Circuit designers often channel RF/microwave signals by means of a variety of different transmission-line technologies, such as microstrip or stripline transmission lines. They may also choose between single-ended and differential (also known as balanced) circuit configurations for certain applications, such as when the benefit of differential transmission lines may be necessary to suppress the influence of outside noise or signal sources. How do these types of transmission lines and circuit configurations differ, and how do they impact the choice of circuit material? Each circuit approach has positive and negative aspects, and the choice of printed-circuit-board (PCB) materials can play a role in the level of performance possible with each circuit configuration.
Perhaps a single-ended microstrip line in its most familiar form features a single transmission line on the top side of a PCB with ground plane underneath. Such transmission lines are commonly used to channel high-frequency sine wave signals, although they can suffer loss of signal energy through the conductive circuit traces and dielectric substrate material. Because this circuit configuration is susceptible to noise from electromagnetic interference (EMI) and other sources, some designers opt for a circuit consisting of a differential pair of circuit traces in close proximity, with a certain amount of coupling between the differential pair which helps to minimize noise. In such circuits, signals are transmitted as complementary signals (positive and negative) along the differential transmission lines, providing a robust and noise-resistant means of transferring signal energy through a circuit.
In microstrip form, differential transmission lines are still somewhat dispersive, and this can be a concern for circuits handling wideband analog or high-speed digital signals. Such signals typically contain high harmonic signal content, with harmonic signal characteristics distorted as a result of dispersion. Fortunately, differential transmission lines can also be fabricated in stripline circuit configurations, where dispersion can be minimized.
Stripline circuits, in either single-ended or differential form, can deliver excellent analog or digital circuit performance, with the most essential tradeoff being added complexity compared to circuits based on microstrip transmission lines. Stripline circuit boards, with their three copper layers and bonding materials, rely on consistency in their materials to achieve good consistency in electrical performance. In comparison, a differential pair of transmission lines fabricated on a PCB as microstrip lines offer relative simplicity, although they tend to be more dispersive. A differential pair of microstrip transmission lines exhibit even- and odd-mode propagation characteristics, with the even-mode electromagnetic (EM) fields propagating through the PCB substrate material while the odd-mode EM fields propagate through the substrate material as well as the air above the substrate. It is this combination of propagation through air and circuit substrate material that results in microstrip circuitry being more dispersive than stripline circuitry, even when comparing differential circuit configurations.
Of course, stripline, with its bonding layer, relies strongly on consistent electrical and mechanical properties of the PCB material for its own consistency, more so than with microstrip transmission lines. Microstrip differential transmission lines, in particular, suffer less lot-to-lot performance variations because of the properties of the circuit substrate material, and microstrip differential circuits can provide fairly consistent performance even when subjected to circuit process variations, with typically less loss than their microstrip single-ended counterparts.
It is the bonding layer used in stripline circuits that often gives rise to inconsistent performance, even when working with differential transmission lines. The gap between the pair of differential signal traces in a stripline circuit, for example, is filled with the bonding material used in that PCB material. If the substrate’s bonding material is glass reinforced, some amount of glass fiber will lie between the two coupled conductors of the differential signal pair. The amount of glass fiber between the differential lines may vary at different points along the transmission lines for a number of reasons, including circuit-to-circuit variations due to normal alignment tolerances of the bonding material to the circuit image on the substrate material. The amount of glass weave can vary a great deal at different points between the two differential conductors, resulting in variations in electrical performance.
This varying amount of glass fiber between transmission lines in a differential stripline circuit can cause variations in performance, depending upon how sensitive the coupling is between the differential lines. Most of the bonding materials used in stripline circuits are anisotropic in nature, because they employ a glass weave “filler” type material. Certainly, more consistent stripline differential circuitry can be achieved if it is possible to fabricate those circuits on PCB materials that do not rely on woven-glass-reinforced bonding materials, essentially by using circuit materials that are more isotropic in nature. One such bondply material that is nearly isotropic is Rogers’ 2929 bondply material.
Rogers’ 2929 bondply material is not woven glass reinforced but still maintains the layer-to-layer integrity needed for robust stripline and multilayer high-frequency circuitry, both for single-ended and differential circuits. Electrically, it features consistent dielectric constant (Dk), with a value of 2.94 in the z-axis when measured at 10 GHz and 2.95 at 10 GHz in the x-y plane when evaluated with the split post dielectric resonator (SPDR) test method.
By departing from the accepted practice of using woven-glass-reinforced bonding materials, the 2929 bondply materials are well suited for bonding a number of different types of circuit materials in multilayer boards. The material is based on an unreinforced hydrocarbon-based thin-film adhesive system with a low loss tangent of less than 0.003 at 10 GHz. It features dielectric strength of 2500 V/mil in the z direction, with coefficient of thermal expansion (CTE) that is equal in x, y, and z directions, at 50 ppm/°C, for operating temperatures from 0 to +150°C. This type of consistency is highly desirable for stripline circuits, whether in single-ended or differential configurations, and can serve to offset some of the added complexities of processing stripline designs for their promise of enhanced performance compared to microstrip circuits, even in their differential configurations.
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