Phase Timing
Transmission delay of the envelope of a modulated signal as it propagates through a printed circuit board trace represents the average time required for the signal energy to travel between two points. This signal group delay is mathematically defined as the derivative of the phase shift with respect to angular frequency. It must remain constant across the entire bandwidth of a high-speed channel to prevent phase distortion and preserve the integrity of the data eye diagram.
Dispersion Effect
When a dielectric material exhibits high loss, the phase velocity of electromagnetic waves varies with frequency, causing different spectral components of a digital pulse to arrive at the receiver at different times. This frequency-dependent signal group delay degrades the signal transition edges and increases inter-symbol interference. High-speed digital systems are highly vulnerable to such dispersion because it closes the eye diagram and increases the bit error rate.
Designers must choose low-loss laminate materials that exhibit a flat dielectric constant over a broad frequency range to minimize this variation.
Impedance Matching
Equalization circuits can compensate for timing offsets by introducing inverse delays on the transmission line. These circuits are designed to counteract the frequency-dependent signal group delay of the trace.
