The process of transmitting high-frequency energy from coaxial connector to printed circuit board (PCB) is often referred to as signal injection, and its features are difficult to describe. Efficiency of energy transmission is different due to the circuit structure. Factors such as PCB materials and their thickness and operating frequency, as well as connector design and their interaction with circuit materials. By understanding of different signal injection settings, performance can be improved by review of some of the optimization of some RF microwave signal injection methods.
Implement effective signal injection and design, general broadband optimization is more challenging than narrowband. Typically high frequency injection is more difficult as the frequency is increased, and it may also increase with the thickness of the circuit material, and there is more problems with the complexity of the circuit structure.
Signal injection design and optimization
Signal injection from the coaxial cable and the connector to the microstrip PCB is shown in Figure 1. The electromagnetic (EM) field distribution through the coaxial cable and the connector is cylindrical, and the EM field distribution in the PCB is a planar or rectangle. From a propagation medium into another medium, the field distribution will change to accommodate the new environment, thereby producing an abnormality. The change depends on the type of media; for example, signal injection is from coaxial cable and connector to microstrip, grounding co-bustic waveguide (GCPW) or line. The type of coaxial cable connector also plays an important role.
Optimization involves several variables. Understanding the distribution of EM field in coaxial cable / connector is useful, but the ground loop must also be regarded as part of the propagation medium. It is usually helpful to achieve smooth impedance transitions that achieve from a propagation medium to another propagation medium. Understand the resistance and sensing resistance of the impedance dissemination, let us understand the circuit performance. If you can perform a three-dimensional (3D) EM emulation, the current density distribution can be observed. In addition, the actual situation associated with radiation loss is also considered.
Although the grounding circuit between the signal transmit 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 electrical conductivity of welded stores connected to different parts and those parts. When RF and microwave frequencies are low, these small differences are usually smaller, but the performance has a large effect when frequency is high. The actual length of the groundback path will affect the transmission quality that can be implemented using a given connector and PCB combination.
As shown in FIG. 2A, when the electromagnetic wave energy is transmitted from the connector pin to the microstrip PCB, the ground circuit returns to the connector housing may be too long for the thick microstrip transmission line. A PCB material with a higher dielectric constant increases the length of the ground circuit, thereby deteriorating the problem. The passage extension will cause problems with frequency dependencies, thereby generating local phase versus and capacitance differences. Both are associated with impedance in the transform area, and will affect it, resulting in a difference in return loss. Ideally, the length of the ground loop should be restored such that the signal injection zone does not have impedance abnormalities. Note that the grounding point of the connector shown in Figure 2a exists only on the bottom of the circuit, and this is a bad case. Many RF connectors are grounded with the same layer. In this case, the ground pad is also designed on the PCB.
Figure 2b shows a grounding coplanar waveguide microstrip signal injection circuit, where the main body of the circuit is a microstrip, but the signal implantation area is a grounded coplanar waveguide (GCPW). Commonly launched microstrip is useful because it can turn ground loopers and also have other useful features. If you have a ground pin on both sides of the signal wire, the ground pin spacing has a significant impact on performance. The distance has been shown to affect the frequency response.
When experimenting with a coplanar waveguide microstrip microstrip based on Rogers 10MIL RO4350B laminate, a connecting connector in which a common waveguide port is different, but other parts are similar (see Figure 3). The ground interval of the connector A is 0.030 ", and the ground interval of the connector B is 0.064". In both cases, the connector is transmitted to the same circuit.
The x-axis represents the frequency, 5 GHz per grid. When the microwave frequency is low (<5 GHz), the performance is quite, but the frequency is higher than 15 GHz, the circuit performance of the ground interval is deteriorated. Similar to the connector, although the two types of pin diameters are slightly different, the pin diameter of the connector B is large and designed for thicker PCB materials. This may also lead to performance differences.
Simple and effective signal injection optimization method is to launch impedance in the signal transmit area. The impact curve is basically due to an increase in inductance, while the impedance curve drops because the capacitance is increased. For the thick microstrip transmission line shown in Fig. 2A (assuming that the dielectric constant of the PCB material is low, about 3.6), the wire is wider-than the inner conductor of the connector. Since the size difference between the circuit wire and the connector wire is large, it will have a strong capacitive mutation. It is usually possible to reduce capacitive mutations by gradually changing the circuit wires to reduce the size gap formed in places where the coaxial connector pin is connected. The PCB wire is narrowed to increase its inductance (or reduce capacity, thereby canceling the capacitive mutation in the impedance curve.
It must be considered for different frequencies. The longer gradient line will have a stronger sensibility of low frequencies. For example, if there is a poor low frequency loss, there is a capacitive impedance spike, and it is more appropriate to use a longer gradient line. Conversely, a shorter gradient line is relatively large.
For coplanar structures, the capacitance is increased when adjacent to the ground. Typically, the sensitive capacity of the signal injection zone is adjusted by adjusting the gradient signal line and the adjacent ground interval size. In some cases, adjacent ground pads of coplanar waveguides are wider on a section of the gradient line to regulate lower frequency bands. Then, the spacing is narrowed in a wider portion of the gradient line, and the portion of the narrowed portion is not long to affect the higher frequency band. In general, the wire gradient is narrowed to increase the sensibility. The length of the progressive line affects the frequency response. Changing the neighboring ground pads of the coplanar waveguide can change the capacity, the weld spacing can change the frequency response, wherein the change in capacity plays a major role.
Example
Figure 4 provides a simple example. Figure 4A is a rough microstrip transmission line having a narrow gradient line. The conversion line is 0.018 "(0.46 mm), 0.110" (2.794 mm) in the plate edge, which is 50 Ω width of a width 0.064 "(1.626 mm). In Fig. 4B and 4C, the length of the gradient line changes. Short. The site-on-site can be compressed terminal connector, not welded, so each of the same inner conductors is used. Microstrip transmission line length 2 "(50.8 mm), processing in a thickness of 30 mil (0.76 mm) RO4350B? Microwave circuit layer On the pressure plate, the dielectric constant is 3.66. In Figure 4A, the blue curve represents insertion loss (S21), which is much fluctuation. Instead, there is less fluctuation of S21 in Figure 4C. These curves indicate that the shorter the gradient line, the higher the performance.
A curve that can be explained in Figure 4 will illustrate the impedance (green curve) of the cable, connector and circuit. Big positive peaks in Figure 4A represent the connector port 1 connected to the coaxial cable, and another peak on the curve represents the connector of the other end of the circuit. The fluctuation on the impedance curve is reduced due to the shortening of the gradient line. Improvement of impedance matches is because the gradient lines of the signal implantation are widened, narrowed; the widening graded line reduces sensibility.
We can learn more about more injected zone circuit sizes from one signal injection design 2, which also uses the same sheet and the same thickness. A coplanar waveguide microstrip circuit produces better results than Figure 4 by using the experience of Figure 4. The obvious improvement is to eliminate the sensibility peaks in the impedance curve, in fact, this is caused by partial sensor peaks and capacitive valley. The use of the correct gradient is reduced to the perceptual peak, while the conjunctional ground pad coupling of the injection zone is used to increase the sensibility. The insertion loss curve of Figure 5 is smoothed in Figure 4c, and the return loss curve also improves. The results of the examples shown in Figure 4 are different from the microstrip circuit using a PCB material having a higher dielectric constant or having a thickness or a microstrip circuit of different types of connector.
Signal injection is a very complex issue, which is affected by many different factors. This example and these guidelines are intended to help designers understand the basic principles.
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Source: Wiku Electronic Market Network
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