The process of delivering high-frequency energy from a coaxial connector to a printed circuit board (PCB) is often referred to as signal injection and is difficult to characterize. The efficiency of energy transfer varies greatly depending on the circuit structure. Factors such as PCB material and its thickness and operating frequency range, as well as connector design and its interaction with circuit materials can affect performance. Performance can be improved with an understanding of different signal injection setups and a review of some optimization cases for RF microwave signal injection methods.
Achieving efficient signal injection is design-dependent, and generally broadband optimization is more challenging than narrowband. High frequency injection is generally more difficult as the frequency increases, and may also be more problematic as the thickness of the circuit material increases and the complexity of the circuit structure increases.
1: Signal injection design and optimization
The signal injection from the coaxial cable and connector to the microstrip PCB is shown in Figure 1. The electromagnetic (EM) field distribution across coaxial cables and connectors is cylindrical, while the EM field distribution within a PCB is flat or rectangular. From one propagating medium into another, the field distribution changes to adapt to the new environment, creating anomalies. The change depends on the medium type: for example, whether the signal injection is from coaxial cables and connectors to microstrip, grounded coplanar waveguide (GCPW), or stripline. The type of coaxial cable connector also plays an important role.
Optimization involves several variables. It is useful to understand the EM field distribution within a coaxial cable/connector, but also ground loops must be considered as part of the propagation medium. It is often helpful to achieve a smooth impedance transition from one propagation medium to another. Knowing the capacitive and inductive reactances at impedance discontinuities allows us to understand circuit behavior. If three-dimensional (3D) EM simulations can be performed, the current density distribution can be observed. In addition, it is best to take into account the actual conditions related to radiation losses.
While the ground loop between the signal launch connector and the PCB may not seem like 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 electrical conductivity of the solder shops connecting the different parts and the metals of those parts. At lower RF and microwave frequencies, the impact of these small differences is generally small, but at higher frequencies the impact on performance is large. The actual length of the ground return path affects the quality of transmission that can be achieved with a given connector and PCB combination.
As shown in Figure 2a, when the electromagnetic wave energy is transferred from the connector pins to the signal conductors of the microstrip PCB, the ground return path back to the connector housing may be too long for thick microstrip transmission lines. Using a PCB material with a higher dielectric constant increases the electrical length of the ground loop, exacerbating the problem. Path lengthening can cause frequency-dependent problems, resulting in localized phase velocity and capacitance differences. Both are related to and affect the impedance within the transform region, resulting in a difference in return loss. Ideally, the length of the ground loop should be minimized so that there are no impedance anomalies in the signal injection area. Note that the ground point of the connector shown in Figure 2a only exists at the bottom of the circuit, which is the worst case. Many RF connectors have ground pins on the same layer as the signal. In this case, the ground pads on the PCB are also designed to be there.
Figure 2b shows the grounded coplanar waveguide to microstrip signal injection circuit, where the main body of the circuit is the microstrip, but the signal injection region is the grounded coplanar waveguide (GCPW). Coplanar emission microstrip is useful because it minimizes ground loops and has other useful properties. If using a connector with ground pins on both sides of the signal conductors, the ground pin spacing can have a significant impact on performance. This distance has been shown to affect the frequency response.
Figure 2. Thick microstrip transmission line circuit and long ground return path to the connector (a) grounded coplanar waveguide to microstrip signal injection circuit (b).
In experiments using a coplanar waveguide to microstrip microstrip based on Rogers' 10 mil thick RO4350B Iaminate, a connector with different coplanar waveguide port ground spacing but otherwise similar parts was used(see Figure 3).Connector A has a ground separation of approximately 0.030', while connector B has a ground separation of 0.064'. In both cases, the connector fires onto the same circuit.
Figure 3. Testing a coplanar waveguide-to-microstrip circuit with a port-like coaxial connector with different ground separations.
The x-axis represents frequency, 5 GHz per division. At lower microwave frequencies (< 5=”” ghz) the performance is comparable, but at frequencies higher than 15=”” ghz=”” the performance of circuits with larger ground separations deteriorates. The connectors are similar, although the 2=”” models have slightly different pin diameters, the connector b=”” has a larger pin diameter and is designed for thicker pcb=””>
A simple and effective signal injection optimization method is to minimize the impedance mismatch in the signal transmitting region. The rise in the impedance curve is basically due to the increase in inductance, while the drop in the impedance curve is due to the increase in capacitance. For the thick microstrip transmission line shown in Figure 2a (assuming a low dielectric constant of the PCB material, about 3.6), the wire is wider - much wider than the connector's inner conductor. Due to the large difference in size of circuit wires and connector wires, there will be strong capacitive abrupt changes during transitions. Capacitive abrupt changes can often be reduced by tapering the circuit wire to reduce the dimensional gap where it connects to the coaxial connector pins. Narrowing the PCB trace will increase its inductive (or decrease its capacitive, thus canceling out the capacitive abrupt change in the impedance curve.
The effect on different frequencies must be considered. Longer gradient lines will have a stronger sensitivity to low frequencies. For example, if the return loss is poor at low frequencies and there is a capacitive impedance spike, a longer gradient line may be appropriate. Conversely, shorter gradient lines have a greater effect on high frequencies.
For coplanar structures, adjacent ground planes increase capacitance when they are close together. Usually, the inductive capacitance of the signal injection area is adjusted in the corresponding frequency band by adjusting the interval between the gradient signal line and the adjacent ground planes. In some cases, the adjacent ground pads of the coplanar waveguide are wider on a section of the gradient line to accommodate lower frequency bands. The spacing is then narrowed in the wider part of the gradient line, and the narrowed part is not long in length to affect the higher frequency bands. In general, the narrowing of the wire gradient will increase the inductance. The length of the gradient line affects the frequency response. Changing the adjacent ground pads of the coplanar waveguide can change the capacitance, and the reason why the pad spacing can change the frequency response is that the change in capacitance plays a major role.
2 Examples
Figure 4 provides a simple example. Figure 4a is a thick microstrip transmission line with long and narrow gradients. The gradient line is 0.018' (0.46 mm) wide at the edge of the board, 0.110' (2.794 mm) long, and finally becomes a 0.064' (1.626 mm) wide 50 Ω line width. In Figures 4b and 4c, the length of the gradient line is shortened. Field crimpable terminal connectors were selected and not soldered, so the same inner conductor was used in each case. The microstrip transmission line is 2' (50.8 mm) long, fabricated on a 30 mil (0.76 mm) thick RO4350B® microwave circuit laminate with a dielectric constant of 3.66. In Figure 4a, the blue curve represents the insertion loss (S21), which fluctuates a lot. Conversely, S21 has the least number of fluctuations in Fig. 4c. These curves show that the shorter the gradient line, the higher the performance.
Figure 4. Performance of 3 microstrip circuits with different gradients; original design with long and narrow gradients (a), reduced gradient length (b), and further reduced gradient length (c).
Perhaps the most telling curve in Figure 4 shows the impedance of the cable, connector, and circuit (green curve). The large forward peak in Figure 4a represents connector port 1, which is connected to the coaxial cable, and the other peak on the curve represents the connector on the other end of the circuit. The fluctuation on the impedance curve is reduced due to the shortening of the gradient line. The improvement in impedance matching is due to the widening and narrowing of the gradient line in the signal injection area; the wider gradient line reduces the inductance.
We can learn more about the size of the circuit in the injection area from an excellent signal injection design 2 that also uses the same board and the same thickness. A coplanar waveguide to microstrip circuit, by exploiting the experience of Fig. 4, produces better results than Fig. 4. The most obvious improvement is the elimination of the inductive peaks in the impedance curve, which, in fact, are partly caused by inductive peaks and capacitive valleys. Using the correct gradient line minimizes inductive peaks while increasing inductance using coplanar ground pad coupling in the injected region. The insertion loss curve of Figure 5 is smoother than that of Figure 4c, and the return loss curve is also improved. The example shown in Figure 4 yields different results for microstrip circuits using PCB materials with higher dielectric constants or different thicknesses, or using different types of connectors.
Note: Signal injection is a complex problem, affected by many different factors.