Delay lines are useful component building-block functions for adjusting signals in both analog and digital circuits on printed-circuit boards (PCBs). High-frequency and high-speed delay lines are characterized by their bandwidths and delay times, as well as their insertion loss across their operating frequency range, their return loss, VSWR, rise time, and their delay stability. Delay lines can be realized with a number of different circuit elements, including coaxial cable assemblies, bulk-acoustic-wave (BAW) devices, and surface-acoustic-wave (SAW) devices, but the choice of PCB material can also play a major role in the final performance of a delay-line design. For example, the consistency of the dielectric constant (Dk) across a PCB and the consistency of the PCB’s thicknesses are critical for consistent and predictable delay-line performance. Quite simply: the better behaved a PCB’s Dk characteristics, and the more consistent the thickness of the material, the better the stability of the delay lines fabricated on that PCB, whether working with stripline or microstrip circuit technologies.
How does a delay line work? It is a function of the propagation medium for electromagnetic (EM) signals. When that medium is air, EM signals travel through air at the speed of light, or 186,280 miles/s. In practical terms for designers working in PCB dimensions, the speed of light is equivalent to 11.8 in./ns. When those signals travel through some other medium, such as a PCB, they slow down as a function of the material’s properties, such as a PCB’s dielectric constant (Dk). All circuit materials have a Dk value greater than 1, with higher values representing greater capacity to store charge and slower travel of an EM wave through that material.
On a PCB trace, EM signals move at a speed equivalent to the speed of light (c) divided by the square root of Dk, or c/(Dk)0.5. The Dk of a vacuum (and approximately of air) is 1, so when the propagation medium is air, it essentially has no effect on the EM propagation speed. For a circuit material like FR-4, with a Dk of 4, the speed of the signals traveling through that PCB is divided by the square root of the material’s Dk value, or 2. As a result, the speed of signals traveling through an FR-4 circuit board is about one-half the speed of light through air or through a vacuum.
For a delay line in an RF/microwave microstrip circuit, the EM field moves through a metal conductor and a combination of dielectric materials, including the PCB dielectric material below the conductive circuit trace and the air above the circuit trace. In an RF/microwave stripline delay line, the EM field moves through PCB dielectric material above and below the circuit traces, typically in multilayer circuit designs with plated through holes (PTHs) connecting the multiple circuit layers. Coplanar-waveguide (CPW) PCB techniques are also applied to the fabrication of RF/microwave delay lines, and variations in the PCB material properties, such as dielectric thickness and even the tolerance of the plated copper conductor thickness, can impact delay line performance.
Of course, circuit fabrication processes and assembly techniques can have a great deal to do with achieving consistent delay-line performance from a particular PCB material. Ideally, the PCB material exhibits consistent thickness within a fairly tight tolerance and consistent Dk value across the material, also within a fairly tight tolerance; variations in these PCB material properties can translate into variations in delay-line performance. Unwanted capacitances, such as circuit junctions, should be minimized since added capacitance also means added delays (above a design target). For good electrical stability, any PCB-based delay-line circuit will have a large ground plane.
For practical delay-line circuits, finding a suitable PCB material starting point will inevitably involve some tradeoffs. For example, in terms of pure performance, RT/duroid® 5880 circuit materials from Rogers Corp. (www.rogerscorp.com) are materials based on polytetrafluoroethylene (PTFE) and reinforced with glass microfibers. The RT/duroid 5880 materials feature an extremely low Dk of 2.20 and impressive Dk tolerance of ±0.02, with low dissipation factor for low loss. They are available in a variety of sheet sizes and thicknesses (as thin as 0.005 in.) with tight thickness control to minimize variations in delay time when fabricating delay lines. But performance generally comes at a price and, with their low Dk value and extremely tight Dk tolerance, these materials are somewhat higher in cost than many PCB materials. They are designed for use in the most challenging circuit applications, including in military electronic systems.
Accepting some tradeoffs in performance and material parameters for a lower cost, the same company’s RO3003™ PCB materials are also based on PTFE but filled with ceramic materials for stability. The RO3003 materials exhibit a Dk of 3.00 with Dk tolerance that is still good, at ±0.04, and also with low dissipation factor and excellent thickness control to minimize delay-line variations. A PCB material that offers a good blend of cost and performance for delay lines is the RO4835™ laminate, with a Dk of 3.48 through the z-axis at 10 GHz and a Dk tolerance that is still quite tight, at ±0.05. In addition to being compatible with lead-free processes (RoHS-compatible), this material offers good thickness tolerance and it can be fabricated using standard FR-4 material processes to minimize production costs. This material is available in a wide range of thicknesses (as thin as 0.0066 in. thick) and different weights of copper cladding to accommodate different design requirements.
Achieving design goals in delay lines often involves more than just the choice of PCB material, and every interface in an RF/microwave circuit design is a potential addition to the delay time of a delay line. For PCBs using coaxial connectors to launch signals, the interfaces between the circuit board and the connectors can introduce variations in the delay time and these interfaces or signal launch points should be as consistent as possible to minimize delay-time variations in the circuit. A circuit material such as RO4835 laminate can provide the tight Dk tolerance, excellent material thickness control, and low-loss performance levels required for consistent delay-line performance.
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