Signal integrity is becoming an increasingly important element of circuit and PCB design. As frequencies used within digital circuits rise, even comparatively short connections act as transmission lines, and they have an effect of the integrity of the signals being carried. Signals that might otherwise be considered as purely digital are modified by effects that may be thought of as applying to the analogue domain. These effects can cause circuits not to work, and accordingly signal integrity is now a major issue for any circuit design.
In view of the importance of signal integrity in any of today's high speed processor designs, it is necessary to incorporate design simulations and checks during the PCB design and layout process. Circuit boards effectively need to undergo signal integrity engineering. If it is not carried out at during the design, then there is little that can be done once a completed board has been built. In view of this the top PCB design software packages incorporate options for including signal integrity engineering and checking software, and this will enable checks to be carried out as the design proceeds. In this way the PCB layout can be optimised to ensure that the signal integrity is correctly engineered and problems occurring once the finished PCB is available for its test will be minimised.
Signal integrity issues
There are four main areas of circuit design and layout that must be taken into consideration to ensure that the signal integrity of a board or circuit design are maintained:
- Transmission line effects
- Impedance matching
- Simultaneous switching effects
To ensure that signal integrity is maintained, all the issues must be addressed to ensure that the signal is not distorted in any way and the data is corrupted. In this way the system is able to operate satisfactorily without errors and at the required speed.
Transmission line effects
At low frequencies a length of track may be considered purely by its DC characteristics. However as frequencies rise, effects including the capacitance and inductance associated with the track start to have a significant impact on the performance of the line. Accordingly it is necessary to consider the tracks as transmission lines, and treat them accordingly, looking at aspects such as the line impedance.
As a result it is necessary to ensure that the line maintains the same characteristic impedance along the length of the line, otherwise discontinuities will be introduced. This may result in signal reflections being created that may give rise to ringing and poor signal integrity.
In order to ensure that the transmission lines are treated correctly. First it is necessary for the lines to have a ground plane underneath them. It is also necessary to calculate the impedance of the line. This is determined from a combination of the line thickness, the distance between the line and the ground plane, and the dielectric constant of the board. If as often may happen, the line needs to traverse between layers and therefore the distance between the line and the ground plane changes. It will be necessary to ensure the line impedance remains the same, possibly by changing the line thickness.
In view of the fact that lines on PCBs act more like transmission lines as the frequencies increase, so too it is necessary to consider the way in which the impedances need to be matched to ensure good signal integrity. When there is a mismatch between the line and the load, not all the energy of the waveform is absorbed by the load. That which is not absorbed is reflected back along the line where it may again not be absorbed if there is a mismatch between the transmitter and the line. This can cause overshoot and ringing which leads to poor signal integrity and giving rise to signal errors.To overcome this problem it is necessary to match the transmission line to the line drivers or transmitters and the line receivers. Many drivers and receivers exits that have suitable input and output impedances. Where this is not possible, say between the transmission line and the receiver, it is possible to put a resistor down to ground. In this way the parallel combination of the line receiver and the resistor can equal the line impedance.
In view of the high speeds involved and the length of some lines, the drive capability of the drivers needs to be higher than some "logic only" chips and special line drivers should be used. They will be able to supply the current required to properly drive the lines.
In some applications, it may be possible to add clamping diodes to reduce the level of overshoot and undershoot, and in this way maintain the levels of signal integrity. However wherever possible it is far better to ensure that proper matching is achieved.
Simultaneous switching effects
One effect that can disrupt the signal integrity on a circuit board occurs when several output lines are switched simultaneously. As stored charge on the outputs needs to be discharged, this gives rise to high levels of transient currents. While the levels of transients are normally adequate for single outputs changing, if several lines are switched simultaneously, especially on the same chip, the transient currents are larger, and this can give rise to problems. Problems with signal integrity arise because a voltage arises between the device ground and the board ground. If the chip ground rises sufficiently it can cause the signal switching levels to be exceeded, thereby causing spurious switching to occur.
To overcome this problem there are a number of measures that can be incorporated. One is to ensure that simultaneous switching does not occur, but this is not always possible, especially when circuits are operated in a synchronous manner. Good grounding is essential: a ground plane must be used to ensure a low resistance ground return. Additionally, sufficient decoupling directly across the chip can assist with some of the related effects.
This aspect of signal integrity arises from the fact that signals appearing on one line appear on nearby lines. This can result in spurious spikes and other signal appearing on nearby lines. This can cause erroneous data or clocking pulses to appear, and these can be very difficult to track down in some circumstances. Poor signal integrity from crosstalk arises from two causes, namely mutual inductance, and mutual capacitance.
The mutual inductance is the effect that is used in transformers. It arises from the fact that a current in one track sets up a magnetic field. Changes in this field then induce a current in a nearby track.
Mutual capacitive occurs as a result of the coupling of the electric fields between two tracks. A voltage appearing on one track creates an electric field which can couple to a second line. Changing voltages, especially fast edges can result in similar edges appearing on nearby lines.
There are several techniques that can be used to overcome these effects. As poor signal integrity from crosstalk arises from mutual inductance and capacitance, the solutions involve taking steps to reduce them. This can be achieved in a number of ways by arranging the layout accordingly. The routing should avoid lines that run parallel to one another. If lines have to cross, this should be achieved at right angles, and using layers as far apart as possible. Line spacing should be as wide as possible, and to reduce mutual capacitance lines should be as thin as possible. Finally, where transmission lines are used, they should be as close to the ground plane as possible. This will reduce coupling to other nearby lines.
There are a number of other ideas that can be implemented to assist maintaining good levels of signal integrity. One area to which particular attention should be paid is the clocking circuitry. As it generates a regular clocking pulse, this can create a background noise if the signal integrity measures are not incorporated. Accordingly it is necessary to ensure that measures to reduce crosstalk on the clock lines are implemented. In particular, signal lines should be kept away from the clock lines, and they should not be routed underneath each other. If this is necessary then the ground or earth plane should be between them. To ensure the signal integrity, it is also necessary to ensure the lines are well matched so that ringing is prevented. This can add additional spikes that may be transmitted around the circuitry.
Another method of improving signal integrity is to ensure that all chips are adequately decoupled. Poor decoupling will add to the noise present on the circuits and this may impact the signal integrity. Each chip should be decoupled in line with the manufacturers guidelines. The decoupling capacitors should also be placed as close to the chips as possible.
Signal integrity engineering is now an integral and essential part of the printed circuit board design process. With the high speeds employed in many of today's circuits, it is no longer possible to design the basic circuit in isolation from the PCB. Instead the PCB design must be part of the overall electrical design. When this approach is adopted, then the possibility of problems arising from poor signal integrity will be minimised.
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