|
Pathology: Transition Fault
Synonyms:
Found by: Transition Faults, Overshoot/Undershoot/Crosstalk:
Disclaimer: This information is provided as
a convenience to the engineering community and is In no way represented as
correct, complete or
error-free in all circumstances. The
reader assumes total responsibility for using or not using any part of this
information for any purpose. Refer to ASA's legal statement for other terms of
use.
Contents:
Summary Diagnosis and Suggested Solution Path
The probable cause of this stair step pattern is reflections
from a poorly matched transmission line. It has the features of too high an
impedance at the driver end.
If what you are measuring is the voltage at a receiver in a
point to point routing topology with source series termination, then you should
reduce the source series resistor at the driver so that the driver impedance
and the resistor impedance match to the line impedance.
If there is no source series resistor than the output
impedance of your driver may be too high to drive this transmission line;
select another driver, add a line driver buffer, or use a higher impedance
transmission line.
As a last resort, add a delay to the clock to wait out the
first shelf, until the received signal level is well above the minimum input
voltage level.
Description
Reflections of signals on transmission lines are the most
common, and easiest to fix, type of signal integrity problem. They arise from
changes in the instantaneous impedance the signal sees on the line. Whenever a
signal encounters a change in the instantaneous impedance, some of it will
reflect, while the rest will continue at a different level.
The magnitude and sign of the reflected signal depends on
the change in impedance. The reflection coefficient is the second impedance,
minus the first, divided by their sum.
Every line has two ends, one at the driver and one at the
receiver. Rarely is either end matched to the 50 Ohms of the transmission line,
unless done on purpose.
The multiple bounces the signal can make between the
impedances of the ends and the impedance of the interconnect cause distortions
in the signal. When the driver impedance is lower than 50 Ohms and the receiver
is high impedance, the waveform at the receiver shows ringing. This is usually
fixed by increasing the series resistance of the driver, for example, by adding
a series resistor. The series resistor plus the output impedance of the driver
should equal the impedance of the line.
When the impedance of the driver is higher than
50 Ohms, less than half the full voltage swing is launched into the
transmission line and when it reflects from the far end, the receiver will see
a voltage below the full signal swing. It may take a few bounces of the signal
to reach the full level. This is usually fixed by reducing the series
resistance of the output driver.
Back to top
Presentation
The
noise signature of a point to point topology with source series resistance can
be easily modeled in any version of SPICE, using the circuit below.
This is the schematic using Agilent’s ADS. The far end of
the line, where the receiver would be, is open and is labeled as V_rx. This is
where the scope probe would be located. The simulated received signal has the
characteristic stair step behavior moving up to a final value, shown below.
In this specific case, what causes the stair stepping up are
the multiple bounces between the high impedance of the receiver at the far end
and the high impedance of the source series resistor.
In this simulation, the line impedance is 50 ohms and the
combination of the driver output impedance and any added source series
resistance is about 79 Ohms. This impedance could be due to a low impedance
driver of 10 ohms, with a series resistor added to the board of 69 Ohms. The
resistor should have been 39 Ohms, but the wrong value was actually assembled.
Back to top
Causes
Reflections are always due to an impedance mismatch at the
driver or receiver with the line. Check the value of the source series
resistor.
The time for the shelf at each level is the round trip time
of flight for the transmission line. Take half the time interval for the shelf,
as measured on the scope. This is the round trip delay. Divide this by 2. This
is the time delay of the transmission line, in nsec. With a signal speed of
roughly 6 inches per nsec, multiply this time delay, in nsec, by 6 and this
should be roughly the physical length of the transmission line, in inches. This
is a good check that the problem is reflection noise from mismatched
terminations.
Back to top
Impact
All signal quality problems at the receiver can affect the
timing of the received signal and create false triggering.
When the output impedance of the driver is too high, there
may be a significant delay before the signal at the receiver rises above the
minimum voltage to designate a 1 or a 0. This delay may push the received
signal outside the set up or hold time. It can result in either a false reading
or in metastablity.
When the output impedance of the driver is too low, there
may be significant ringing. This can bring the signal at the receiver either
below or above the threshold for a 1 or a 0.
The impact of the ringing or stair step behavior will depend
on the relative timing of the received clock signal. Jitter in the clock may
bring this reflection noise in and out of a problem area.
Back to top
Related Pathologies
Other anomalies with similar presentations.
All impedance mismatches the signal encounters can
contribute to reflection noise and appear as shelves or ringing.
Distortions will also occur if the routing topology has
branches, or long stubs. If stubs are longer in inches, than the rise time in
nsec, distortions will occur. For example, if the rise time is 0.25 nsec and
stubs are longer than 0.25 inches, ringing, shelves and distortion will happen.
Even in a well terminated source series topology, a shelf
may appear in the measured signal at about half the signal swing level. This is
usually due to measuring the voltage not at the receiver, but at a location
some distance from the receiver. The duration of the shelf is the round trip
time delay from the measurement location to the input of the receiver. If this shelf is measured, it is not an indication of a
problem, but an artifact of where the measurement is taken.
Back to top
Solutions
The general approach to minimize reflection noise is to:
- use
controlled impedance interconnects
- route
in a linear topology without branches and stubs
- use
either a source series or far end parallel termination
- verify
the signal quality expected before you commit to hardware using accurate
driver models, accurate interconnect models and a circuit simulation tool
such as SPICE.
Back to top
Related Topics
See also topics related to
-
shelf at the mid point of the voltage swing
- ringing
- branches
- stubs
- far end parallel termination
Author Information
Dr. Eric Bogatin
Be The Signal.com
Back to top
Would you like to contribute?
|
