Signal Distortion
Frequency dependent variations in the velocity of electromagnetic waves cause pulses to spread out as they travel along a transmission line. Phase dispersion occurs because the dielectric constant of the circuit board material changes across the operating bandwidth. This effect results in different frequency components of a complex signal reaching the receiver at slightly different times.
High speed digital designs must account for this phenomenon to avoid data errors and timing jitter.
Dielectric Interaction
The molecular structure of the resin and glass reinforcement determines how the material responds to rapidly oscillating electric fields. In most fiberglass-based substrates, the polarization of the molecules cannot keep pace with very high frequencies, leading to a decrease in the effective dielectric constant. Phase dispersion is the observable result of this change, appearing as a smearing of the signal edges in the time domain.
Because the high frequency harmonics of a digital pulse travel faster than the low frequency components, the sharp rising edge of the signal becomes rounded. This loss of signal definition complicates the task of clock recovery and reduces the available timing margin for data sampling. Advanced laminate materials use low loss resins and specialized glass weaves to minimize the frequency dependence of the dielectric properties.
Material Selection
Choosing substrates with a flat dielectric constant over a wide frequency range preserves the pulse shape during transmission. Low dissipation factor materials reduce the impact of these timing shifts in multi-gigahertz circuits.
