[hsdd] High-Speed Digital Design Newsletter MITIGATING CROSSTALK
- From: "Dr. Howard Johnson" <howiej@xxxxxxxxxx>
- To: <hsdd@xxxxxxxxxxxxx>
- Date: Fri, 24 Jan 2003 08:49:07 -0800
MITIGATING CROSSTALK
HIGH-SPEED DIGITAL DESIGN - online newsletter -
Vol. 6, Issue 01
January 23, 2003
Welcome to the New Year!
My family and I have returned from a semester spent
at the University of Oxford, where I tutored
undergraduates, studied signal integrity issues on-
chip, and delivered my High-Speed Digital Design
seminar at several European venues. Travel is
wonderful, but it is great to be home, too.
Those of you interested in the release of my long-
awaited new book on signal integrity need not wait
much longer. The copy-editing and page layout
processes are now complete. It is currently
available for pre-sales at www.barnesandnoble.com. I
expect the first printing in late February or early
March of 2003.
The new book is titled, "High-Speed Signal
Propagation: More Black Magic". It is a companion to
the original book, High-Speed Digital Design: A
Handbook of Black Magic, Prentice-Hall, 1993. The
two books may be used separately or together. They
cover different material.
______________________________________________________
MITIGATING CROSSTALK
(Continued from vol. 5, #11)
In the last newsletter, Mr. Manivassakam asked what
level of crosstalk would be acceptable in his
design. This note considers what can be done to
reduce the amount of crosstalk in his board.
______________________________________________________
Dr. Johnson replies:
For crosstalk to perturb the operation of a state
machine, the following sequence of events must take
place.
1. First, your system must generate an aggressive
signal. This is usually an intentional signal,
meaning that it appears in the system for some
useful purpose other than just to bother the victim.
2. The aggressive signal couples through some
mechanism into the victim circuit.
3. The coupled signal arrives at a time when the
victim circuit is receptive to noise.
4. The coupled signal exceeds the available noise
margin in the victim circuit at the time of
reception.
5. In cases where the noise from one aggressor is
not enough to cause data errors, the aggregate noise
from multiple simultaneous aggressors may cause
errors.
This basic outline of the crosstalk scenario
suggests a number of ways to reduce the impact of
crosstalk.
(1) SHRINK THE AGGRESSOR
This is not always possible with digital logic;
however, you can adopt a policy of at least
separating groups of nets according to their signal
amplitude. This policy prevents large-voltage nets
(e.g., 3.3-V) from affecting smaller-voltage nets
(e.g., 1.5-V).
For example, in the design of a connectorized
interface you might specify operating voltages for
the synchronous data at a level lower than the
asynchronous control signals.
(2) REDUCE THE COUPLING
A solid reference plane is the most powerful tool
available for reducing crosstalk within a pcb. The
reference plane may be a ground plane or a power
plane; either can be equally effective in reducing
crosstalk between digital signals.
Once a solid reference plane is in place, the
crosstalk between two parallel nets may be
dramatically reduced by either spacing the nets
further apart, or by pressing either trace closer to
the plane.
If H represents the height of a trace above the
nearest reference plane, and if the trace-to-trace
separation S exceeds H, then crosstalk plummets
roughly quadratically with increased separation.
Doubling the spacing cuts the crosstalk to roughly
1/4 its original level.
Pressing the traces closer to the planes also
reduces crosstalk, with the caveat that as the
traces are pressed closer to the planes the trace
width must be correspondingly reduced to maintain
constant impedance.
Noise couples differently inside an integrated
circuit. Increased separation does not necessarily
decrease all forms of crosstalk (especially the
substrate noise that occurs in a chip with a low-
resistance substrate). Increased separation
definitely reduces the direct parasitic capacitance
from track-to-track, but since the metal tracks in a
chip are held relatively far up away from any other
bits of grounded metal the severity of the drop-off
in crosstalk is not quadratic.
Differential signaling can reduce the coupling
between nets, but the gains achieved are not as
dramatic for pcb traces as for twisted-pair cables,
because the pcb traces are not twisted.
Differential signaling works particularly well as a
countermeasure against the changing ground voltages
apparent on either side of a connector.
(3) CHANGE THE TIMING
Regarding crosstalk onto a synchronous net, the
crosstalk only matters within a small window around
the moment of clocking. If you can adjust the clock
feeding the aggressor so that it changes state at a
time when the victim is NOT being clocked, then the
interaction between the two circuits disappears.
Some mixed-signal chip architectures provide
separate time slots for analog and digital
processing, pinging back and forth continuously
between analog and digital modes.
(4) IMPROVE THE RECEIVER NOISE MARGINS
The noise margin available at the receiver is the
difference, in volts, between intentional signal
presented to the receiver and the internal threshold
used within the receiver. An additive noise signal
smaller than this margin cannot possibly create a
data error.
Uncertainty in the exact setting of the receive
threshold directly reduces the available noise
margin, which in turn directly reduces the degree of
additive crosstalk your system can tolerate.
For example, in a 3.3-volt system a driver going LOW
might be expected to produce a signal no greater
than 0.3 volts, while the receiver is guaranteed to
respond to any signal less than 0.8 volts. The
difference (the noise margin) in the low state is
0.5 volts.
Had the receiver threshold been guaranteed between
1.6 and 1.7 volts, the noise margin in the low state
given the same transmitter would have been 1.5
volts, THREE TIMES BIGGER.
Differential receivers generally have very well
placed thresholds, which partly accounts for their
popularity in noisy systems.
In a highly attenuated system (like a 2.5-Gb/s
backplane), any increase in the received signal
level obtained by using a better transmitter
(perhaps with pre-emphasis), or a less lossy
transmission media, will directly increase the noise
margin.
Most digital logic operates so far above the
intrinsic internal noise floor of the semiconductor
structures used to build the receiver that cooling
the receiver doesn't help greatly, however in the
design of some infra-red receivers the reduction in
noise achieved by cooling the receiving elements is
worth the cost of the cooling apparatus.
(5) REDUCE THE NUMBER OF AGGRESSORS
While it is admittedly difficult to modify one's
architecture in order to reduce the number of
aggressors, it is in some cases possible to modify
the number of aggressors that change state
simultaneously.
This is accomplished in some systems by skewing the
clock used on various banks of logic such that the
aggressors change state at slightly different times.
The crosstalk resulting from such a system displays
a sequence of smaller blobs of crosstalk, rather
than one grand superposition of all signals
switching at once.
It may in some cases be possible to reduce the
number of simultaneously switching signals with data
coding. This approach is used in the coding of
address lines to reduce the power required and noise
generated when addressing memory sequentially.
I hope this structured examination of crosstalk has
been helpful to you.
Best Regards,
Dr. Howard Johnson
______________________________________________________
Join us at our upcoming seminar in San Jose, Feb. 4-
5, 2002. A full schedule of cities and dates appears
at: http://sigcon.com.
Best Regards,
Jennifer Epps
Marketing Director, Signal Consulting, Inc.
PO Box 698 Twisp, WA 98856
t: 509.997.0505
f: 509.997.0566
www.sigcon.com
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