Exploring the performance of high-frequency millimeter wave frequencies in different high-frequency materials
The millimeter wave frequency band is gradually being more applied due to its greater bandwidth advantage. Circuit developers for many emerging applications such as 5G wireless networks and ADAS cars are facing the challenge of designing and producing practical and feasible 30 to 300GHz circuit solutions. Here, we explore the different circuit technologies commonly used at microwave frequencies and how different circuit materials affect the performance of high-frequency millimeter wave circuit boards.
The signal frequency in the application of automotive radar varies between 30 and 300 GHz, or even as low as 24 GHz. These signals are transmitted through different transmission line technologies such as microstrip line, stripline, substrate integrated waveguide (SIW) and grounded Coplanar waveguide (GCPW) by virtue of different circuit functions. These transmission line technologies (Figure 1) are typically used at microwave frequencies and sometimes also at millimeter wave frequencies, requiring the use of circuit laminate materials specifically designed for these high-frequency conditions. As the simplest and most commonly used transmission line circuit technology, microstrip lines can achieve high circuit qualification rates using conventional circuit processing techniques. But increasing the frequency to millimeter wave frequency may not be the best circuit transmission line. Each transmission line has its own advantages and disadvantages. For example, although microstrip lines are easy to process, they must solve the problem of high radiation loss when used at millimeter wave frequencies.
Figure 1. When transitioning to millimeter wave frequencies, microwave circuit board designers need to face the choice of at least four transmission line technologies at microwave frequencies
The open structure of microstrip lines, although convenient for physical connection, can also cause some problems at higher frequencies. In microstrip transmission lines, electromagnetic (EM) waves propagate through conductors and dielectric substrates of circuit materials, but there are still some electromagnetic waves that propagate through the surrounding air. Due to the low Dk value of air, the effective Dk value of the circuit is lower than the Dk value of the circuit material, which must be considered in circuit simulation. Circuits made from high Dk value materials tend to hinder the transmission of electromagnetic waves and reduce their propagation rate compared to low Dk values. Therefore, low Dk value circuit materials are commonly used in millimeter wave circuit boards.
Because there is a certain amount of electromagnetic energy in the air, microstrip line circuits will radiate outward into the air, similar to antennas. This will cause unnecessary radiation losses to microstrip line circuits, which will increase with frequency, and also pose a challenge for circuit designers studying microstrip lines to limit circuit radiation losses. To reduce radiation loss, microstrip lines can be processed using circuit materials with higher Dk values. However, the increase of Dk will slow down the electromagnetic wave propagation rate (relative to air), resulting in signal phase shift. Another method is to reduce radiation loss by processing microstrip lines using thinner circuit materials. However, compared to thicker circuit materials, thinner circuit materials are more susceptible to the surface roughness of copper foil, which can also cause a certain signal phase shift.
Although the microstrip line circuit configuration is simple, precision tolerance control is required for microstrip line circuits in the millimeter wave frequency band. For example, the conductor width that needs to be strictly controlled, and the higher the frequency, the stricter the tolerance will be. Therefore, microstrip lines in the millimeter wave frequency band are very sensitive to changes in processing technology, as well as the thickness of the dielectric material and copper in the material. The tolerance requirements for the required circuit size are very strict.
Stripline is a reliable circuit transmission line technology, which can play a good role in millimeter wave frequency. However, compared with the microstrip line, the conductor of the stripline is surrounded by the medium, so it is not easy to connect the connector or other input/output ports to the stripline for signal transmission. The stripline can be regarded as a kind of flat coaxial cable, in which the conductor is wrapped by a dielectric layer and then covered by a stratum. This structure can provide high-quality circuit isolation while maintaining signal propagation within the circuit material (rather than in the surrounding air). The electromagnetic wave always propagates through the circuit material, and the stripline circuit can be simulated according to the characteristics of the circuit material, without considering the influence of electromagnetic wave in the air. However, circuit conductors surrounded by dielectric are vulnerable to changes in processing technology, and the challenge of signal feeding makes it difficult for the stripline to cope, especially under the condition of smaller connector size at millimeter wave frequency. Therefore, except for some circuits used in automobile radar, stripline is not usually used in millimeter wave circuits.
Based on SIW technology, active and passive circuits can be designed and have been used in automotive radar and other millimeter wave applications, such as resonators and filters. This type of circuit can achieve low loss signal propagation at higher frequencies, but like other circuit technologies, SIW technology also needs to balance the advantages and challenges at millimeter wave frequencies.
In the SIW structure, a circuit signal pathway is formed by utilizing an upper metal layer, a lower layer, and several rows of electroplated through holes (PTH) between the metal layer and the layer. In fact, it forms a compact rectangular waveguide filled with dielectric materials. It can maintain low loss characteristics at millimeter wave frequencies, but PTH must be within very strict tolerances, especially at higher frequency conditions. Therefore, SIW is susceptible to changes in circuit processing technology. At the same time, SIW circuits in the millimeter wave frequency band require the use of circuit materials with the smallest Dk variation, and during circuit processing, precise drilling aperture and position are required, while maintaining strict drilling tolerances. Therefore, achieving SIW circuits under millimeter wave frequency conditions also has certain difficulties.
In contrast, circuits made using GCPW structures and low Dk circuit materials are widely used in circuits with a wide RF/microwave/millimeter wave frequency range, such as in experimental/measurement applications. The symmetrical structure of the dielectric and copper conductor can achieve low losses at higher frequencies. Millimeter wave circuits based on GCPW are usually used in combination with low-frequency microstrip line circuits, such as the intermediate frequency circuit of the receiver, so materials that meet the technical requirements of these two circuits need to be used.
GCPW circuits can achieve repeatable and consistent performance at millimeter wave frequencies, but they also require strict control of circuit processing variables and use in combination with low loss circuit materials to achieve optimal results. Usually, the GCPW conductor is assumed to be rectangular in design, but the actual processed conductor is usually trapezoidal. At millimeter wave frequencies, changes in conductor shape and thickness may cause changes in signal phase (Figure 2). The requirements for processing technology in GCPW circuits have some similarities with microstrip lines and SIW. For example, like microstrip lines, it is necessary to minimize changes in conductor width and thickness as much as possible; Same as SIW, GCPW PTH must be accurately positioned to minimize impedance changes and losses and form a consistent and continuous propagation path.
Figure 2. The simulation of GCPW circuit conductor design is rectangular (as shown in the figure above), but the processed conductor is trapezoidal (as shown in the figure below), which will have different effects on millimeter wave frequency.
For many emerging millimeter wave circuit applications that are sensitive to signal phase response (such as automotive radar), the causes of phase inconsistency should be minimized. Millimeter wave frequency GCPW circuits are susceptible to changes in materials and processing techniques, including changes in material Dk values and substrate thickness. Secondly, circuit performance may be affected by the thickness of copper conductors and the surface roughness of copper foil. Therefore, the thickness of copper conductors should be maintained within a strict tolerance, while minimizing the surface roughness of copper foil as much as possible. Once again, the selection of surface coating on GCPW circuits may also affect the millimeter wave performance of the circuit. For example, in circuits using chemical nickel gold, nickel has more losses than copper, and the nickel plated surface layer will increase the losses on GCPW or microstrip lines (Figure 3). Finally, due to the small wavelength, the change of coating thickness will also cause the change of phase response, and the influence of GCPW is greater than that of microstrip line.