Crosstalk Measurement Techniques for Multi-Channel and Multi-Rate High Speed
Serial Communication Systems
By (Jeff Hushley, Senior Applications Engineer, Cypress Semiconductor Corp. and
Palani Subbiah, Senior Staff Applications Engineer, Cypress Semiconductor Corp.)
Executive Summary
Due to the pitfalls facing parallel systems at high operating frequencies (skew, timing budget, and layout constraints), many
systems are converting to a serial interface to transport information. These serial interfaces may be designed to support
multiple standards (e.g. SD-SDI and HD-SDI in digital video broadcast systems, USB and Firewire in data transfer systems,
video streams with different frame resolutions/rates in dual HDMI/DVI systems) and thus, multiple data rates. In fact, there are
serial interfaces that can simultaneously transmit different standards across multiple channels, integrated into one device (e.g.
Quad Independent-channel SERDES). Therefore, the device will have high-speed signals switching at different rates. This
situation raises the question: “Will there be any interference between these signals?”
Crosstalk is the effect on a signal caused by the high-speed switching of a nearby signal. This effect can manifest itself as
jitter, which is the deviation of a signal’s edge from its expected location. A large amount of jitter can cause a timing budget
failure in a parallel system or it can cause a clock and data recovery PLL to incorrectly recover the data in a serial system.
Due to the deleterious effects of crosstalk, it is important to determine the amount of crosstalk that exists during the worst case
scenarios. Currently, there are no standard crosstalk measurement techniques for the serial domain. This article will describe
effective measurement techniques and how to determine if the amount of crosstalk is acceptable for reliable data transfer.
The techniques described will include measuring the device’s jitter output with a wide-bandwidth oscilloscope and spectral
output with a high-bandwidth spectrum analyzer. Also, the configurations that will yield the highest crosstalk scenarios will be
described. Real measurement data of a multi-channel, independent rate device will be provided. The measurements are
performed at video serial digital interface (SDI) data rates because of the common application of transmitting multiple video
standards. However, the measurement techniques apply to any high-speed standard.
Crosstalk
Crosstalk is the effect on a signal trace caused by cross-coupling with a neighboring trace. The trace that is being measured is
referred to as the victim. The trace that is cross-coupled with the victim trace is referred to as the aggressor. A picture of this
relationship is shown in Figure 1 .
Figure 1. Victim and Aggressor Traces
Aggressor trace
Crosstalk
Victim trace
Measuring
Equipment
Crosstalk Measurement Techniques for Multi-Channel and Multi-rate High Speed Serial Communication Systems
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Crosstalk is dependent upon the edge rate of the aggressor signal; a faster edge induces more crosstalk. When the operating
frequency of a transmitter is increased, the transmitter usually increases its edge rate to improve the signal noise margin.
Therefore, to test the worst case the aggressor channels should switch at the highest frequency (See advanced crosstalk
measurement section for more detailed ananlysis ).
Jitter
Crosstalk is a concern because it can be a major contributor to the amount of jitter present in a device. Simply stated, jitter is
the deviation of an edge from its expected location. A large amount of jitter in a serial communications link may cause bit
errors in the received serial bit stream.
Phase-Locked Loops
Within any serializer/deserializer (SERDES) device, there is a transmit PLL and a receive PLL. A quad independent channel
SERDES has four transmit and receive pairs, where each pair has its own reference clock. A concern is that when adjacent
PLL’s are switching at different frequencies, additional crosstalk may occur. This section will briefly describe the structure of
the transmit PLL and the tests that will be performed to measure its performance. The receive PLL has similar frequency
response characteristics and will be tested similarly.
The transmit PLL is a clock multiplier unit (CMU). It receives an input clock (REFCLK) and outputs a bit clock that has ten
times the frequency of REFCLK. A diagram of a transmit PLL is shown in Figure 2 . The amount of jitter that propagates
through a PLL depends on where the jitter entered the PLL. If it enters via the REFCLK input, only low frequency components
will pass through because the PLL acts like a low-pass filter. Low-frequency jitter has a smaller impact on receiver
performance because a clock and data recovery (CDR) PLL will track the low-frequency jitter and correctly receive the data.
However, if jitter is injected inside the loop (by crosstalk between PLLs, for example), the system acts like a high-pass filter.
Therefore, crosstalk between the PLLs can be a cause of harmful, high-frequency jitter. To test the performance of the PLLs,
three frequency variations between victim and aggressor REFCLKs will be performed: large frequency offsets (>100 MHz),
small frequency offsets (< 1 MHz), and nearly identical frequencies (<1 kHz) from different sources.
Figure 2. Transmit PLL Block Diagram
SHIFTER
REFCLK
VCON
PHASE UP
FREQ DOWN FILTER
DET
VCO/10
VCO
SERIAL
DATA
BIT CLOCK
DIVIDE BY 10
Jitter and Crosstalk Measurement Techniques
Measuring crosstalk can be performed in two ways: Jitter in the time domain and crosstalk in the frequency domain.
Measuring Crosstalk in the Time Domain
To measure jitter in the time domain, the eye diagram of the victim channel’s output can be observed on a wide-bandwidth
oscilloscope. An eye diagram is formed by a superposition of waveforms. The phase of these waveforms with respect to one
another is determined by the phase difference with the trigger signal. The amount of jitter will be obtained by analyzing the
histogram formed at the eye crossing (shown in Figure 3). The eye crossing is the point at which the positive edge and
negative edge intersect. When the waveform intersects the histogram window, a “hit” is recorded in the histogram. The
histogram formed in Figure 3 resembles a Gaussian distribution.
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Figure 3. Jitter Measurements at the Eye Crossing
Peak-peak
Jitter
Histogram
Window
Histogram
The two important values obtained from this measurement are the peak-to-peak jitter and the root mean square (RMS) value.
The peak-to-peak jitter is the difference between the minimum and maximum time of hits in the histogram. This value is nondeterministic because of the nature of random jitter, but can provide helpful information if the test is performed for a long
period of time. The RMS value (or standard deviation) converges to a stable value quickly. Assuming a Gaussian distribution,
12
it can be proven that the peak-to-peak jitter will be greater than fourteen times the RMS value less than once in 10 bit
-12
periods. Therefore, to ensure a 10 bit error rate, the jitter tolerance of the receiver should be greater than 14 times the RMS
value.
When testing for crosstalk in the time domain, the data pattern can be any pattern (as opposed to specific periodic sequences
mentioned in the frequency domain section). This way, the true jitter budget in a real-world system can be tested.
To determine the effect of crosstalk in the time domain, measure the increase in jitter on the victim channel between no
aggressors and with aggressors. The figure below shows an example of this measurement. In this example, the victim
channel is transmitting a pseudo-random Built-In Self Test (BIST) sequence at 1485Mbps. The aggressor channel (when
enabled) is transmitting the highest possible frequency signal, which for this device is a 1010 pattern at 1500Mbps . The RMS
jitter (aka Standard Deviation) increased from ~13.5ps with no aggressors to ~14.2ps with aggressors. This translates to an
-12
increase in total jitter of ~10ps when targeting a BER of 10 .
Figure 4. Time Domain Comparison Between Aggressors Disabled (Left) and Aggressors Enabled (Right)
Victim Channel with No Aggressors Enabled
Victim Channel with Aggressors Enabled
Crosstalk Measurement Techniques for Multi-Channel and Multi-rate High Speed Serial Communication Systems
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Measuring Crosstalk in the Frequency Domain
To measure crosstalk in the frequency domain, a high bandwidth spectrum analyzer can be used. The amplitude of the
victim’s fundamental frequency can be compared with the amplitude of the aggressor’s fundamental frequency. The
fundamental frequency of a signal is the lowest natural frequency and is also called the first harmonic. For example, a perfect
square wave 20-MHz clock has a fundamental frequency of 20 MHz. Furthermore, it is composed of a sum of sinusoids at
multiples (or harmonics) of the fundamental frequency. The most prevalent harmonics of a square wave are the odd
harmonics. The first (20 MHz, in this example), third (60 MHz), and fifth (100 MHz) harmonics contribute to most of the shape
of the square wave. Thus, if the crosstalk components are significantly smaller than the fifth harmonic, their impact will be
negligible.
When looking at the frequency response of a fairly random data pattern, the spectrum will not only have peaks at the odd
harmonics. Instead, the energy will be more distributed across the frequency range. In this case, it will be difficult to see the
effects of crosstalk because the spectral energy floor may be higher than the crosstalk energy. Therefore, when measuring
crosstalk on serial data transmitters, the most effective pattern is a 1010 periodic pattern. This pattern will look like a square
wave and thus, will have a low energy floor when not at the odd harmonics.
Figure 5 shows the spectrum of a victim channel that is transmitting a 1111100000 pattern at 270Mbps (SD-SDI serial
signalling rate) while the aggressor channel is transmitting a 1010101010 pattern at 1485Mbps (HD-SDI serial signalling rate).
In this case, the victim channel is equivalently transmitting a 27MHz square wave (270Mbps / 10bits per cycle) and the
aggressor is equivalently transmitting a 742.5MHz square wave (1485Mbps / 2bits per cycle). The top graph in Figure 5
shows the energy (in dB) vs. frequency from 0 to 800MHz. Every gridline on the energy axis represents 7dB. Since this a realworld signal, there are some even harmonics, but they all have < -50dB (0.3%) the energy of the fundamental. The bottom
graph in Figure 5 is zoomed in on the 125-165MHz region to show the largest crosstalk peak in the victim channel. This peak
th
is at 148.5MHz, which is the character rate of the aggressor channel. The other peak is the 5 harmonic of the victim signal at
135MHz (5 * 27MHz). In this case, the affect of crosstalk is almost negligible because the largest crosstalk peak has ~ -28dB
(4%) as much energy as the fifth harmonic. In the top graph in Figure 5, the aggressor’s serial data fundamental frequency
(742.5MHz) can also be seen. It has approximately the same energy as the peak at 148.5MHz. All other crosstalk peaks at
>1GHz are smaller.
In the above example, it shows the effect of crosstalk from the aggressor’s character clock and from the aggressor’s serial
data. Crosstalk from the aggressor’s character clock may be because the character clock traces are routed to close together.
Crosstalk from the aggressor’s serial data may be because the serial I/Os are not isolated well enough or that the serial PCB
traces are too close together. If there was crosstalk at the aggressor’s bit clock frequency(in this case, 1485MHz), then the
PLLs may be interfering with each other. This information can help the board or chip designer narrow down the cause of the
crosstalk.
Figure 5. Spectral Plot of Victim Channel
5th
harmonic
Crosstalk
Crosstalk Measurement Techniques for Multi-Channel and Multi-rate High Speed Serial Communication Systems
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Crosstalk Measurement Equipment Setup
Figure 6 shows an example of a crosstalk measurement equipment setup for a multi-channel high-speed serial transmission
device, which in this case is a Cypress CYV15G0404DXB Independent Channel Serializer/Deserializer. The reference
character clock for each channel (REFCLKx) will be supplied by a different Agilent 8133A Pulse Generator. The RMS jitter
from the pulse generator should be low because its jitter directly relates to the jitter of the serial outputs. Therefore, minimizing
the reference clock jitter will make it easier to see the effect of crosstalk on the serial data path. The Agilent 8133A’s RMS jitter
is less than 5 ps (1 ps typical). The measurements performed on the transmit side will be taken at the serial output OUTA+.
The other channels can operate at independent data rates.
The time-domain jitter measurement would be performed with an Agilent 86100A Wide-Bandwidth Oscilloscope. The
frequency-domain measurement would be performed with an Agilent E4407B Spectrum Analyzer. The screenshot in Error!
Reference source not found. is from the oscilloscope and the screenshot in Figure 5 is from the spectrum analyzer.
To determine the worst case amount of crosstalk, all channels should be enabled and the transmitted signal should be looped
back into the receiver to maximize the amout of crosstalk. When debugging the cause of the crosstalk, the approach may be to
enable one channel at a time and observe the configuration that causes the largest increase in jitter or causes the highest
energy peak at a frequency related to the aggressor channel.
Figure 6. Crosstalk Measurement Equipment Setup Example
Agilent 8133 Pulse Generator
8133A
Cypress CYV15G0404DXB
Quad Independent Channel
SERDES Evaluation Board
3 GHz
Puls e Ge ne ra tor
FLASE / DAT A
ME MOR Y
TRI GGE R
FLASE
Agilent 86100 Oscilloscope
SETTING
861 00A
infiniium DCA
W ide-Bandwidth
Os cillosc ope
Smp
Single
Run
Clear
D is play
Hori zontal
EXTERN AL
IN PU T
O
CHAN NEL 2
OU TPUT
CHAN NEL 1
OU TPUT
OU TPUT
OU TPUT
Default
Setup
OU TPUT
Cal
I
OUT
Aut o
Scale
Local
1
Eye/Mask
Mode
2
3
Agilent 83494A
SELECT
REFCLK
B
Os cillos cope
Mode
B
Op ti cal
TRI GGE R
OUTPU T
T rigger
Level
+
IN
Puls e Generator
FLASE / DAT A
Cl oc k
Auxili ary Outputs
In put
3 GHz
ME MOR Y
Vertica l
Data
Quick
Meas ure
M ar kers
Source
8133A
4
86106A
SIN GLE MODE CLOC K R EC OVER Y MOD ULE
TDR/TDT
Mode
Free R un
Front Panel
Left M odule
R ight Modul e
P robe Power
10mW Max Opt ic al
+
_ 2V Max D C
50 Ω
+
_ 2V Max
_
FLASE
SETTING
EXTERN AL
IN PU T
O
CHAN NEL 2
OU TPUT
CHAN NEL 1
OU TPUT
OU TPUT
OU TPUT
OU TPUT
I
OUT
REFCLK
A
8133A
A
IN
3 GHz
Puls e Generator
FLASE / DAT A
ME MOR Y
TRI GGE R
Agilent E4407 Spectrum
Analyzer
FLASE
E4407B
ESA -E SE RIES SPEC TR UM ANA LY ZER
SETTING
CONTROL
F REQUENCY
Channel
O
CHAN NEL 2
OU TPUT
SYSTEM
In Out /
Output
MODE
A MPLITUDE
Y S cale
Display
BW /
A vg
Tr ig
Save
MEA S URE
Menu
Se tup
Sing e
l
Sw eep
Ma rker
Restar t
Menu
Co ntr ol
Sour ce
Mod e
Setup
Syst em
Freq
Coun t
Pr eset
Fil e
S PA N
X Sca e
l
Ω
EXTERN AL
IN PU T
Pr n
it
Se tup
Coup e
l
Pr n
it
CHAN NEL 1
OU TPUT
OU TPUT
OU TPUT
OU TPUT
I
Peak
Se ar ch
MARKE R
OUT
REFCLKD
8
9
4
5
6
1
2
3
0
.
+/-
Rx
Sp
On
Help
D
Next
W ind ow
Zo o m
IF IN PU T
RF OUT 50 Ω
9 kH z - 3GHz
50 VDC MA X
R EV PW R
8133A
7
Marker
LO OUTP UT
INPUT 50 Ω
Ent er
PR OBE POWER
EXT
KEY BOARD
V OLUME
TE RM 1M 50 Ω
IN
3 GHz
Puls e Generator
FLASE / DAT A
ME MOR Y
TRI GGE R
FLASE
SETTING
EXTERN AL
IN PU T
O
CHAN NEL 2
OU TPUT
CHAN NEL 1
OU TPUT
OU TPUT
OU TPUT
OU TPUT
I
OUT
REFCLKC
C
IN
Conclusion
Using the time-domain measurement methodology, we can get a better idea of how crosstalk is impacting the performance of
the system. Since jitter is the cause of bit errors, this measurement will help to determine the jitter budget of the system. Also,
the data pattern can be any normal data pattern (e.g. PRBS 23), which enables real-world system analysis.
The frequency-domain measurement methodology is a useful tool for determining the cause of the crosstalk. The spectrum
analyzer screenshots provide an easy way of detecting peaks that are not part of the original signal. The frequency of those
peaks can be used to determine which aggressor signals are causing the largest impact on the system and to determine
where (PLL, signal traces, I/O buffers, etc.) the crosstalk is occurring.
Crosstalk Measurement Techniques for Multi-Channel and Multi-rate High Speed Serial Communication Systems
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References
HOTLink™ Jitter Characteristics. Cypress Semiconductor application note. March 11, 1999. <http://www.cypress.com/
cfuploads/support/app_notes/jitter.pdf>
Measuring Crosstalk in LVDS Systems. Cole, Elliot. Texas Instruments. January 2000. <http://focus.ti.com/lit/an/
slla064/slla064.pdf>
Cypress Semiconductor
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San Jose, CA 95134-1709
Phone: 408-943-2600
Fax: 408-943-4730
http://www.cypress.com
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