Oscilloscope Bandwidth
Requirements for Emerging
Serial Data Interfaces
Bandwidth Requirements
Page 1
February 2008
Bandwidth Requirements
What best determines bandwidth requirements?
5th harmonic?
Or spectral content of the signal, which is related to rise
time?
Rise time measurements on controlled signals
What rise time does a typical PCIe 2.0 signal actually exhibit?
What effect do fixtures have on rise times?
Is there such a thing as too much bandwidth?
Bandwidth Requirements
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Oscillioscope BW Requirements
February 2008
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What Do You Think?
Which do you think best predicts the required scope rise time:
•The data rate?
•The rise time of the signal?
Bandwidth Requirements
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February 2008
Two Examples
620 Mb/s
10 Gb/s
Bandwidth Requirements
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Oscillioscope BW Requirements
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10 Gb/s
Bandwidth Requirements
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February 2008
620 Mb/s
Bandwidth Requirements
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Oscillioscope BW Requirements
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Spectra
620 Mb/s
10 Gb/s
Bandwidth Requirements
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February 2008
10 Gb/s Spectrum
Bandwidth Requirements
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Oscillioscope BW Requirements
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10 Gb/s Fifth Harmonic
Bandwidth Requirements
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February 2008
620 Mb/s Fifth Harmonic
Bandwidth Requirements
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Oscillioscope BW Requirements
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5
620 Mb/s 25th Harmonic
Bandwidth Requirements
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February 2008
Changing Bit Rate With Constant Rise Time
Rise time = 35 ps
4 Gb/s
5th Harmonic = 4%
6 Gb/s
5th Harmonic = 1.4%
1 Gb/s
5th Harmonic = 8%
Bandwidth Requirements
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Oscillioscope BW Requirements
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Changing Rise Time With Constant Bit Rate
Bit rate = 4 Gb/s
Rise time = 50ps
5th Harmonic = 4.1%
Rise time = 66ps
5th Harmonic = 2.1%
Rise time =15ps
5th Harmonic = 7.4%
Bandwidth Requirements
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February 2008
Conclusion
The rise time is a better predictor of the required bandwidth than
the bit rate.
Bandwidth Requirements
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Oscillioscope BW Requirements
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7
Measurements
Signals with known, controlled rise times generated by test
sources
32 ps, 44 ps, 87 ps
Mean, standard deviation, range
At various oscilloscope bandwidths
An actual PCI Express 2.0 device using the PCI SIG 2.0 CBB
fixture
A mated pair of PCIe gen2 test fixtures
A mated pair of SATA test fixtures
Bandwidth Requirements
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February 2008
Controlled Rise Times
Nominal rise Scope and Mean
time
Bandwidth Error
Range
Std dev
DCA
reference
32 ps
9.83 ps
1.53 ps
31.79 ps
Tek 20 GHz
34.76 ps
9%
44 ps
87 ps
Tek 13 GHz
36.18 ps
4.75 ps
0.788 ps
31.79 ps
Agilent 13 GHz
14%
36.04 ps
7.57 ps
1.21 ps
32.07 ps
Tek 20 GHz
12%
45.5 ps
10.74 ps
1.56 ps
43.91 ps
Tek 13 GHz
4%
45.98 ps
7.41 ps
1.07 ps
43.91 ps
Agilent 13 GHz
5%
45.99 ps
8.33 ps
1.25 ps
44.13 ps
Tek 6 GHz
4%
86.7 ps
9.02 ps
1.4 ps
86.97 ps
Agilent 6 GHz
0.03%
84.34 ps
8.07 ps
1.29 ps
86.95 ps
3%
Bandwidth Requirements
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Oscillioscope BW Requirements
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Controlled Rise Times
Nominal rise Scope and Mean
time
Bandwidth Error
Range
Std dev
DCA
reference
32 ps
9.83 ps
1.53 ps
31.79 ps
Tek 20 GHz
34.76 ps
9%
44 ps
87 ps
Tek 13 GHz
36.18 ps
4.75 ps
0.788 ps
31.79 ps
Agilent 13 GHz
14%
36.04 ps
7.57 ps
1.21 ps
32.07 ps
Tek 20 GHz
12%
45.5 ps
10.74 ps
1.56 ps
43.91 ps
Tek 13 GHz
4%
45.98 ps
7.41 ps
1.07 ps
43.91 ps
Agilent 13 GHz
5%
45.99 ps
8.33 ps
1.25 ps
44.13 ps
Tek 6 GHz
4%
86.7 ps
9.02 ps
1.4 ps
86.97 ps
Agilent 6 GHz
0.03%
84.34 ps
8.07 ps
1.29 ps
86.95 ps
3%
Bandwidth Requirements
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February 2008
Controlled Rise Times
Nominal rise Scope and Mean
time
Bandwidth Error
Range
Std dev
DCA
reference
32 ps
9.83 ps
1.53 ps
31.79 ps
Tek 20 GHz
34.76 ps
9%
44 ps
87 ps
Tek 13 GHz
36.18 ps
4.75 ps
0.788 ps
31.79 ps
Agilent 13 GHz
14%
36.04 ps
7.57 ps
1.21 ps
32.07 ps
Tek 20 GHz
12%
45.5 ps
10.74 ps
1.56 ps
43.91 ps
Tek 13 GHz
4%
45.98 ps
7.41 ps
1.07 ps
43.91 ps
Agilent 13 GHz
5%
45.99 ps
8.33 ps
1.25 ps
44.13 ps
Tek 6 GHz
4%
86.7 ps
9.02 ps
1.4 ps
86.97 ps
Agilent 6 GHz
0.03%
84.34 ps
8.07 ps
1.29 ps
86.95 ps
3%
Bandwidth Requirements
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Oscillioscope BW Requirements
February 2008
9
Controlled Rise Times
Nominal rise Scope and Mean
time
Bandwidth Error
Range
Std dev
DCA
reference
32 ps
9.83 ps
1.53 ps
31.79 ps
Tek 20 GHz
34.76 ps
9%
44 ps
87 ps
Tek 13 GHz
36.18 ps
4.75 ps
0.788 ps
31.79 ps
Agilent 13 GHz
14%
36.04 ps
7.57 ps
1.21 ps
32.07 ps
Tek 20 GHz
12%
45.5 ps
10.74 ps
1.56 ps
43.91 ps
Tek 13 GHz
4%
45.98 ps
7.41 ps
1.07 ps
43.91 ps
Agilent 13 GHz
5%
45.99 ps
8.33 ps
1.25 ps
44.13 ps
Tek 6 GHz
4%
86.7 ps
9.02 ps
1.4 ps
86.97 ps
Agilent 6 GHz
0.03%
84.34 ps
8.07 ps
1.29 ps
86.95 ps
3%
Bandwidth Requirements
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February 2008
Controlled Rise Times
Nominal rise Scope and Mean
time
Bandwidth Error
Range
Std dev
DCA
reference
32 ps
9.83 ps
1.53 ps
31.79 ps
Tek 20 GHz
34.76 ps
9%
44 ps
87 ps
Tek 13 GHz
36.18 ps
4.75 ps
0.788 ps
31.79 ps
Agilent 13 GHz
14%
36.04 ps
7.57 ps
1.21 ps
32.07 ps
Tek 20 GHz
12%
45.5 ps
10.74 ps
1.56 ps
43.91 ps
Tek 13 GHz
4%
45.98 ps
7.41 ps
1.07 ps
43.91 ps
Agilent 13 GHz
5%
45.99 ps
8.33 ps
1.25 ps
44.13 ps
Tek 6 GHz
4%
86.7 ps
9.02 ps
1.4 ps
86.97 ps
Agilent 6 GHz
0.03%
84.34 ps
8.07 ps
1.29 ps
86.95 ps
3%
Bandwidth Requirements
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Oscillioscope BW Requirements
February 2008
10
Spectrum Analyzer Measurements – 35 ps
Bandwidth Requirements
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February 2008
Spectrum Analyzer Measurements – 35 ps
Bandwidth Requirements
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Oscillioscope BW Requirements
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Spectrum Analyzer Measurements – 35 ps
Bandwidth Requirements
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February 2008
Spectrum Analyzer Measurements – 45 ps
Bandwidth Requirements
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Oscillioscope BW Requirements
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Spectrum Analyzer Measurements – 45 ps
Bandwidth Requirements
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February 2008
What Are You Working On?
Bit rate
Rise time (20-80)
<2 Gb/s
<35 ps
2-4 Gb/s
35-45 ps
4-8 Gb/s
45-65 ps
8-10 Gb/s
>65 ps
>10 Gb/s
Bandwidth Requirements
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Oscillioscope BW Requirements
February 2008
13
PCIe Gen2 Device
In this section we will examine “real world” signals from a PCIe
Gen2 device, using a PCI SIG PCIe2.0 CBB as the test fixture.
We will examine the signal with both an oscilloscope and a
spectrum analyzer.
The device is a prototype PCIe Gen2 X16 graphics card.
The signal is the PCIe 40-bit compliance test pattern.
Bandwidth Requirements
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February 2008
Rise Time Measurements vs Bandwidth
Scope
Bandwidth
Measured mean
20-80 rise time
13 GHz
10 GHz
8 GHz
6 GHz
80 ps
80 ps
80 ps
86 ps
Bandwidth Requirements
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Oscillioscope BW Requirements
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Spectrum Analysis of PCIe Gen2 Signal
Bandwidth Requirements
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February 2008
Spectrum Analysis of PCIe Gen2 Signal
Bandwidth Requirements
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Oscillioscope BW Requirements
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Mated Pair of PCIe Gen2 Test Fixtures
PCIe Gen2
CLB
PCIe Gen2
CBB
Bandwidth Requirements
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February 2008
Mated Pair of PCIe Gen2 Test Fixtures
Bandwidth Requirements
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Oscillioscope BW Requirements
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Mated Pairs of SATA Test Fixtures
Bandwidth Requirements
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February 2008
S21, Mated Pairs of SATA Test Fixtures
SDD21 (Gain) vs. Frequency
0
-0.5
-1
-1.5
Gain (dB)
-2
-2.5
-3
-3.5
-4
-4.5
02_Comax_eSATA_01080078_pair65_TDT.s4p
03_Comax_eSATA_01110077_pair65_TDT.s4p
04_Comax_eSATA_01120079_pair65_TDT.s4p
05_Comax_eSATA_01130081_pair65_TDT.s4p
01_Comax_iSATA_00060005_pair65_TDT.s4p
-5
1
2
3
4
5
6
Frequency (GHz)
7
8
9
10
Bandwidth Requirements
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Oscillioscope BW Requirements
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Transmission Across FR-4
Bandwidth Requirements
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February 2008
10.5-inch Trace on FR-4
Bandwidth Requirements
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Oscillioscope BW Requirements
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3.5-inch Trace on FR-4
Bandwidth Requirements
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February 2008
Typical Rise Times of Emerging Standard
Interfaces
SATA, SAS 6 Gb/s
60 ps
PCIe Gen2
60-70 ps
Fibre Channel 8G
60 ps
Bandwidth Requirements
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Oscillioscope BW Requirements
February 2008
19
Formulas for Recommended Bandwidth
For oscilloscopes with a “brickwall” response:
Error in rise time
Bandwidth
3%
0.56/(20%-80% rise time)
10%
0.48/(20%-80% rise time)
20%
0.4/(20%-80% rise time)
Bandwidth Requirements
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February 2008
What About Transmitter Measurements?
Agilent DSO81304B,
bandwidth set to 13 GHz
Tek DSA72004, bandwidth
set to 20 GHz
19 ps 20-80 rise time
Bandwidth Requirements
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Oscillioscope BW Requirements
February 2008
20
Excess Bandwidth – Is It Better?
Standard deviation and range of a rise time measurement on a signal with
a nominal rise time of 85 ps, as a function of oscilloscope bandwidth
Oscilloscope
Tektronix
bandwidth setting DSA72004
Agilent
DSO81304B
Standard
deviation
Range
Standard
deviation
Range
20 GHz
3.35 ps
19.24 ps
13 GHz
1.72 ps
12.55 ps
2.2 ps
14.52 ps
6 GHz
1.4 ps
9.02 ps
1.29 ps
8.02 ps
Bandwidth Requirements
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February 2008
How Noise Affects Time Interval Measurements
Graphical representation of voltage noise appearing as time uncertainty
Bandwidth Requirements
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Oscillioscope BW Requirements
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Noise As a Function of Bandwidth
13 GHz
8 GHz
Bandwidth Requirements
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February 2008
Noise As a Function of Bandwidth in the
Frequency Domain
BW = 8 GHz
BW = 13 GHz
Bandwidth Requirements
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Oscillioscope BW Requirements
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22
Eye Diagram Measurements
Agilent 13 GHz
Tek 13 GHz
5 Gb/s serial data signal
with 35 ps rise time
Tek 20 GHz
Bandwidth Requirements
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February 2008
Case Study: USB2.0 Eye Patterns From the Same
Device.
Device Under Test:
USB2.0 Hi-Speed (480Mbps) Hub
(Downstream port)
Bandwidth Requirements
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Oscillioscope BW Requirements
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Case Study: Excess Bandwidth Is the Cause
The Answer：
• The top waveform is a USB2.0 eye pattern
measured with the scope bandwidth set to
3GHz.
• The bottom waveform is the same USB2.0
eye pattern measured with the scope
bandwidth set to 12GHz.
• The noise difference in the two waveforms
is the “high frequency noise” from a scope
front end.
Measured with 3GHz Scope
• Too much bandwidth only increases the
noise.
Measured with 12GHz Scope
Bandwidth Requirements
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February 2008
Case Study: Excess Bandwidth Adds Unnecessary
Noise to the Waveform
Measured with 12GHz Scope
FFT Analysis on
the Transition
Bits
Measured with 3GHz Scope
FFT
FFT analysis
analysis reveals
reveals the
the cause.
cause.
Select
noise.
Select an
an optimum
optimum bandwidth
bandwidth for
for each
each application
application to
to avoid
avoid capturing
capturing unnecessary
unnecessary
noise.
Bandwidth
Requirements
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Oscillioscope BW Requirements
February 2008
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Bandwidth Requirements for Popular Applications
Popular applications
Signal Rate
Fundamental Freq
Rise Time
Base
CEM/Probing Pts
Optimum Bandwidth
Brickwall
Gaussian
USB2.0
480 Mbps
240 MHz
500 ps (10-90%)
1.5 GHz
2.0 GHz
DDR2
< 800MT/s
400 MHz
288 ps (10-90%)
2.4 GHz
3.3 GHz
DDR3
< 1.6GT/s
800 MHz
120 ps (10-90%)
5.8GHz
7.9 GHz
Serial ATA 1
1.5 Gbps
750 MHz
100 ps
5.6 GHz
7.9 GHz
Serial ATA 2
3.0 Gbps
1.5 GHz
67 ps
8.4 GHz
11.3 GHz
Serial ATA 3
6.0 Gbps
3 GHz
35 to 45 ps (estimate)
12.5 GHz
17 GHz
SAS 150
1.5 Gbps
750 MHz
67 ps
8.4 GHz
11.3 GHz
SAS 300
3.0 Gbps
1.5 GHz
67 ps
8.4 GHz
11.3 GHz
SAS 600
6.0 Gbps
3 GHz
47.7 ps
12.5 GHz
15.9 GHz
PCI Express Gen 1
2.5 Gbps
1.25 GHz
50 ps
100 ps
5.6 GHz
7.9 GHz
PCI Express Gen 2
5.0 Gbps
2.5 GHz
45 ps
80 ps (estimate)
12.5 GHz
17 GHz
ExpressCard
2.5 Gbps
1.25 GHz
50 ps
100 ps
5.6 GHz
7.9 GHz
Fibre Channel 4G
4.25 Gbps
2.125 GHz
75 ps
7.5 GHz
10.1 GHz
Fibre Channel 8G
8.5 Gbps
4.25 GHz
60 ps
9.3 GHz
12.7 GHz
XAUI
3.125 Gbps
1.5625 GHz
60 ps
9.3 GHz
12.7 GHz
HDMI 1.3b
3.4Gbps
1.7 GHz
75ps
7.5 GHz
10.1 GHz
DisplayPort
2.7 Gbps
1.35 GHz
75 ps
7.5 GHz
10.1 GHz
FBD 1 / AMB 1
4.8 Gbps
2.4 GHz
12.5 GHz
17 GHz
35 ps
45 ps
Bandwidth Requirements
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February 2008
Conclusions
There is an optimum bandwidth for any signal – too much
bandwidth leads to excess noise and degraded measurement
repeatability.
For signals with rise times >35 ps, 13 GHz bandwidth is
adequate for accurate measurements. More bandwidth only
leads to more noise and degraded measurement repeatability.
Signals used in high speed serial interfaces currently under
development typically have rise times significantly slower than
35 ps.
Both Agilent and Tektronix high-performance real-time scopes
offer the capability to control bandwidth.
Bandwidth Requirements
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Oscillioscope BW Requirements
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Resources
Signal Integrity – Simplified, by Eric Bogatin
Understanding Oscilloscope Frequency Response and Its Effect
on Rise-Time Accuracy, Agilent Technologies Application Note
1420, http://cp.literature.agilent.com/litweb/pdf/59888008EN.pdf
Signal integrity information: www.agilent.com/find/si
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