the ground plane is annularly interlaced 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 at design time. Of course, this mismatch can also cause return loss, which must be minimized to avoid noise and signal interference.

The management of impedance problems within a board is not a negligible design issue. The impedance starts at the surface of the board, then passes through a solder joint to the connector and ends at the coaxial cable. Since the impedance varies with frequency, the higher the frequency, the harder it is to manage impedance. The problem of using higher frequencies to transmit signals over broadband appears to be a major problem in design.