Oscilloscope BW Reqmts for Emerging Serial Data Intfcs · Oscillioscope BW Requirements 1 Bandwidth...

Oscillioscope BW Requirements 1

Bandwidth Requirements

February 2008

Oscilloscope Bandwidth Requirements for Emerging

Serial Data Interfaces


February 2008Page 2


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?



February 2008Page 3

What Do You Think?

Which do you think best predicts the required scope rise time:

•The data rate?

•The rise time of the signal?


February 2008Page 4

Two Examples

10 Gb/s

620 Mb/s



February 2008Page 5

10 Gb/s


February 2008Page 6

620 Mb/s



February 2008Page 7

Spectra

10 Gb/s

620 Mb/s


February 2008Page 8

10 Gb/s Spectrum



February 2008Page 9

10 Gb/s Fifth Harmonic


February 2008Page 10

620 Mb/s Fifth Harmonic




620 Mb/s 25th Harmonic



Changing Bit Rate With Constant Rise Time

1 Gb/s5th Harmonic = 8%

4 Gb/s5th Harmonic = 4%

Rise time = 35 ps

6 Gb/s5th Harmonic = 1.4%




Changing Rise Time With Constant Bit Rate

Rise time =15ps 5th Harmonic = 7.4%

Rise time = 50ps 5th Harmonic = 4.1%

Rise time = 66ps 5th Harmonic = 2.1%

Bit rate = 4 Gb/s



Conclusion

The rise time is a better predictor of the required bandwidth than the bit rate.




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



Controlled Rise Times

43.91 ps1.07 ps7.41 ps45.98 ps5%

Tek 13 GHz

44.13 ps1.25 ps8.33 ps45.99 ps4%

Agilent 13 GHz

86.97 ps1.4 ps9.02 ps86.7 ps0.03%

Tek 6 GHz87 ps

32.07 ps1.21 ps7.57 ps36.04 ps12%

Agilent 13 GHz

43.91 ps1.56 ps10.74 ps45.5 ps4%

Tek 20 GHz44 ps

1.29 ps

0.788 ps

1.53 ps

Std dev

86.95 ps

31.79 ps

31.79 ps

DCA reference

8.07 ps84.34 ps3%

Agilent 6 GHz

4.75 ps36.18 ps14%

Tek 13 GHz

9.83 ps34.76 ps9%

Tek 20 GHz32 ps

RangeMeanError

Scope and Bandwidth

Nominal rise time





43.91 ps1.07 ps7.41 ps45.98 ps5%

Tek 13 GHz

44.13 ps1.25 ps8.33 ps45.99 ps4%

Agilent 13 GHz

86.97 ps1.4 ps9.02 ps86.7 ps0.03%

Tek 6 GHz87 ps

32.07 ps1.21 ps7.57 ps36.04 ps12%

Agilent 13 GHz

43.91 ps1.56 ps10.74 ps45.5 ps4%

Tek 20 GHz44 ps

1.29 ps

0.788 ps

1.53 ps

Std dev

86.95 ps

31.79 ps

31.79 ps

DCA reference

8.07 ps84.34 ps3%

Agilent 6 GHz

4.75 ps36.18 ps14%

Tek 13 GHz

9.83 ps34.76 ps9%

Tek 20 GHz32 ps

RangeMeanError

Scope and Bandwidth

Nominal rise time




43.91 ps1.07 ps7.41 ps45.98 ps5%

Tek 13 GHz

44.13 ps1.25 ps8.33 ps45.99 ps4%

Agilent 13 GHz

86.97 ps1.4 ps9.02 ps86.7 ps0.03%

Tek 6 GHz87 ps

32.07 ps1.21 ps7.57 ps36.04 ps12%

Agilent 13 GHz

43.91 ps1.56 ps10.74 ps45.5 ps4%

Tek 20 GHz44 ps

1.29 ps

0.788 ps

1.53 ps

Std dev

86.95 ps

31.79 ps

31.79 ps

DCA reference

8.07 ps84.34 ps3%

Agilent 6 GHz

4.75 ps36.18 ps14%

Tek 13 GHz

9.83 ps34.76 ps9%

Tek 20 GHz32 ps

RangeMeanError

Scope and Bandwidth

Nominal rise time




Spectrum Analyzer Measurements – 35 ps










What Are You Working On?

Bit rate

<2 Gb/s

2-4 Gb/s

4-8 Gb/s

8-10 Gb/s

>10 Gb/s

Rise time (20-80)

<35 ps

35-45 ps

45-65 ps

>65 ps




PCIe Gen2 Device

In this section we will examine “real world” signals from a PCIeGen2 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.



Rise Time Measurements vs Bandwidth

80 ps8 GHz86 ps6 GHz

80 ps10 GHz80 ps13 GHz

Measured mean 20-80 rise time

Scope Bandwidth




Spectrum Analysis of PCIe Gen2 Signal



Spectrum Analysis of PCIe Gen2 Signal




Mated Pair of PCIe Gen2 Test Fixtures

PCIe Gen2 CLB

PCIe Gen2 CBB



Mated Pair of PCIe Gen2 Test Fixtures




Mated Pairs of SATA Test Fixtures



S21, Mated Pairs of SATA Test Fixtures

1 2 3 4 5 6 7 8 9 10-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

Frequency (GHz)

Gai

n (d

B)

02_Comax_eSATA_01080078_pair65_TDT.s4p03_Comax_eSATA_01110077_pair65_TDT.s4p04_Comax_eSATA_01120079_pair65_TDT.s4p05_Comax_eSATA_01130081_pair65_TDT.s4p01_Comax_iSATA_00060005_pair65_TDT.s4p

SDD21 (Gain) vs. Frequency




Transmission Across FR-4



10.5-inch Trace on FR-4




3.5-inch Trace on FR-4



Typical Rise Times of Emerging Standard Interfaces

60 psFibre Channel 8G

60-70 psPCIe Gen2

60 psSATA, SAS 6 Gb/s




Formulas for Recommended Bandwidth

0.4/(20%-80% rise time)20%

0.48/(20%-80% rise time)10%

0.56/(20%-80% rise time)3%

BandwidthError in rise time

For oscilloscopes with a “brickwall” response:



What About Transmitter Measurements?

Agilent DSO81304B, bandwidth set to 13 GHz

Tek DSA72004, bandwidth set to 20 GHz

19 ps 20-80 rise time




Excess Bandwidth – Is It Better?

9.02 ps

12.55 ps

19.24 ps

Range

1.29 ps

2.2 ps

Standard deviation

Agilent DSO81304B

RangeStandard deviation

8.02 ps1.4 ps6 GHz

14.52 ps1.72 ps13 GHz

3.35 ps20 GHz

Tektronix DSA72004

Oscilloscope bandwidth setting

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



How Noise Affects Time Interval Measurements

Graphical representation of voltage noise appearing as time uncertainty




Noise As a Function of Bandwidth

8 GHz13 GHz



Noise As a Function of Bandwidth in the Frequency Domain

BW = 8 GHz

BW = 13 GHz




Eye Diagram Measurements

Agilent 13 GHz

Tek 13 GHz

Tek 20 GHz

5 Gb/s serial data signal with 35 ps rise time



Device Under Test:USB2.0 Hi-Speed (480Mbps) Hub (Downstream port)

Case Study: USB2.0 Eye Patterns From the Same Device.




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.

• Too much bandwidth only increases the noise.

Measured with 3GHz Scope




Case Study: Excess Bandwidth Adds Unnecessary Noise to the Waveform

FFT analysis reveals the cause. Select an optimum bandwidth for each application to avoid capturing unnecessary noise.

FFT analysis reveals the cause. Select an optimum bandwidth for each application to avoid capturing unnecessary noise.


Measured with 12GHz ScopeFFT Analysis on the Transition

Bits




Bandwidth Requirements for Popular Applications

Popular applications Signal Rate Fundamental Freq Rise Time Optimum Bandwidth

Base CEM/Probing Pts 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 35 ps 45 ps 12.5 GHz 17 GHz



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.




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/5988-8008EN.pdf

Signal integrity information: www.agilent.com/find/si

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