PCB News - PCB substrate is the key to determining the stability of the delay line

PCB substrate is the key to determining the stability of the delay line

In the PCB circuit, the delay line is a very useful structural element used to adjust the signal in the analog circuit and the digital circuit. The main characteristics of high-frequency high-speed delay lines include working bandwidth, delay time, insertion loss within the working bandwidth, return loss, standing wave ratio, rise time and delay stability. The delay line can be realized by different circuit components, such as coaxial lines, bulk acoustic wave components, surface acoustic wave components, etc., but the choice of PCB material will have a significant impact on the final performance of the delay line. The uniformity of the intermediate point constant of the entire PCB and the uniformity of the PCB thickness will greatly affect the performance uniformity and expected effect of the delay line. Quite simply, whether for stripline or microstrip circuit, the better the consistency of the dielectric constant Dk in the PCB, the better the consistency of the material thickness, the better the stability of the delay line in the PCB.

The main function of the delay line in the circuit is to act as an electromagnetic signal transmission medium. When the transmission medium is air, the propagation speed of electromagnetic signals is equal to the speed of light 300,000 kilometers per second. Taking into account the PCB size that designers usually use, the speed of light can be calculated with 11.8 inches/nanosecond or 300 mm/nanosecond. When electromagnetic signals propagate through other media, such as PCB, the propagation rate of the signal will slow down due to the effect of material properties such as the dielectric constant of the PCB. The dielectric constant of all circuit materials is greater than 1, and when electromagnetic waves propagate in the circuit material, a higher dielectric constant means a larger charge capacity and a lower propagation rate.

For signal lines on a PCB, the propagation rate of electromagnetic signals is equal to the speed of light divided by the square root of the dielectric constant. The dielectric constant Dk of both vacuum and air is considered to be 1. Therefore, when air is used as the propagation medium, the propagation rate of electromagnetic signals does not change. For FR-4 material with a dielectric constant of 4, when an electromagnetic signal propagates in it, the propagation rate is equal to the speed of light divided by the square root of the dielectric constant, that is, divided by 2. Therefore, the propagation speed of the signal in FR-4 material is half of the propagation speed in air or vacuum.

For the radio frequency microstrip delay line, the electromagnetic field passes through the metal conductor and the combination of dielectric materials, including the PCB dielectric material under the circuit conductor and the air above the circuit. For the radio frequency strip delay line, the electromagnetic field will pass through the air above the circuit and the dielectric material of the PCB below the circuit. This is especially true for multilayer circuits that use vias to connect different circuit layers. Coplanar waveguides are also commonly used in radio frequency microwave delay lines, and differences in PCB material characteristics such as dielectric material thickness and copper wire thickness tolerance will have a significant impact on the performance of the delay line.

Of course, for a particular PCB material, circuit processing technology and assembly technology are closely related to the performance consistency of the delay line. Ideally, even if the deviation of the thickness and dielectric constant of the PCB material is very small and the consistency is very good, the difference in the characteristics of different PCB materials will be converted into the difference in the performance of the delay line. Undesirable capacitance effects caused by factors such as circuit connection points should be minimized, because an increase in capacitance will lead to an increase in delay time. In order to ensure good electrical performance stability, the PCB delay line must be designed with a large area ground plane.

In the actual delay line circuit design, finding suitable PCB materials must comprehensively weigh multiple factors. From the perspective of excellent performance, the RT 5880 circuit material of Rogers Corporation, which is represented by Shiqiang, is based on PTFE and cured by glass micro-optical fibers. The RT 5880 circuit material has a polar dielectric constant and a very small tolerance, Dk is 2.2, and the tolerance is 0.02. At the same time, its loss factor is also very small, with a variety of laminate sizes and thicknesses (the thinnest can be up to 0.005 inches). When using RT 5880 to design a delay line, the thickness can be strictly controlled to reduce the impact on the delay line. Of course, good performance often means higher costs. PCB materials with low Dk value and ultra-low Dk tolerance are somewhat more expensive than other materials. These materials are often used in the most challenging circuit equipment, such as military electronic equipment.

Considering the performance and cost of the material, Rogers' RO3003 material is also based on PTFE and reinforced by ceramics. RO3003's dielectric constant is 3.00, tolerance is 0.04, its loss factor is also very small and it is easy to accurately control the thickness to reduce the impact on the performance of the delay line. Rogers is a very cost-effective PCB material for the laminate to the delay line. At 10GHz, the Z-axis dielectric constant of RO4835 is 3.48, and the tolerance is 0.05. In addition to being compatible with lead-free processes, the thickness deviation of the material is also very small, and it can be processed with FR-4 material standard process to reduce product costs. In order to meet different design requirements, the applicable thickness of the material and the thickness of the copper foil have a large range, and the thinnest thickness can be 0.0066 inches.

In order to achieve the design goal of the delay line, in addition to the choice of PCB material, many factors need to be considered. Each interface of the radio frequency microwave circuit may cause the delay time of the delay line to increase. For PCB circuit boards that use coaxial connectors for signal transmission, the interface between the circuit board and the connector will introduce delay time changes. These interfaces or signal transition points should be as consistent as possible at both ends to reduce the delay time variation of the circuit. RO4835 laminate material can provide extremely small Dk tolerance, precise thickness control and low loss performance level to meet the performance consistency requirements of the delay line.