When we speak of “signal integrity,” or SI, we refer to a idealized digital signal that offers clean, fast transitions; stable and valid logic levels; accurate placement in time; and freedom from transients. However, in the real world, it’s becoming extremely difficult for system designers to produce and maintain complete, unimpaired signal quality in digital systems. The reasons for this are manifold: bus cycle times are as much as a thousand times faster than they were 20 years ago. Transactions that once took microseconds now take nanoseconds. Edge speeds are now 100 times faster than in simpler times.
But circuit-board technology hasn’t kept pace because of physical limitations. The propagation times of inter-chip buses is no faster. Sure, geometries have shrunk and chips are smaller, but they still take up space on boards, as do all the other components like passives and connectors. That translates into longer wires, and thus into signal delays. The edge speed, or rise time, of a digital signal can carry much higher-frequency components than its repetition rate might imply. It’s these higher-frequency components that create the desired fast transitions in a digital signal. On today’s high-speed serial buses, there is often significant energy at the fifth harmonic of the clock rate.
Thus, PCB traces of only six inches in length become transmission lines when driven with signals exhibiting edge rates below 4 to 6 ns. These traces are no longer simple conductors. At lower frequencies, they act mostly like resistors. With rising frequencies, they start acting more like a capacitor. Finally, at the highest frequencies, inductance becomes an issue.
SI problems become much worse at higher frequencies. Transmission-line effects become critical. Impedance discontinuities along the signal path create reflections, which degrade signal edges. Crosstalk increases. Power-supply decoupling becomes far less effective as ground planes and power planes become inductive and act like transmission lines.
Electromagnetic interference (EMI) increases as faster edge speeds produce shorter wavelengths relative to the bus length, which results in unintended radiated emissions. These emissions create crosstalk and can cause a digital device to fail electromagnetic compliance (EMC) testing.
Faster edge speeds generally also require higher currents to produce them, and higher currents tend to cause ground bounce, especially on wide buses in which many signals switch at once. Those higher currents increase the amount of radiated magnetic energy and result in more crosstalk.
So with data rates rising to the gigabit range and beyond, digital designers are increasingly frustrated with the physics of high-frequency design. Ideal digital pulses are cohesive in time and amplitude and free from deviations and jitter, providing fast, clean transitions. But as system speeds increase, these ideal characteristics become more and more difficult to achieve.
(With permission of Tektronix Inc.)