Signal Integrity
an overview the methods used during PCB and circuit design to ensure
signal integrity.
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
- Crosstalk
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.
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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.
Impedance matching
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.
Crosstalk
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.
Further ideas
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.
Summary
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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