Texas Instruments | LVDS Signal Quality: Cable Drive Measurements using Eye Patterns Test Report #3 | Application notes | Texas Instruments LVDS Signal Quality: Cable Drive Measurements using Eye Patterns Test Report #3 Application notes

Texas Instruments LVDS Signal Quality: Cable Drive Measurements using Eye Patterns Test Report #3 Application notes
DS90C031,DS90LV027
Application Note 1088 LVDS Signal Quality: Cable Drive Measurements using
Eye Patterns Test Report #3
Literature Number: SNLA004
National Semiconductor
Application Note 1088
July 1998
HOW FAST? AND HOW FAR?
The above two questions should be asked together because
they are related to one another. Each one is a critical part of
the complete system. (See Figure 1). The first question concerns the physical layer integrated circuits (ICs) referred to
as drivers and receivers. The second question concerns the
media interface between the driver(s) and receiver(s). Each
question deserves its own application note to specify the
particulars of issues associated with its section of the system. However, it is not enough to understand each part of the
system, but understanding the relationship between the respective parts of the system is very important. This report is
concerned with the relationship between the two questions
which include measurement techniques, trade-offs, typical
characteristic curves, and recommendations based on empirical data from LVDS drivers and receivers.
option may be chosen over another, but it is typically application dependent. Type of equipment available, application criteria, transmitting scheme, and receiving scheme all play a
part in the decision process. Eye pattern testing was selected for this report for equipment and previous data comparison reasons.
MEASUREMENTS
Before beginning measurements, some decisions are made
about the test conditions and the environment. Obviously,
the ideal setup would be to take the measurements on the
actual system application. In this case, a general setup is
used since this data is for general use. (See Figure 5.) For
the same reason, room temperature, nominal power supply,
and general off-the-shelf equipment are used for the experiment. Further experiment details will follow but now, one out
of several measurement techniques must be selected. For
example, Bit Error Rate Testing (BERT), eye pattern testing,
and slew rate evaluation are some of the different ways to
evaluate system performance. There are reasons why one
WHY EYE PATTERNS?
Eye pattern measurements may be used to measure the
amount of jitter versus the unit interval (bit width) to establish
the data rate versus cable length curves and therefore is a
very accurate way to measure the expected signal quality in
the end application and furthermore addresses the concerns
of this report. The eye pattern may be used to measure the
effects of inter-symbol interference (ISI) of 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 Non-Return to Zero (NRZ) data which does
not guarantee periodic 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 of the
cable. Figure 2 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 2 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 disscussions on eye
patterns).
AN100109-1
FIGURE 1. Typical Interface System Components
Additionally, whenever the system is used to supply both
clock and data, using NRZ coding, and the clock frequency
equals the data rate (clk_Hz = data_bps); then the period of
the clock, not the unit interval of the clock, should be used to
determine the maximum cable length. Although the clock’s
unit interval (UI) is half the data’s UI, the same maximum
cable length limit for data also apply to the clock. This is due
to the fact that the clock’s periodic waveform is not prone to
distortion from inter symbol distortion as is a data line.
LVDS Signal Quality: Cable Drive Measurements using Eye Patterns Test Report #3
LVDS Signal Quality: Cable
Drive Measurements using
Eye Patterns Test Report
#3
AN-1088
© 1999 National Semiconductor Corporation
AN100109
www.national.com
AN100109-9
FIGURE 2. Formation of an Eye Pattern by Superposition
Figure 3 shows the measurement locations for jitter. Peakto-Peak Jitter is the width of the signal crossing the optimal
receiver threshold level. For a differential receiver, that would
correspond to zero Volts (differential).
AN100109-10
FIGURE 3. NRZ Data Eye Pattern
IC configurations, by-pass capacitors, and additional SMB
connectors for waveform monitoring purposes. The PCB
along with the equipment listed in Table 1 were all used in
the experiment setup. The complete test set up is shown in
Figure 5.
TEST MEASUREMENT SETUP
The test setup used is a general setup since the data acquired from it is for general purpose use. The equivalent PC
board configuration is shown in Figure 4. However, the actual test board has components not shown, like jumpers for
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2
AN100109-2
FIGURE 4. Equivalent PCB for Test Measurement Setup
TABLE 1. Test Measurement Equipment
Equipment
Manufacturer
Power Supply
Hewlett-Packard
Model Number
6626A
Function Generator
Tektronix
HFS9009
Oscilloscope
Tektronix
11801B
Probes
Tektronix
SD-14
CAT 3 Cable
Beldon
9842
AN100109-3
FIGURE 5. Test Equipment Setup
used. The power supply was connected to both the driver
and receiver side of the PCB and grounds were tied together, therefore common mode voltage range is not being
tested. The oscilloscope was used to probe the resulting eye
pattern, measured differentially at both the output of the
TEST PROCEDURE
A pseudo-random bit sequence (PRBS) was programmed
into the function generator and connected to the driver input
via a 50Ω cable with a SMB connector. The sequence was
512 (29) bits long. The length is restricted by the equipment
3
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and 100 meters) to generate the curves in the data and results section of this report. Category 3 cables were tested.
The CAT 3 cable was a Beldon 9842 shielded twisted pair
cable. This cable is 24 gauge with tinned copper conductors
and polyethylene insulation. Testing was done at room temperature and the power supply was set to 3.3V. Only LVDS
style devices (DS90LV027, DS92LV101(test device similar
to DS92LV010A with separate driver and receiver), and
DS92LV010A) were tested.
driver and the input of the receiver. A 100Ω resistor was used
to terminate the signals at the far end of the cable. Only the
measurements taken at the far end of the cable, at the receiver’s input, are used for the jitter analysis in this report.
The frequency of the input signal was increased until the
measured jitter (ttcs) in Figure 3, equaled 20% with respect to
the unit interval (ttui) for the particular cable length under test.
The coding scheme used was NRZ. Jitter was measured at
the zero volt differential voltage of the differential eye pattern. Five different cable lengths were tested (2, 10, 20, 50,
DATA AND RESULTS
TABLE 2. 20% Jitter Table @ 0V Differential with CAT 3
Cable Length — (meter)
Data Rate — (Mbps)
Unit Interval — tui (ns)
Jitter — tcs (ns)
2
337
2.97
0.594
10
325
3.08
0.616
20
280
3.57
0.714
50
135
7.41
1.482
100
35
28.57
5.714
Note the data in and Table 2 is for all devices tested. The
DS90LV027 is a 3 mA current source driver. The DS92LV010
is a high drive current source driver capable of delivering
about 5.5 mA with a 100Ω load. No difference in the measurement was noticeable between 3 mA and 5.5 mA current
drivers, as the transition times of both devices is similar. This
is also a result of the measurement technique. Since jitter
was measured horizontal at zero volts differential (crossing
point), which does not vary too much with small current drive
difference, the measurements remain constant across all devices. Examiming the data in Table 2, shows the CAT 3 cable
has good performance (above 100 Mbps with 20% jitter) to
50 meters. The data for Table 2 is plotted in Figure 6. Note
that the data is plotted in log vs. log scale. This is done to
display the roll off point better.
will have the opposite effect on the system. Twenty percent
jitter is a reasonable place to start with many system designs.
LVDS Data Rate vs Cable Length
AN100109-5
FIGURE 6. CAT 3 Cable Length vs Frequency
Figure 6 shows very good typical performance curve that
can be used as design guidelines. Increasing the jitter percentage increases the curve respectively. Allowing the device to transmit faster over longer cable lengths. This relaxes
the jitter tolerance of the system allowing more jitter into the
system which could reduce the reliability and efficiency of
the system. Alternatively, decreasing the jitter percentage
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4
vide VOD by IOD to obtain the maximum resistance. Second, when the resistance per unit (unit may be feet, meter,
etc.) of the cable is divided into the maximum resistance, the
result is the maximum cable length in units. For instance,
434 mV ÷ 1.5 mA results in a DC resistance of 150Ω. Assuming 100Ω for the termination load, 50Ω (150Ω–100Ω) is
left for the cable resistance. Since this is a differential device,
25Ω will be used for source side and 25Ω for the sink side.
The cable used has ≈ 10Ω/100 meters. Therefore, the device
should theoretically be able to drive 250 meters. Remember,
this is a DC drive capability only. Figure 8 shows a 5V VOD
vs IOD curve for comparison. This curve was taken using the
DS90C031. Notice that the higher VCC level produces higher
cable drive as expected. Recall, this is because the larger
VCC is capable of producing a larger VOD which allows the
device to sustain a constant output current over longer
cables.
LVDS VOD vs IOD
To show this, Figure 4 in AN-977 shows the same information as, except the data is for the DS90C031 using CAT 3
cable and was only taken up to 10 meters. Notice that the
DS90C031 shows better performance over CAT 3 cable than
the DS92LV027 in this report. The only difference is the
DS90C031 is a 5V device versus a 3V device. To explain
this, as the cable length is increased so does the resistance.
Therefore, the VOD must increase in order to keep the current near constant. If VOD continues to increase, the output
stage transistors go from operating in the saturation region
to the linear region due to a decrease in drain to source voltage. This is why a 5V DS90C031 can drive over longer cable
lengths than a 3V DS90LV027.
AN100109-6
FIGURE 7. 3V DS90LV027 VOD vs IOD
LVDS VOD vs IOD
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 visa versa.
However, different systems can tolerate amounts of jitter,
commonly 5%, 10%, or 20% is selected, with 20% being the
recommended maximum. Jitter in the system that is greater
than 20% tends to close down the eye opening, and error
free recovery of NRZ data becomes increasingly more difficult. This report shows typical performance of 3V LVDS devices across long CAT 3 cables. This data is intended to be
used as a recommendation guideline for system designs.
Importantly, system applications should always be evaluated
individually and independently. Other parameters may limit
cable length prior to the jitter. The operating region of the
system should be determined by complete application and
system parameters. It may be assumed that to operate below typical performance curves, given in this report are reliable and to operate beyond the curves is less reliable based
on jitter alone.
Although operation up to 100 meters is obtainable with LVDS
drivers, approximately 30 meters would be a recommended
maximum cable distance. Beyond 30 meters dealing with
other concerns like common mode will complicate system
design. Is ± 1V common mode enough (application dependent) for reliable operation beyond 30 meters? Also the frequency roll off is steep beyond 30 meters. TIA/EIA-485-A
may be a better choice for below 50 Mbps operation because of the ± 7V common mode and the 1000 meter cable
drive capability of most 485 drivers.
AN100109-7
FIGURE 8. 5V DS90C031 VOD vs IOD
In addition to the AC analysis of jitter, a DC analysis was also
completed. Figure 7 and Figure 8 shows the VOD vs IOD
(Voltage Output Differential vs Current Output Differential)
curve for the DS90LV027. This analysis is beneficial whenever the part is being operated at very low frequencies or
DC. This may be the case for a control line or static line. This
curve is generated by increasing the resistance and measuring voltage and current at the output of the device. The increasing resistance is used to model increasing cable
length. Capacitance and inductance are being ignored since
the part is operating at or near DC. The experiment was
done across VCC to show the impact of delta VCC. The curve
may be divided into two regions, the first where the curve is
almost vertical and the second where the curve is near horizontal. The first region is where a large delta VOD results in
a very small delta IOD. The second region is where a small
delta VOD results in a very large delta IOD. Since the LVDS
devices are current mode, with a constant output current,
they should operate in the first region where current is more
stable. Also, if the system will operate over the full power
supply range then the lowest VCC curve should be used to
determine the operating range. For example in Figure 7, if a
horizontal line is drawn at VOD = 434 mV and a vertical line
is drawn at IOD = 2.89 mA, they will intersect the 3V VCC
curve at the intersection of the two regions. This point of intersection may be used to calculate a cable length. First, di-
5
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LVDS Signal Quality: Cable Drive Measurements using Eye Patterns Test Report #3
AN-1088
AN-916 A Practical Guide to Cable Selection
AN-1040 LVDS Performance: Bit Error Rate (BER) Testing
Test Report #2
AN-977 LVDS Signal Quality: Jitter Measurements Using
Eye Patterns Test Report #1
For LVDS devices the power supply voltage plays a role in
long cable application (above 10 meters). This is because
the devices are current source devices. The higher VCC, the
longer the cable the device will be capable of driving.
REFERENCES
The following National Semiconductor Application Notes are
recommended readings.
AN-808
Long Transmission Lines and Data Signal Quality
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