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Printed Circuit Board Traces and Transmission Line Theory
A transmission line is a system of conductors, such as wires, waveguides, coaxial cables or PCB traces suitable for transferring an electromagnetic field between two or more locations. To meet the challenges of high-speed digital processing, transmission lines within a PCB must perform the following sample list:
- Reduce propagation delay between devices.
- Manage transmission line reflections and crosstalk (signal integrity).
- Reduce signal losses.
- Permits higher density interconnections.
A transmission line allows a signal to propagate from one device to another at or near the speed of light, generally 60% within FR-4. The slowing down of the propagation speed is due to both the value of the dielectric present (core, prepreg, soldermask, or free space), plus the characteristic transmission line capacitance value. In a transmission line, an electromagnetic field is the component of intelligence, not voltage or current (metric units of measurements that describe the electric and magnetic field properties of the propagating wave).
If a transmission line is not properly terminated, circuit functionality (signal integrity) and EMI can occur. Concerns include voltage drop, reflections, ringing, timing margins, overshoot, and undershoot in addition to the development of common-mode RF currents, to name a few.
Transmission line effects must be considered when the round-trip propagation delay exceeds switching-current transition time. Faster logic devices and their corresponding increase in edge rates are becoming more common in the sub-nanosecond range approaching the femto-second range. A very long trace in a PCB can become an antenna for radiating RF currents or cause functionality problems if proper circuit design techniques are not used early in the design cycle.
When dealing with transmission line effects, the impedance of the transmission line becomes an important factor in designing for optimal performance. A signal that travels down a PCB trace will be absorbed at the far end if, and only if, the trace is terminated in its characteristic impedance. If proper termination is not provided, a portion of the transmitted signal will be reflected back to the source. If improper termination is present, multiple reflections may occur, resulting in a longer signal-settling time because due to multiple overshoots and undershoots within the transmission line. This condition is known as ringing.
When a high-speed electrical signal travels through a transmission line, a propagating electromagnetic wave will move down the line (e.g., a wire, coaxial cable, or PCB trace). A PCB trace looks very different to the signal source at high signal speeds than it does at DC or at low signal speeds. The characteristic impedance of the transmission line is identified by the letter Zo. For a lossless line, where the full energy is received at the load, characteristic impedance is equal to the square root of L/C, where L is the inductance per unit length divided by C, the capacitance per unit length. Impedance is also the ratio of the line voltage to the line current per Ohm's law. In the equation below, we see subscripts for the line voltage and the line current. The ratio of line voltage to line current is constant with respect to the line distance x only for a matched termination. The (x) subscript indicates that variations in V and I will exist along the line, except for special cases.
The primary area of concern with designing a PCB with fast edge rate transitions (note, not frequency), is to ensure that a propagated signal travels from the source to load in an optimal manner, without degradation of signal integrity or loss of power absorbed by the dielectric properties of the PCB material. Disruptions in the transmission path, such as vias, changes in trace width, jumping routing layers and traveling through connectors all impact the integrity of the transmission line.
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