PCB Tech - PCB company's analysis of stripline design in millimeter wave frequency band

PCB company's analysis of stripline design in millimeter wave frequency band

Although the design and manufacture of Printed Circuit Board (PCB) at millimeter wave frequencies start from considering the circuit materials, the choice of transmission line technology plays a considerable role in the circuit performance at high frequencies. As cellular and wireless communications continue to occupy the RF/microwave frequency bands, resulting in narrower bandwidths, and millimeter waves can provide sufficient bandwidth, scientific researchers are more concerned about short-range, low-power systems (such as automotive radars and fifth-generation (5G) wireless networks). Interest in millimeter wave frequencies continues to grow. As a commonly used transmission line technology at millimeter wave frequencies, circuit designers may first think of microstrip lines, grounded coplanar waveguides (GCPW) or even rectangular waveguides, but what about the stripline performance? In compact and dense circuits, striplines perform well at 24 GHz (many 5G base stations will operate at higher frequencies) or higher frequencies. There are a few things to keep in mind when designing and constructing stripline circuits at millimeter wave frequencies.

The structure of the stripline is relatively unique and is often compared with flat coaxial cables. It has a multi-layer structure: the middle conductor is surrounded by two upper and lower dielectric layers (circuit materials), and the outside of the dielectric layer is surrounded by metal shielding layers on the top and bottom. These laminated structures increase the circuit complexity, but provide good isolation between the conductor and the transmission line, so that extremely small circuits can be implemented at RF, microwave, and millimeter wave frequencies (depending on the characteristics of the PCB material).

Although the complexity of the stripline increases manufacturing time and cost, it also exhibits some outstanding advantages. In addition to high isolation and miniaturization, the top and bottom ground planes of stripline circuits help reduce radiation loss, especially in the millimeter wave frequency band. The high radiation loss of microstrip circuits sometimes makes them unnecessary antennas. . Stripline may not be as simple as microstrip line or GCPW, but for certain millimeter wave circuit designs, it may be the best choice for transmission lines, especially in high-performance (no interference) densely packaged circuits, or circuits that are undesirable Radiation and electromagnetic interference (EMI) sensitive applications.

Fortunately, the excellent performance of stripline PCB can always be "applied" at 77GHz or higher frequency through the design and manufacturing techniques that have proven good results through several experiments. If you need to quickly understand the microstrip line and GCPW, you can click the previous technology microschool video "Comparison of the performance of microstrip line and grounded coplanar waveguide in millimeter wave frequency band" (click to jump directly) for more information.

Like other transmission line formats, stripline circuits will shrink as the frequency increases to adapt to circuits with small wavelengths such as millimeter waves. However, due to its unique multilayer structure, the circuits will always be able to maintain high isolation. Stripline circuits also have a wider bandwidth, so a single millimeter wave circuit]design can support multiple applications. When designing and implementing stripline circuits at millimeter wave frequencies, appropriate precautions must be taken to achieve the best possible performance to avoid unnecessary signals, such as parasitic signal patterns related to broadband coverage. The choice of PCB material plays a key role in the performance of stripline circuits at millimeter wave frequencies.

Matters needing attention

Since the wavelength of the millimeter wave circuit is short, a thin laminate is usually used. However, even with very thin dielectric materials, strip lines and their multilayer circuits are generally thicker than microstrip or GCPW circuits at a given frequency. At higher frequencies, the consistency of PCB dielectric materials is critical to the consistency of signal propagation (computer-aided simulation). At millimeter wave frequencies, the multilayer dielectric material structure in stripline circuits will have higher dielectric loss and insertion loss than microstrip and GCPW circuits. However, by selecting low dielectric loss or low loss factor (Df) circuit materials, even at millimeter wave frequencies, the stripline insertion loss can be controlled and minimized.

For stripline circuits at millimeter wave frequencies, due to the small wavelength and usually processed on thinner dielectric materials, the surface roughness of copper foil conductors may be a concern. Compared with the smoother copper foil conductor surface, the rougher copper foil conductor surface will slow down the propagation of electromagnetic waves in the conductor. In addition, the inconsistency of the surface roughness of the conductor and the PCB will cause the electromagnetic propagation characteristics of the signal on the PCB to change, especially the change of the phase characteristics at the millimeter wave frequency.

The change of copper surface roughness will cause the dispersion of PCB material to change. The dispersion of the PCB is a function of the conductor and dielectric materials. Inconsistent dispersion may not affect the circuit at RF or even microwave frequencies, but it will cause changes in the phase response of some circuits that are sensitive to this at millimeter wave frequencies.

Compared with the relatively simple signal transition from a coaxial connector to a microstrip or GCPW circuit, a stripline circuit requires proper preparation to achieve an effective signal transition from a coaxial connector to a PCB. In a microstrip circuit, assuming that the connector center conductor and the circuit transmission line with a single ground plane have the same impedance (for example, 50Ω), a direct connection can usually effectively transmit signal energy from the connector to the circuit.

Because the signal plane of the stripline circuit is not on the surface, the signal transition from the coaxial connector to the stripline circuit requires multiple attempts. To connect the center conductor of the connector with the strip line circuit conductor, it can only be achieved by means of metalized vias (PTH). Due to the small wavelength of the operating frequency, the signal feed or transition from the connector center conductor to the stripline signal plane usually passes through metallized vias with extremely small diameters. In order to form a uniform ground plane in a stripline circuit, similar PTH vias are usually used to connect the top and bottom ground planes of the circuit to minimize the possibility of current density differences in different ground planes. Of course, it is important to minimize the length of the transition PTH. In a stripline circuit, any unnecessary length in the signal path may cause reflection and return loss to be reduced, and even generate parasitic or harmonic signals.

Which type of laminate is most suitable for stripline circuits at millimeter wave frequencies? Rogers’ RO3003™ laminate is an example, which is a ceramic-filled polytetrafluoroethylene (PTFE) composite material. The dielectric constant of the entire material is kept within 3.00±0.04, which has the consistency required by the 77GHz automotive radar millimeter wave frequency band circuit. RO3003 laminate has a Df as low as 0.0010 at 10GHz and has excellent temperature stability. At the same time, the material also has a consistent coefficient of thermal expansion (CTE) on the three axes. The CTE consistency can ensure that the extremely small vias in the strip line at the millimeter wave frequency can maintain integrity and high throughout the temperature range. Reliability.