Analysis of PCB Signal Injection Method for Printed Circuit Board

The process of transferring high-frequency energy from coaxial connector to printed circuit board (PCB) is usually called signal injection, which is difficult to describe. The efficiency of energy transfer will vary greatly due to different circuit structures. PCB material and its thickness, operating frequency range and other factors, as well as connector design and its interaction with circuit materials will affect performance. Through the understanding of different signal injection settings and the review of some optimization cases of RF and microwave signal injection methods, the performance can be improved.

Realizing effective signal injection is related to design. Generally, broadband optimization is more challenging than narrowband. Generally, high-frequency injection becomes more difficult with the increase of frequency. At the same time, there may be more problems with the increase of the thickness of circuit materials and the complexity of circuit structure.

Signal injection design and optimization

The signal injection from coaxial cable and connector to microstrip PCB is shown in Figure 1. The electromagnetic (EM) field distribution through coaxial cable and connector is cylindrical, while the EM field distribution in PCB is planar or rectangular. From one propagation medium to another, the field distribution will change to adapt to the new environment, resulting in anomalies. The change depends on the media type; For example, whether the signal is injected from coaxial cable and connector to microstrip, grounded coplanar waveguide (GCPW), or strip line. The type of coaxial cable connector also plays an important role.

Optimization involves several variables. It is useful to understand the EM field distribution in coaxial cables / connectors, but the grounding loop must also be considered as part of the propagation medium. It is usually helpful to achieve a smooth impedance transition from one propagation medium to another. Understanding the capacitive reactance and inductive reactance at the impedance discontinuity allows us to understand the circuit performance. If three-dimensional (3D) EM simulation can be carried out, the current density distribution can be observed. In addition, it is better to take into account the actual situation related to radiation loss.

Although the ground loop between the signal transmitting connector and the PCB may not seem to be a problem, the ground loop from the connector to the PCB is very continuous, but it is not always the case. There is usually a small surface resistance between the metal of the connector and the PCB. There is also a small difference in the conductivity between the welding shop connecting different parts and the metal of these parts. At low RF and microwave frequencies, the impact of these small differences is usually small, but at high frequencies, it has a great impact on performance. The actual length of the ground return path will affect the transmission quality that can be achieved with a given connector and PCB combination.

As shown in Fig. 2a, when the electromagnetic wave energy is transmitted from the connector pin to the signal wire of the microstrip PCB, the grounding loop back to the connector housing may be too long for a thick microstrip transmission line. The use of PCB materials with high dielectric constant will increase the electrical length of the grounding circuit, which will worsen the problem. Path extension will lead to frequency dependent problems, resulting in local phase velocity and capacitance differences. Both of them are related to the impedance in the conversion region and will affect it, resulting in the difference of return loss. Ideally, the length of the grounding loop should be minimized so that there is no impedance anomaly in the signal injection area. Note that the ground point of the connector shown in Figure 2A exists only at the bottom of the circuit, which is the worst case. The ground pin of many RF connectors is on the same layer as the signal. In this case, the grounding pad will also be designed on the PCB.

Fig. 2B shows a grounded coplanar waveguide to microstrip signal injection circuit. Here, the main body of the circuit is microstrip, but the signal injection region is grounded coplanar waveguide (GCPW). Coplanar emission microstrip is useful because it minimizes the ground loop and has other useful characteristics. If a connector with ground pins on both sides of the signal wire is used, the ground pin spacing has a significant impact on performance. It has been shown that this distance affects the frequency response.

When the coplanar waveguide to microstrip microstrip based on Rogers' 10mil thick ro4350b laminate is used for the experiment, connectors with different grounding spacing of coplanar waveguide ports but similar to other parts are used (see Fig. 3). The grounding interval of connector A is about 0.030 ", while the grounding interval of connector B is 0.064". In both cases, the connector emits to the same circuit.

The x-axis represents the frequency, 5 GHz per grid. When the microwave frequency is low (

Analysis of PCB Signal Injection Method for Printed Circuit Board 1

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