What is Loss Tangent

What is Loss Tangent

Loss tangent, or dissipation factor (Df), is a parameter that measures the amount of signal loss as the signal travels along the transmission line. It is a dimensionless value that signifies the level of electromagnetic wave absorption by the dielectric material used in the PCB. The loss tangent, denoted as tan(δ), is influenced by various factors such as the material’s structure and composition, particularly the glass-resin composition.

A lower loss tangent indicates reduced signal loss, allowing a greater portion of the original transmitted signal to reach its intended destination. This characteristic is especially significant in designs involving transceivers and the transmission of high-frequency signals across long backplane channels. The loss tangent is typically measured in decibels per inch (dB/in) and can be calculated using the attenuation equation.

When selecting a suitable material for a PCB, it is ideal to choose one with a lower loss tangent to minimize signal loss. It is important to consider the tradeoff between material performance and cost, as materials with lower loss tangents often come at a higher price. Therefore, evaluating the cost-performance ratio by comparing the signal attenuation for different material options at the target data rate and reach required is recommended.