Agilent EEsof EDA
Eye Diagram Measurements in Advanced Design
System
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Eye Diagram Measurements in
Advanced Design System
Page 1
Group/Presentation Title
Agilent Restricted
Month ##, 200X
Agenda
• Why an Eye Diagram ?
• Eye Diagram Measurements in ADS.
• Frequency Domain Analysis of an Interconnect.
• Optimization of an Equalizer Performance.
Eye Diagram Measurements in ADS
Page 2
Why create an Eye Diagram?
Difficult to Characterize a Waveform
Eye Diagram Measurements in ADS
Page 3
Interconnect/Transmitter- Good or Bad?
Eye Diagram Measurements in ADS
Page 4
Eye Diagram Functions In ADS
Two eye diagram functions in ADS
• eye(node_name, data_rate, number_of_cycles, delay)
• FrontPanel_eye( node_name, data_rate, number_of_cycles, delay)
Difference between two eye functions
Unit Interval
…
Eye Diagram Measurements in ADS
Page 5
Characterizing an Eye Diagram
Eye diagram parameters
• Eye level 1 & level 0
• Eye rise/ fall time
• Eye opening
• Eye width
50 psec
delay
• Eye height
• Eye amplitude
• Eye signal to noise ratio
• Peak to peak & RMS jitter
• …
FrontPanel_eye( node_name, data_rate, number_of_cycles, delay)
Eye Diagram Measurements in ADS
Page 6
Eye Binning Function
321 Rows
Eqn = eye_binning( eye_plot,num_x_bins,num_y_bins)
451 Columns
Number of Traces
Inner Independent
Outer Independent
Eye Diagram Measurements in ADS
Page 7
Histogram Plots
Majority of Eye calculations are based on histogram plots
Two types of histograms
• Horizontal histogram - For every time point,
sum up the number of traces across vertical
bins
FrontPanel_eye_horizontal_histogram()
• Vertical histogram – For every amplitude
point, sum up the number of points across time
axis
FrontPanel_eye_vertical_histogram_index()
Statistical calculation
• Mean value, Standard Deviation ( 1 σ ), and
Three Standard Deviation ( 3 σ )
Frontpanel_get_histogram_mean_stddev ()
Eye Diagram Measurements in ADS
Page 8
Calculate Eye Delay
Eqn=FrontPanel_eye_delay(Waveform_Data,Default_DataRate,Data_Type)
Function calculates the time delay required to position eye crossing at
the center of the UI (Only applicable to NRZ type data)
Creates Eye Diagram
Convert Eye data to bin data
Determine Max and Min amplitude
Create horizontal histogram in a narrow strip
Find Mean value of horizontal histogram
Compare Mean position as compared with UI/2
Calculated Delay
Delay calculation is required for automated eye parameter measurements.
Binning the eye diagram makes this calculation easy.
Eye Diagram Measurements in ADS
Page 9
Eye Crossing Measurements
Create horizontal
histogram across a
narrow central strip
The mean value
provides eye crossing
time
Create vertical
histogram @ mean
crossing time points
The mean value will
provide eye crossing
amplitude
Mean Value of Horizontal Histogram
Give Eye Crossing Time Value
Mean Value (Eye Crossing Amplitude Point)
Eye Diagram Measurements in ADS
Page 10
Measurements of Eye Level One/Zero
Determine Eye
Crossing points ( t1, t2)
Level one
Upper half
Determine
(40-60% ) Eye
Boundaries
Calculate vertical
histograms in upper half
region
Measurement boundaries
0% 40% 60% 100%
Calculate Level One
Mean & Standard
Deviation
Lower half
Level zero
Calculate vertical
histograms in lower half
region
Calculate Level Zero
Mean & Standard
Deviation
Eye Diagram Measurements in ADS
Page 11
Measurements of Eye Amplitude, Height, and S/N
40-60% Region
Eye Height
Eye Amplitude
One level
histogram
mean
Zero level
histogram
mean
1σ
Upper half
3σ
3σ
1σ
Lower half
Eye Amplitude = Level One – Level Zero
Eye Height = (Eye level one- 3σ)- (Eye level zero+3σ)
Eye S/N= (Eye level one-Eye level zero)
1σ level one+1σ level zero
Eye Diagram Measurements in ADS
Page 12
Measurement of Eye Width
Calculate Eye Crossing
points
Calculate Horizontal
Histograms around Eye
Crossing Amplitude
Calculate Mean value
and Standard Deviation
t2
t1
3σ
Eye Width
3σ
Calculate Eye Width
Eye width = (Eye crossing@t2- 3σ)- (Eye crossing@t1+3σ)
Eye Diagram Measurements in ADS
Page 13
Calculating Rise Time
Plot single Eye Diagram
with delay
Determine Level One
and Level Zero
Get the Rising Edges
which qualifies for
all three thresholds
Calculate Horizontal
Histogram for Qualified
Edge @ all three
threshold values
Calculate Mean value
for all three histograms
Rise time is the
difference between the
Mean value of
Histograms @ low and
high threshold
Horizontal histogram
plot for 20% threshold
Horizontal histogram
plot for 50% threshold
Horizontal histogram
plot for 80% threshold
Eye Diagram Measurements in ADS
Page 14
Jitter Histogram
Jitter (p-p)
Jitter Histogram - Horizontal histogram across Eye Crossing point.
Standard Deviations - Represents RMS Jitter ( If only Random Jitter is present)
Peak to Peak Jitter is the histogram width
Eye Diagram Measurements in ADS
Page 15
Eye Diagram FrontPanel
• Automated measurements using Eye Diagram FrontPanel
In addition to eye diagram measurements, it performs many waveform
parameter measurements
Eye Diagram Measurements in ADS
Page 16
Compare Eye FP to DCA on same waveform
ADS Eye Diagram FrontPanel
1G Data Rate through 20” trace
Eye Diagram FrontPanel on the DCA output file.
DCA Measurement
FrontPanel
DCA typ
DCA min/max
Level 1
212mV
211.3mV
211.3/213.5
Level 0
-177.4mV
-177.5mV
-177.5/-178.5
Rise Time
219pS
222pS
219/226
Fall Time
212pS
215pS
211/219
Eye Amp
389.4mV
389mV
386.3/389.4
Eye Height
274.7mV
274.7mV
273.9/274.9
Eye S/N
10.18
10.07
10.01/10.23
Jitter p-p
22.2pS
22.2pS
22.2/22.2
Jitter rms
5.8pS
5.7pS
5.7/5.8
[NOTE: FrontPanel data is from a single trace.
DCA measurements are averaged with 16
measurements.]
Eye Diagram Measurements in ADS
Page 17
Case Study
Design an equalizer to improve eye diagram performance
– Frequency domain simulator to predict channel performance
– Simulation of a channel with equalizer
– Optimization of an equalizer for eye performance
Eye Diagram Measurements in ADS
Page 18
Channel Simulation
Various interconnect
models
Eye Diagram Measurements in ADS
Page 19
Time Domain Simulation (Convolution)
S-Parameter representation
of the channel
For this example, channel is just a
passive interconnect.
Can one use frequency domain
simulators?
Eye Diagram Measurements in ADS
Page 20
Why use Frequency Domain Simulators?
• No exclusive time domain components (IBIS model)
• Fast simulation
• Accurately accounts for all the frequency domain effects
• Availability of frequency domain bit sequence source
• No convergence issue
• Voltage at any node can be converted to time domain
• Eye diagram measurements can be accomplished
• Extremely fast and accurate simulation
Requirement: S-parameter/models should have sufficient bandwidth to
satisfy the time sampling requirement
Limitation: Cannot be used with IBIS models
(Rise and fall tables are defined as the timing waveforms )
Eye Diagram Measurements in ADS
Page 21
AC Simulation of Channel Model
What we are trying to accomplish?
Frequency domain simulation Æ Time domain characterization
AC simulation setup
Eye Diagram Measurements in ADS
Page 22
Comparing Time and Frequency Domain
Simulation Results
Time Domain Simulation
Frequency Domain Simulation
Comparison shows good
correlation.
Calculate eye parameters?
Eye Diagram Measurements in ADS
Page 23
Characterizing Eye Diagram using
ADS FrontPanel
Eye diagram
measurements
Can one measure eye parameters during simulation?
What are the advantages?
Eye Diagram Measurements in ADS
Page 24
Eye Diagram Measurements during Simulation
Eye diagram measurement equations defined
on the schematic page.
Eye Diagram Measurements in ADS
Page 25
What we are achieving?
Frequency domain simulation Æ Time Domain Characterization
• Running AC simulation
• Plotting time domain waveform
• Plotting eye diagram measurements
Advantage
• Fast and accurate channel simulation
• No convergence issues
• Measure eye diagram performance
Download example project from ADS knowledge website.
Provides simulation setup and measurement equations.
Eye Diagram Measurements in ADS
Page 26
Eye Diagram Measurements
Is Eye Performance Acceptable?
How to improve eye diagram performance?
Eye Diagram Measurements in ADS
Page 27
Why Equalization?
As data rate increases, the frequency response and attenuation
characteristics of PCB traces degrade signals
Enables channel to operate at higher data rates
faster, better, cheaper
How to negate channel
effects?
Eye Diagram Measurements in ADS
Page 28
Consider Passive Equalizer Design
A high pass filter
How to determine equalizer parameters?
Eye Diagram Measurements in ADS
Page 29
Comparing Simulation Results
Without Equalizer
With Equalizer
Eye height and Jitterp-p is improved.
Can equalizer design be further improved?
Eye Diagram Measurements in ADS
Page 30
Optimize Equalizer Design for Eye Opening
The goal is to optimize equalizer design to
maximize Eye Opening
Parameter to be
optimized
Goal: Improve Eye Opening Factor from 0.59 to 0.9
Eye Diagram Measurements in ADS
Page 31
Eye Parameter Measurements and Optimization
Frequency Domain
Simulation
Optimizing Eye
Opening Factor
Eye Measurements
Change in the reactive element value will change the flight time.
Will the optimization goals works with varying flight time?
Eye Diagram Measurements in ADS
Page 32
Advantages
Frequency domain simulation Æ Time domain characterization
Æ Time domain optimization
• Works well even if the flight time delay is changed due to change
in the reactive element value.
• Automatically calculates delay required for eye positioning
• Automatically detects eye crossing point and 40-60% region
• Optimize eye diagram performance
Any eye diagram parameter such as eye opening factor, eye height,
peak to peak jitter, rise time … can be used as an optimization goal.
• Unique and extremely powerful
• Essential for state of the art serial link design
Eye Diagram Measurements in ADS
Page 33
Optimizing Eye Diagram Parameters
Limitation: One cannot send Eye Diagram Measurements to data
display during optimization
Deactivate output Node Voltage and Measurement Equations while
performing optimization ( Defined in Simulator Controller)
Eye Diagram Measurements in ADS
Page 34
Optimized Value of Equalizer Components
Eye Diagram Measurements in ADS
Page 35
Comparing Simulation Result
ADS Eye Diagram plots
Eye Diagram without equalizer
Eye Diagram with optimized equalizer
Improvement in Jitter
Eye Diagram Measurements in ADS
Page 36
ADS Waveform in DCA – without Equalization
ADS simulated waveform was
sent to DCA for post processing
ADS Eye Diagram
DCA Eye Diagram display showing ADS
simulated waveform
Eye Diagram Measurements in ADS
Page 37
ADS Waveform in DCA with Equalization
DCA Eye Diagram display showing ADS
simulated waveform
ADS Eye Diagram
Eye Jitterp-p
15.7 ps
Eye Signal to Noise
Eye Rise Time
4.75
33 psec
Eye Diagram Measurements in ADS
Page 38
Comparing Simulation Results
Channel+ Equalizer before Optimization
Channel+ Equalizer after Optimization
Optimizer type
– Random optimizer
Number of iterations – 40
Optimization time
– 15 Minutes
Eye Diagram Measurements in ADS
Page 39
Conclusion
• ADS has been used for SI design for over 20 years
• ADS has a multitude of accurate built in models
• ADS allows you to build accurate physical models
• ADS brings IP, simulation and measurement together
• Reviewed Eye Diagram measurement algorithms in ADS
• ADS allows you to predict time domain performance using frequency
domain simulators
• Allows you to characterize and optimize Eye Diagram Performance
You can optimize eye diagram performance not only using the AC simulator,
but ADS also allows you to optimize interconnects for eye diagram
performance using numeric, time and other frequency domain simulators.
Eye Diagram Measurements in ADS
Page 40
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Revised: March 27, 2008
Product specifications and descriptions
in this document subject to change
without notice.
© Agilent Technologies, Inc. 2008
Printed in USA, May 26, 2006
5989-9453EN