Oscilloscope Bandwidth Requirements for Emerging Serial Data Interfaces

Transcription

1 Oscilloscope Bandwidth Requirements for Emerging Serial Data Interfaces Page 1 What best determines bandwidth requirements? 5 th 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? Page 2 Oscillioscope BW Requirements 1

2 What Do You Think? Which do you think best predicts the required scope rise time: The data rate? The rise time of the signal? Page 3 Two Examples 620 Mb/s 10 Gb/s Page 4 Oscillioscope BW Requirements 2

3 10 Gb/s Page Mb/s Page 6 Oscillioscope BW Requirements 3

4 Spectra 620 Mb/s 10 Gb/s Page 7 10 Gb/s Spectrum Page 8 Oscillioscope BW Requirements 4

5 10 Gb/s Fifth Harmonic Page Mb/s Fifth Harmonic Page 10 Oscillioscope BW Requirements 5

6 620 Mb/s 25 th Harmonic Page 11 Changing Bit Rate With Constant Rise Time 4 Gb/s 5 th Harmonic = 4% Rise time = 35 ps 6 Gb/s 5 th Harmonic = 1.4% 1 Gb/s 5 th Harmonic = 8% Page 12 Oscillioscope BW Requirements 6

7 Changing Rise Time With Constant Bit Rate Bit rate = 4 Gb/s Rise time = 50ps 5 th Harmonic = 4.1% Rise time = 66ps 5 th Harmonic = 2.1% Rise time =15ps 5 th Harmonic = 7.4% Page 13 Conclusion The rise time is a better predictor of the required bandwidth than the bit rate. Page 14 Oscillioscope BW Requirements 7

8 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 Page 15 Controlled Rise Times Nominal rise Scope and time Bandwidth 32 ps 44 ps 87 ps Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 6 GHz Agilent 6 GHz Mean Error ps 9% ps 14% ps 12% 45.5 ps 4% ps 5% ps 4% 86.7 ps 0.03% ps 3% Range 9.83 ps 4.75 ps 7.57 ps ps 7.41 ps 8.33 ps 9.02 ps 8.07 ps Std dev 1.53 ps ps 1.21 ps 1.56 ps 1.07 ps 1.25 ps 1.4 ps 1.29 ps DCA reference ps ps ps ps ps ps ps ps Page 16 Oscillioscope BW Requirements 8

9 Controlled Rise Times Nominal rise Scope and time Bandwidth 32 ps 44 ps 87 ps Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 6 GHz Agilent 6 GHz Mean Error ps 9% ps 14% ps 12% 45.5 ps 4% ps 5% ps 4% 86.7 ps 0.03% ps 3% Range 9.83 ps 4.75 ps 7.57 ps ps 7.41 ps 8.33 ps 9.02 ps 8.07 ps Std dev 1.53 ps ps 1.21 ps 1.56 ps 1.07 ps 1.25 ps 1.4 ps 1.29 ps DCA reference ps ps ps ps ps ps ps ps Page 17 Controlled Rise Times Nominal rise Scope and time Bandwidth 32 ps 44 ps 87 ps Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 6 GHz Agilent 6 GHz Mean Error ps 9% ps 14% ps 12% 45.5 ps 4% ps 5% ps 4% 86.7 ps 0.03% ps 3% Range 9.83 ps 4.75 ps 7.57 ps ps 7.41 ps 8.33 ps 9.02 ps 8.07 ps Std dev 1.53 ps ps 1.21 ps 1.56 ps 1.07 ps 1.25 ps 1.4 ps 1.29 ps DCA reference ps ps ps ps ps ps ps ps Page 18 Oscillioscope BW Requirements 9

10 Controlled Rise Times Nominal rise Scope and time Bandwidth 32 ps 44 ps 87 ps Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 6 GHz Agilent 6 GHz Mean Error ps 9% ps 14% ps 12% 45.5 ps 4% ps 5% ps 4% 86.7 ps 0.03% ps 3% Range 9.83 ps 4.75 ps 7.57 ps ps 7.41 ps 8.33 ps 9.02 ps 8.07 ps Std dev 1.53 ps ps 1.21 ps 1.56 ps 1.07 ps 1.25 ps 1.4 ps 1.29 ps DCA reference ps ps ps ps ps ps ps ps Page 19 Controlled Rise Times Nominal rise Scope and time Bandwidth 32 ps 44 ps 87 ps Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 20 GHz Tek 13 GHz Agilent 13 GHz Tek 6 GHz Agilent 6 GHz Mean Error ps 9% ps 14% ps 12% 45.5 ps 4% ps 5% ps 4% 86.7 ps 0.03% ps 3% Range 9.83 ps 4.75 ps 7.57 ps ps 7.41 ps 8.33 ps 9.02 ps 8.07 ps Std dev 1.53 ps ps 1.21 ps 1.56 ps 1.07 ps 1.25 ps 1.4 ps 1.29 ps DCA reference ps ps ps ps ps ps ps ps Page 20 Oscillioscope BW Requirements 10

11 Spectrum Analyzer Measurements 35 ps Page 21 Spectrum Analyzer Measurements 35 ps Page 22 Oscillioscope BW Requirements 11

12 Spectrum Analyzer Measurements 35 ps Page 23 Spectrum Analyzer Measurements 45 ps Page 24 Oscillioscope BW Requirements 12

13 Spectrum Analyzer Measurements 45 ps Page 25 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 ps ps >65 ps Page 26 Oscillioscope BW Requirements 13

14 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. Page 27 Rise Time Measurements vs Bandwidth Scope Bandwidth 13 GHz 10 GHz 8 GHz 6 GHz Measured mean rise time 80 ps 80 ps 80 ps 86 ps Page 28 Oscillioscope BW Requirements 14

15 Spectrum Analysis of PCIe Gen2 Signal Page 29 Spectrum Analysis of PCIe Gen2 Signal Page 30 Oscillioscope BW Requirements 15

16 Mated Pair of PCIe Gen2 Test Fixtures PCIe Gen2 CLB PCIe Gen2 CBB Page 31 Mated Pair of PCIe Gen2 Test Fixtures Page 32 Oscillioscope BW Requirements 16

17 Mated Pairs of SATA Test Fixtures Page 33 S21, Mated Pairs of SATA Test Fixtures 0 SDD21 (Gain) vs. Frequency Gain (db) _Comax_eSATA_ _pair65_TDT.s4p 03_Comax_eSATA_ _pair65_TDT.s4p 04_Comax_eSATA_ _pair65_TDT.s4p 05_Comax_eSATA_ _pair65_TDT.s4p 01_Comax_iSATA_ _pair65_TDT.s4p Frequency (GHz) Page 34 Oscillioscope BW Requirements 17

18 Transmission Across FR-4 Page inch Trace on FR-4 Page 36 Oscillioscope BW Requirements 18

19 3.5-inch Trace on FR-4 Page 37 Typical Rise Times of Emerging Standard Interfaces SATA, SAS 6 Gb/s PCIe Gen2 Fibre Channel 8G 60 ps ps 60 ps Page 38 Oscillioscope BW Requirements 19

20 Formulas for Recommended Bandwidth For oscilloscopes with a brickwall response: Error in rise time 3% 10% 20% Bandwidth 0.56/(20%-80% rise time) 0.48/(20%-80% rise time) 0.4/(20%-80% rise time) Page 39 What About Transmitter Measurements? Agilent DSO81304B, bandwidth set to 13 GHz Tek DSA72004, bandwidth set to 20 GHz 19 ps rise time Page 40 Oscillioscope BW Requirements 20

21 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 bandwidth setting Tektronix DSA72004 Agilent DSO81304B Standard deviation Range Standard deviation Range 20 GHz 3.35 ps ps 13 GHz 1.72 ps ps 2.2 ps ps 6 GHz 1.4 ps 9.02 ps 1.29 ps 8.02 ps Page 41 How Noise Affects Time Interval Measurements Graphical representation of voltage noise appearing as time uncertainty Page 42 Oscillioscope BW Requirements 21

22 Noise As a Function of Bandwidth 13 GHz 8 GHz Page 43 Noise As a Function of Bandwidth in the Frequency Domain BW = 8 GHz BW = 13 GHz Page 44 Oscillioscope BW Requirements 22

23 Eye Diagram Measurements Agilent 13 GHz Tek 13 GHz 5 Gb/s serial data signal with 35 ps rise time Tek 20 GHz Page 45 Case Study: USB2.0 Eye Patterns From the Same Device. Device Under Test: USB2.0 Hi-Speed (480Mbps) Hub (Downstream port) Page 46 Oscillioscope BW Requirements 23

24 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 Measured with 12GHz Scope Page 47 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 analysis reveals the cause. Select an optimum bandwidth for each application to avoid capturing unnecessary noise. Page 48 Oscillioscope BW Requirements 24

25 for Popular Applications Popular applications Signal Rate Fundamental Freq Rise Time Optimum Bandwidth Base CEM/Probing Pts Brickwall Gaussian USB 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 Gbps 750 MHz 100 ps 5.6 GHz 7.9 GHz Serial ATA Gbps 1.5 GHz 67 ps 8.4 GHz 11.3 GHz Serial ATA Gbps 3 GHz 35 to 45 ps (estimate) 12.5 GHz 17 GHz SAS Gbps 750 MHz 67 ps 8.4 GHz 11.3 GHz SAS Gbps 1.5 GHz 67 ps 8.4 GHz 11.3 GHz SAS Gbps 3 GHz 47.7 ps 12.5 GHz 15.9 GHz PCI Express Gen Gbps 1.25 GHz 50 ps 100 ps 5.6 GHz 7.9 GHz PCI Express Gen 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 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 Gbps 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 Gbps 2.4 GHz 35 ps 45 ps 12.5 GHz 17 GHz Page 49 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. Page 50 Oscillioscope BW Requirements 25

26 Resources Signal Integrity Simplified, by Eric Bogatin Understanding Oscilloscope Frequency Response and Its Effect on Rise-Time Accuracy, Agilent Technologies Application Note 1420, Signal integrity information: Page 51 Oscillioscope BW Requirements 26