| Attenuation |
Attenuation
is a reduction of signal amplitude during transmission. At high
frequencies, attenuation is a problem even when a signal travels
just a few inches along a transmission line. |
 |
| Capacitance |
Almost anything
conductive (e.g., transmission lines, vias, connectors, IC package’
pins, input into receivers, stubs) has capacitance. Drivers
must be capable of charging the capacitance fast enough to preserve
the edge speed of signals. |
|
| Crosstalk |
Crosstalk is
unwanted energy coming from adjacent aggressor traces into a
victim trace under question. |
|
| Dielectric
loss |
At high frequency,
dielectric loss dominates and becomes a major contributor to
the increase in rise and fall time of a signal’s edge. |
|
| Dispersion |
Since different
frequency components of digital waveform propagate with varying
speeds, a signal will be smeared in time. This is typically
not a major issue. |
|
| EMI/EMC |
Electromagnetic
interference (EMI) and electromagnetic compatibility (EMC) issues
need to be addressed early to prevent emission and reception
of unwanted electromagnetic energy. |
|
| Ground
bounce |
Ground bounce
results when a current surge occurs in a circuit with an inductive
ground connection. |
|
| Ground
distribution |
In order to
achieve good ground distribution, ground needs to be a continuous
plane and provide a low impedance path to return currents. |
|
| Impedance
discontinuities |
Impedance discontinuities
are undesirable changes in characteristic impedance (Zo). Capacitance
will usually decrease, and inductance will typically increase
the Zo of a transmission line. |
|
| Inductance |
The larger
the loop of current, the larger the inductance. |
|
| IR
Drop |
IR drop is
a voltage reduction that occurs when current flows through resistance
of power rails. IR drop can cause major timing delays and should
be minimized. |
|
| Jitter |
A jitter is
a time variation of clock or data from a desired or average
frequency. Jitter decreases window margin. |
|
| Line delay |
A line delay
is the time delay a signal incurs while traveling from a driver
to a receiver. |
|
| Lossy
lines |
At high frequencies,
a PCB trace exhibits dielectric and resistive loss. Dielectric
loss will dominate. |
|
| Non-monotonic
edges |
A non-monotonic
edge is an edge that is not straight and can cause double clocking
and/or errors. Edges of data and specially clocks should be
monotonic. |
|
| Overshoot
& undershoot |
Overshoot and
undershoot is typically caused by reflection from impedance
discontinuity. Undershoots are usually more dangerous. |
|
| Parasitics |
Parasitics
include unwanted Zo discontinuity, additional capacitance, and
inductance of connectors, packages, vias, and other components. |
|
| Power
distribution |
It is important
to be able to distribute power to all components with minimum
voltage loss. Today’s Vdd voltages are getting lower and
power consumption of IC’s is getting higher. Power distribution
is becoming increasingly more difficult and needs careful consideration
in design, since many SI problems can be traced to problematic
power distribution systems. |
|
| RC
delay |
The RC (resistance-capacitance)
delay, such as that caused by the pins in IC packages, in combination
with input capacitance of circuits directly on a silicon chip,
must be taken into account in calculating the total time delay
of a signal. |
|
| Reflections |
If a transmission
line does not terminate in its characteristic impedance, energy
will be reflected back to the driver. |
|
| Return
current path |
Return current
path is usually a ground. Current will return to the source
through the lowest impedance, which implies through lowest inductance.
|
|
| Ringing |
If the transmission
line is not well terminated, electromagnetic energy will be
reflected and re-reflected along the line. |
|
| Rise
and fall time edge degradation |
Higher-frequency
components of the edge are attenuated more than lower-frequency
components. This will cause the edge to slow down and may result
in inter-symbol interference. |
|
| Skin
depth |
At high frequencies,
current flows only in the outer skin of a conductor resulting
in rising impedance of a transmission line and increased losses. |
|
| Stub length |
Stub length
is the electrical length of an unterminated transmission line. |
|
| Termination |
A transmission
line must be terminated into its Zo in order to prevent reflection
of electromagnetic energy. |
|
| Transmission
line |
If the propagation
delay of a line from the driver to the receiver takes one-third
or more time than the rise time of the signal, then the line
must be considered a transmission line. |
|
| Vdd
collapse |
Vdd collapse
is usually caused by bypass capacitors and/or power distribution
rails that do not have low enough impedance at a given frequency
for the current demands of the ICs and terminating resistors.
In reality, it is a bypass capacitor, especially the capacitance
of power distribution planes that will supply instantaneous
current needs. The power supply acts like a DC charger of this
capacitance. |
