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 Page 2 Oscillioscope BW Requirements February 2008 1 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 Page 3 February 2008 Two Examples 620 Mb/s 10 Gb/s Bandwidth Requirements Page 4 Oscillioscope BW Requirements February 2008 2 10 Gb/s Bandwidth Requirements Page 5 February 2008 620 Mb/s Bandwidth Requirements Page 6 Oscillioscope BW Requirements February 2008 3 Spectra 620 Mb/s 10 Gb/s Bandwidth Requirements Page 7 February 2008 10 Gb/s Spectrum Bandwidth Requirements Page 8 Oscillioscope BW Requirements February 2008 4 10 Gb/s Fifth Harmonic Bandwidth Requirements Page 9 February 2008 620 Mb/s Fifth Harmonic Bandwidth Requirements Page 10 Oscillioscope BW Requirements February 2008 5 620 Mb/s 25th Harmonic Bandwidth Requirements Page 11 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 Page 12 Oscillioscope BW Requirements February 2008 6 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 Page 13 February 2008 Conclusion The rise time is a better predictor of the required bandwidth than the bit rate. Bandwidth Requirements Page 14 Oscillioscope BW Requirements February 2008 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 Page 15 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 Page 16 Oscillioscope BW Requirements February 2008 8 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 Page 17 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 Page 18 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 Page 19 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 Page 20 Oscillioscope BW Requirements February 2008 10 Spectrum Analyzer Measurements – 35 ps Bandwidth Requirements Page 21 February 2008 Spectrum Analyzer Measurements – 35 ps Bandwidth Requirements Page 22 Oscillioscope BW Requirements February 2008 11 Spectrum Analyzer Measurements – 35 ps Bandwidth Requirements Page 23 February 2008 Spectrum Analyzer Measurements – 45 ps Bandwidth Requirements Page 24 Oscillioscope BW Requirements February 2008 12 Spectrum Analyzer Measurements – 45 ps Bandwidth Requirements Page 25 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 Page 26 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 Page 27 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 Page 28 Oscillioscope BW Requirements February 2008 14 Spectrum Analysis of PCIe Gen2 Signal Bandwidth Requirements Page 29 February 2008 Spectrum Analysis of PCIe Gen2 Signal Bandwidth Requirements Page 30 Oscillioscope BW Requirements February 2008 15 Mated Pair of PCIe Gen2 Test Fixtures PCIe Gen2 CLB PCIe Gen2 CBB Bandwidth Requirements Page 31 February 2008 Mated Pair of PCIe Gen2 Test Fixtures Bandwidth Requirements Page 32 Oscillioscope BW Requirements February 2008 16 Mated Pairs of SATA Test Fixtures Bandwidth Requirements Page 33 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 Page 34 Oscillioscope BW Requirements February 2008 17 Transmission Across FR-4 Bandwidth Requirements Page 35 February 2008 10.5-inch Trace on FR-4 Bandwidth Requirements Page 36 Oscillioscope BW Requirements February 2008 18 3.5-inch Trace on FR-4 Bandwidth Requirements Page 37 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 Page 38 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 Page 39 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 Page 40 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 Page 41 February 2008 How Noise Affects Time Interval Measurements Graphical representation of voltage noise appearing as time uncertainty Bandwidth Requirements Page 42 Oscillioscope BW Requirements February 2008 21 Noise As a Function of Bandwidth 13 GHz 8 GHz Bandwidth Requirements Page 43 February 2008 Noise As a Function of Bandwidth in the Frequency Domain BW = 8 GHz BW = 13 GHz Bandwidth Requirements Page 44 Oscillioscope BW Requirements February 2008 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 Page 45 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 Page 46 Oscillioscope BW Requirements February 2008 23 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 Page 47 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 Page 48 Oscillioscope BW Requirements February 2008 24 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 Page 49 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 Page 50 Oscillioscope BW Requirements February 2008 25 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 Bandwidth Requirements Page 51 Oscillioscope BW Requirements February 2008 26