Transmission line effect in PCB Design
In this article we are going to discuss about
is any net & its current return to GND/PWR supply. Parallel conductors can couple a signal onto each other if the signal on one conductor is a time varying signal. (A time varying signal has EM fields that can couple on the other conductor). The conductors are not physically connected, but are Electromagnetically connected, this is known as the transmission line effect. Short signal transition times & high clock rates of PCB traces need to be considered as TM lines. In this application note we look at 2 configurations of TM line.
The circuit in Fig. is an example of a single-ended TM line. The single-ended TM line is probably the commonest way to connect 2 devices. In this case a single conductor connects the source of one device to the load of another device. The reference GND plane provides the signal return path. This is an example of an unbalanced line. The signal & return lines differ in geometry — the cross-section of the signal conductor is different from that of the return GND plane conductor. The impedance value is determined by the dimensions of the trace, the value of the board dielectric constant & thickness of the dielectric.
These structures are illustrated in the following diagrams. Note that in the following diagrams the signal trace is actually trapezoidal in profile & width ‘W’ refers to the trace width nearest the upper surface,
W1 reference to the trace width nearest the lower surface. There are 2 basic configurations of micro strip, surface or exposed & embedded.
The simplest configuration, the surface/exposed micro strip, shown in Fig, consists of a signal line, the top & sides exposed to air, on the surface of a board of dielectric constant Er & referenced to a PWR/GND plane. Surface micro strip SHOULD be implemented by etching one surface of double-sided PCB material. The diagram shows the characteristic micro strip impedance attributes: the micro strip impedance references a single plane the impedance trace is cab be on an outer layer.The effects of Er: The value of Er, the dielectric constant of the board material, a significant component of the value of the characteristic impedance of the line. Designers therefore sometimes just specify the value of trace impedance & rely on the board mfrs to control their processes to conform the impedance of the trace to the designer’s specification. Note that in the surface micro strip configuration the signal conductor is exposed to the air so the effective dielectric constant will be somewhere between 1 (that of air ) & about 4 (the Er of G-10 or FR-4 substrate). This will also have an effect on the signal propagation velocity. Propagation velocity reduces (from the speed of light in air) as Er increases so the surface micro strip configuration provides the highest propagation velocity. However, the trade off is that radiation from the surface micro strip is higher than from embedded types.
The equations for characteristic impedance require very complex mathematics, usually using field solving methods including boundary element analysis.
Controlled impedance PCBs are usually produced using micro strip or strapline TM lines in single-ended (unbalanced) or differential (balanced) configurations. The differential mode of operation is shown in figure below
The differential configuration is used when better noise immunity & improved timing are required in critical applications. This configuration is an example of a balanced line— the signal & return paths have similar geometry. The lines are driven should be a pair with one line transmitting a signal waveform of the opposite polarity to the other. Fields generated in the 2 lines will tend to cancel each other, so EMI & RFI will be lower than with the unbalanced line & problems with external noise are reduced.