Texas Instruments | LVDS Signal Quality: Jitter Measurements Using Eye Patterns Test Report #1 | Application notes | Texas Instruments LVDS Signal Quality: Jitter Measurements Using Eye Patterns Test Report #1 Application notes

Texas Instruments LVDS Signal Quality: Jitter Measurements Using Eye Patterns Test Report #1 Application notes
DS90C031,DS90C032
LVDS Signal Quality: Jitter Measurements Using Eye Patterns Test Report #1
Literature Number: SNLA166
National Semiconductor
Application Note 977
John Goldie
Syed Huq
October 1994
LVDS SIGNAL QUALITY
This report provides data rate versus cable length recommendations for LVDS drivers and receivers in a typical application for a particular twisted pair cable. The questions of:
How Far? and How Fast? seem simple to answer at first, but
after detailed study their answers become quite complex.
This is not a simple device parameter specification. But
rather, a system level question, and to be answered correctly
a number of other parameters besides the switching characteristics of the drivers and receivers must be known. This includes the measurement criteria for signal quality that has
been selected, and also the pulse coding that will be used
(NRZ for example). Additionally, other system level components should be known too. This includes details about the
cable, connector, and information about the printed circuit
board (PCB). Since the purpose is to measuring signal quality, it should be done in a test fixture that matches the end
environment as close as possible, or even better in the actual application if possible. Eye pattern measurements may
be used to measure the amount of jitter versus the unit internal to establish the data rate versus cable length curves and
therefore are a very accurate way to measure the expected
signal quality in the end application. This test report assumes: maximum jitter allotment of 20%, measurements
taken at 0V (differential zero) for minimum jitter, measurements taken at ± 100 mV for maximum jitter, and then provides the corresponding data rate versus cable length recommendations.
When line drivers (generators) are supplying symmetrical
signals to clock leads, the period of the clock, rather than the
unit interval of the clock waveform, should be used to determine the maximum cable lengths (e.g., though the clock rate
is twice the data rate, the same maximum cable length limits
apply). This is due to the fact that a periodic waveform is not
prone to distortion from inter symbol distortion as is a data
line.
WHY EYE PATTERNS?
The eye pattern is used to measure the effects of inter symbol interference on random data being transmitted through a
particular media. The transition time of the signal is effected
by the prior data bits, this is especially true for NRZ data
which does not guarantee transitions on the line. For example in NRZ coding, a transition high after a long series of
lows has a slower rise time than the rise time of a periodic
(010101) waveform. This is due to the low pass filter effects
that the cable causes. Figure 1 illustrates the superposition
of six different data patterns. Overlaid they form the eye pattern that is the input to the cable. The right hand side of Figure 1, illustrates the same pattern at the end of the cable.
Note the rounding of the formerly sharp transitions. The
width of the crossing point is now wider, and the opening of
the eye is also now smaller (see AN-808 for an extensive discussion on eye patterns).
AN012338-1
FIGURE 1. Formation of an Eye Pattern by
Superposition
Figure 2 describes the measurement locations for minimum
jitter. Peak-to-Peak Jitter is the width of the signal crossing
the optimal receiver thresholds. For a differential receiver,
that would correspond to 0V (differential). However, the receiver is specified to switch between −100 mV and +100 mV.
Therefore for a worse case jitter measurement, a box should
be drawn between ± 100 mV and jitter measured between
LVDS Signal Quality: Jitter Measurements Using Eye Patterns Test Report #1
LVDS Signal Quality: Jitter
Measurements Using Eye
Patterns Test Report #1
AN-977
© 1998 National Semiconductor Corporation
AN012338
www.national.com
the first and last crossing at ± 100 mV. If the vertical axis
units in Figure 2 was 100 mV/division, the worse case jitter
± 100 mV levels.
Cable: Cable used for this testing was Berk-Tek part number
271211. This is a 105Ω (Differential Mode) 28 AWG stranded
twisted pair cable (25 Pair with overall shield) commonly
used on SCSI applications. This cable represents a common
data interface cable. For this test report the following cable
lengths were tested: 1, 2, 3, 5, and 10 meter(s). Cables
longer than 10 meters were not tested, but may be employed
at lower data rates.
PCB#2: DS90C032 LVDS Quad Receiver soldered to the
PCB with matched PCB traces between the device (located
near the edge of the PCB) to the connector. The connector is
an AMP amplite 50 series connector. A 100Ω surface mount
resistor was used to terminate the cable at the receiver input
pins.
TEST PROCEDURE
A pseudo-random (PRBS) generator was connected to the
driver input, and the resulting eye pattern, measured differentially at TP' was observed on the oscilloscope. Different
cable lengths (L) were tested, and the frequency of the input
signal was increased until the measured jitter equaled 20%
with respect to the unit interval for the particular cable length.
The coding scheme used was NRZ. Jitter was measured
twice at two different voltage points. First, jitter was measured at the 0V differential voltage (optimal receiver threshold point) for minimum jitter, and second at the maximum receiver threshold points ( ± 100 mV) to obtain the worst case
or maximum jitter at the receiver thresholds. Occasionally jitter is measured at the crossing point alone, this will result in
a much lower jitter point, but ignores the fact that the receivers may not switch at that very point. For this reason this signal quality test report measured jitter at both points.
AN012338-2
FIGURE 2. NRZ Data Eye Pattern
EYE PATTERN TEST CIRCUIT
LVDS drivers and receivers are intended to be primarily used
in an uncomplicated point-to-point configuration as is shown
in Figure 3. This figure details the test circuit that was used
to acquire the Eye pattern measurements. It includes the following components:
PCB#1: DS90C031 LVDS Quad Driver soldered to the PCB
with matched PCB traces between the device (located near
the edge of the PCB) to the connector. The connector is an
AMP amplite 50 series connector.
AN012338-3
FIGURE 3. LVDS Signal Quality Test Circuit
www.national.com
2
ent cable, a different wire gauge (AWG), etc., will create a
different relationship between maximum data rate versus
cable length.
RESULTS AND DATA POINTS
20% Jitter Table @ 0V Differential
(Minimum Jitter)
Cable
Length
Data
Rate
Unit Interval tui
Jitter tcs
(meter)
(Mbps)
(ns)
(ns)
1
400
2.500
0.490
2
391
2.555
0.520
3
370
2.703
0.524
5
295
3.390
0.680
10
180
5.550
1.160
As described above, Jitter was measured at the zero volt differential point. For the case with the 1 meter cable, 490 ps of
jitter at 400 Mbps was measured, and 1.160 ns of jitter at 180
Mbps and with the 10 meter cable.
AN012338-4
FIGURE 4. Data Rate versus Cable Length
20% Jitter Table @ ± 100 mV
(Maximum Jitter)
Cable
Length
Data
Rate
Unit Interval tui
(meter)
(Mbps)
(ns)
(ns)
1
200
5.000
1.000
2
190
5.263
1.053
3
170
5.882
1.176
5
155.5
6.431
1.286
10
100
10.000
2.000
CONCLUSIONS
Eye patterns provide a useful tool to analyze jitter and thus
the resulting signal quality as it captures the effects of a random data pattern. They provide a method to determine the
maximum cable length for a given data rate or vice versa.
However, different systems can tolerate different amounts of
jitter, commonly 5%, 10%, or 20% is selected, with 20% being the maximum allowed. Jitter in the system that is greater
than 20% tends to close down the eye opening, and error
free recovery of NRZ data is increasing more difficult. This
report illustrates typical maximum cable lengths for a common data interface cable at 20% jitter, for data rates between
100 Mbps and 200 Mbps. Selecting a premium cable, a category 5 cable for example, will extend the curve significantly.
While selecting a lower limit for jitter, 5% for example will decrease the maximum cable length.
Jitter tcs
The second case measured jitter between ± 100 mV levels.
For the 1 meter cable, 1 ns of jitter was measured at
200 Mbps, and for the 10 meter cable, 2 ns of jitter occurred
at 100 Mbps.
REFERENCES
To probe further the following National Semiconductor Application Notes are recommended which are all located in the
INTERFACE: Data Transmission Databook:
AN-808 Long Transmission Lines and Data Signal Quality
AN-903 A Comparison of Differential Termination Techniques
AN-916 A Practical Guide to Cable Selection
Figure 4 is the graphical representation of the relationship
between data rate and cable length for the application under
test. Both curves assume a maximum allotment of 20% jitter
with respect to the unit interval. Basically data rates between
200–400 Mbps are possible on the shorter lengths, and data
rates of 100–200 Mbps are possible at 10 meters. It should
be noted that employing a different coding scheme, a differ-
For additional information on cables contact: Berk-Tek @
1-800-237-5835 (USA), 1-717-354-6200.
3
www.national.com
LVDS Signal Quality: Jitter Measurements Using Eye Patterns Test Report #1
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