EE539: Electromagnetic
Compatibility
LAB: Effect of PCB Trace Design on
Crosstalk-Report
Date: 4/20/11
Authors: Arun Kumar, Camerin Hahn
Table of Contents
1.
Introduction .......................................................................................................................................... 1
2.
Lab Setup............................................................................................................................................... 2
3.
Results ................................................................................................................................................... 3
3.1
Straight Through ........................................................................................................................... 4
3.2
“Snake” Line (Sharp Corners) ........................................................................................................ 5
3.3
Triangular Snake (Angular Bends) ................................................................................................. 6
3.4
Mismatched Line ........................................................................................................................... 7
3.5
Round & Square Corner ................................................................................................................ 9
4.
Conclusions ......................................................................................................................................... 11
5.
Recommendations .............................................................................................................................. 12
6.
References .......................................................................................................................................... 12
Effect of PCB Trace Design on Crosstalk
1. Introduction
Crosstalk refers to the “unintended electromagnetic coupling between wires and PCB (Printed
Circuit Boards) lands that are in close proximity” (Paul, 2006). Crosstalk is a near field coupling
issue that is faced primarily between wires and PCB . Crosstalk between wires in cables or
between PCB traces on microstrip type boards. Here since that source and the receptor are
within the system, the problem of crosstalk deals with the third concern in EMC design: the
design of the product such that it does not interfere with itself. Higher clock speeds, and lesser
real estate due gives rise to complex PCB trace routing with traces being at very close proximity
of one another. This causes unwanted coupling between lines and hence interference within.
We have studied these phenomena with simple setups that replicate the most basic PCB traces
across a board frequency range (3 MHz to 1 GHz) to narrow the effect of the design of PCB
routing on crosstalk.
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Effect of PCB Trace Design on Crosstalk
2. Lab Setup
The basic theory behind cross talk measurements is shown in Figure 2.1 below. A signal is fed into one of
the two parallel traces with the other end of that trace being terminated with a 50 Ohm load. The other
parallel trace input is terminated with a 50 Ohm load and its output connected to a spectrum analyzer.
The reason behind is setup is to see how much of the neighboring signal is coupled to the unintended
trace. Ideally we should see none of the power in the trace that is connected to the spectrum analyzer,
but due to coupling, especially at higher frequencies, we “see” some of the power on that trace as the
power increases as we go higher in frequency. The frequency we were sweeping from approximately
1.25 MHz to 1000 MHz. The equipment we used for this lab were: (1) Fluke signal generator, that
provided a sine wave with 0 dBm power swept from 1 MHz to 1000 MHz, (2) HP Spectrum Analyzer, (3)
Computer with Agilent VEE software that interfaces with the spectrum analyzer to obtain test data. Data
post processing was done with Matlab program.
Fluke Function Generator (0
dBm, Sine Wave, 1 MHz to
1000 MHz)
HP Spectrum Analyzer (50
mSec Sweep, 1 MHz to 1000
MHz)
PORT 1
SMA Termination
SMA Termination
DUT (Device Under Test)
PORT 2
Figure 2.1: Test Setup
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Effect of PCB Trace Design on Crosstalk
Fig 2.2: Actual Test Setup (Control Line Pictured)
3. Results
Key parameters we measured was the attenuation in power 2. Ideally the power level should be as close
to the noise floor (around 67 dBm for the particular Network Analyzer we used), but due to unwanted
coupling, crosstalk occurs and there is some power seen at port 2. We want this to be as close to the
noise floor as possible. We ran the test to different types of PCB traces to see which type(s) of design
were most prone to cross talk.
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Effect of PCB Trace Design on Crosstalk
3.1
Straight Through
This was the control setup. Both lines were of same impedance and ran parallel to one another.
Fig 3.1.1: Control Line PCB
The amount of power seen by the line that is affected by cross talk is shown below.
Fig 3.1.2: Control Link Crosstalk
We can see that although at the lower frequencies the amount of crosstalk seen is closer to the noise
floor of about -61 dBm, at higher frequencies; it increases significantly to about -36.51 dBm.
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Effect of PCB Trace Design on Crosstalk
3.2
“Snake” Line (Sharp Corners)
The PCB shown below was used to analyze traces with sharp bends and curves, along with a straight
trace.
Fig 3.2.1: PCB with sharp “snake” line bends
Fig 3.2.2: Crosstalk of PCB with “Snake” Lines
The results for the “snake” line show poorer isolation between the control and the line being studied.
The power seen throughout the band is higher than the control line, although at the highest frequency
they are very close to one another.
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Effect of PCB Trace Design on Crosstalk
3.3
Triangular Snake (Angular Bends)
The PCB shown below was used to analyze traces with sharp bends and curves, along with a straight
trace.
Fig 3.3.1: PCB with angular line bends
Fig 3.3.2: Crosstalk of PCB with angular bends
The results for the PCB traces with angular bends show a much poorer isolation between the lines,
resulting is much more power being seen on the control line. The lines were, at certain points, much
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Effect of PCB Trace Design on Crosstalk
closer to the control line carrying the power signal, which might have had a much more significant
power coupling effect.
3.4
Mismatched Line
The PCB shown below was used to analyze traces with a mismatch it between, along with a straight
trace. The impedance of the VSWR 1:2 mismatch shown in between was approximately 100 ohms.
Fig 3.4.1: PCB with angular line bends
Fig 3.4.2: Crosstalk of PCB with mismatch line
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Effect of PCB Trace Design on Crosstalk
The results for the PCB traces with the mismatch in between were higher through the frequency band.
At lower frequencies the crosstalk was very close to that of the control setup but at higher frequencies
they gradually increased.
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Effect of PCB Trace Design on Crosstalk
3.5
Round & Square Corner
The PCB shown below was used to analyze traces with a two types of corners, along with a straight
trace. These traces were to emulate those traces that not necessarily were along each other, but “met”
each other at certain points of the board. The two types were a square corner and a more round corner.
They were tested separated with the other ports not used for testing terminated with a 50 Ohm load.
Fig 3.5.1: PCB with square and round corners
Fig 3.5.2: Crosstalk of PCB with mismatch line
The results above are for the PCB traces with the different types of corners. Since the traces analyzed
were on the same PCB, the results were compared with each other, rather than with the control line.
Though there was crosstalk along both lines they were very close to one another with the square line
being more susceptible to crosstalk than the round line. It was noticed that both these corners had
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Effect of PCB Trace Design on Crosstalk
significantly less crosstalk than the control setup used initially. This might be due to the fact that both
these corners did not run the whole length along the signal carrying trace and only “met” with it at
certain points.
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Effect of PCB Trace Design on Crosstalk
4. Conclusions
The results from all the different traces studied above are summarized below.
Maximum
Minimum
Trace
Frequency
Amplitude
Trace
Frequency Amplitude
Type
(HZ)
(dBm)
Type
(HZ)
Miss-match
(dBm)
972500000
-41.07
Miss-match
20000000
-61.6
1000000000
-46.18
Round Corner
45000000
-61.96
Snake
757500000
-36.15
Snake
12500000
-61.19
Square Corner
957500000
-44.27
Square Corner
37500000
-61.31
1000000000
-36.24
Straight
17500000
-61.24
997500000
-29.65
Triangular Snake
12500000
-60.65
Round Corner
Straight
Triangular Snake
Table: 4.1 Summary of Results
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Effect of PCB Trace Design on Crosstalk
5. Recommendations
6. References

Paul, C.R (2006). Electromagnetic compatibility . New Jersey: Wiley.
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