© H. Heck 2008Section 2.61 Module 2:Transmission Line Basics Topic 6:Measurement Equipment OGI EE564 Howard Heck

Measurement Equipment EE 564 © H. Heck 2008 Section 2.62 Where Are We? 1.Introduction 2.Transmission Line Basics 1.Transmission Line Theory 2.Basic I/O Circuits 3.Reflections 4.Parasitic Discontinuities 5.Modeling, Simulation, & Spice 6.Measurement: Basic Equipment 7.Measurement: Time Domain Reflectometry 3.Analysis Tools 4.Metrics & Methodology 5.Advanced Transmission Lines 6.Multi-Gb/s Signaling 7.Special Topics

Measurement Equipment EE 564 © H. Heck 2008 Section 2.63 Contents Why Measure? Basic Measurement Equipment List Pulse Generators Oscilloscopes  Bandwidth  Sampling & Nyquist’s Theorem Probes:  Bandwidth  Loading  Attenuation  Types  Grounding System Validation & Margin Testing Summary References

Measurement Equipment EE 564 © H. Heck 2008 Section 2.64 Why Measure? How do we prove to ourselves that our design works?  We simulate the design, but simulators have limitations.  Our models may not be accurate (garbage in, garbage out). Measurement gives us confidence in our design.  By determining system margin (timing, noise). Measure under environmental conditions (temperature, voltage, etc.), and vary reference and termination voltages. Limitation: can’t measure worst case extremes.  By characterizing the pieces of the system – I/O buffers, packages, PCBs, connectors, etc. Build confidence in our models by correlating to measurement. Use the models to extrapolate behavior to worst case conditions (or to project defect rates, i.e. statistical design). Another reason to measure: debug.  May be necessary to find errors in protocol or with I/O circuits.  Measurement is the only way to prove to someone else that your design is NOT the cause when a problem exists.

Measurement Equipment EE 564 © H. Heck 2008 Section 2.65 Equipment Needs Equipment list for a modern signal integrity lab: High speed digitizing oscilloscope with pulse generator  Measure system signals & use for TDR (time domain reflectometry) and TDT (time domain transmission) High performance probes Calibration standards Vector Network Analyzer (VNA)

Measurement Equipment EE 564 © H. Heck 2008 Section 2.66 Pulse Generator Function: generate waveforms for characterizing our design. Allows control over:  Wave shape (sine, ramp, arbitrary)  Rise time  Output voltage  Bit pattern Example: Tektronics AWG 610 Arbitrary Waveform Generator

Measurement Equipment EE 564 © H. Heck 2008 Section 2.67 Oscilloscopes Function: capture signals for viewing and analysis. Key Parameters: Bandwidth & Sampling Rate If your scope has inadequate bandwidth, high frequency components of the signal being measured will be filtered out, and key timing and/or noise information may be inaccurate. If your scope has inadequate sampling rate, edges cannot be located precisely enough and high frequency noise components will be missed.

Measurement Equipment EE 564 © H. Heck 2008 Section 2.68 Oscilloscope Bandwidth Rule of Thumb: The minimum scope bandwidth should be at least 5 times the highest frequency component being sampled. [2.6.1] [2.6.2] In general, the greater the scope bandwidth, the smaller the errors in measuring time intervals. Rule of thumb: scope rise time should be < 1/3 of the time interval to be measured. Input Pulse Waveform seen on oscilloscope

Measurement Equipment EE 564 © H. Heck 2008 Section 2.69 Oscilloscope Bandwidth Example A 17.5 ps signal edge measured with… 700 MHz scope 2 GHz scope 1 GHz scope Insufficient bandwidth degrades the measured rise time and filters out noise.

Measurement Equipment EE 564 © H. Heck 2008 Section Oscilloscope Sampling – Types of Sampling Real Time: “Single shot”, on the fly acquisition. All data acquired in one cycle After many samples, scope reconstructs signal. Requires repetitive signal. Sequential: One sample digitized per trigger. Each successive trigger delays sampling further. Random Repetitive: Signal is constantly sampled & digitized at rate determined by scope’s sample rate. Requires repetitive signal.

Measurement Equipment EE 564 © H. Heck 2008 Section Oscilloscope Sampling Rate Affect on timing measurements: Higher: good characterization of edge. Too Low: edge cannot be located precisely. Glitch detection: Higher: better chance of detection. Too Low: can miss glitch.

Measurement Equipment EE 564 © H. Heck 2008 Section Sampling Rate: Nyquist’s Theorem Nyquist’s Theorem: If a signal is sampled at a frequency 2 f, there is no information in the samples about the components of the signal at frequencies above f. The effect on time domain measurement uncertainty of a sample rate 2 f is equivalent to a band-limiting filter with a sharp cut-off at frequency = f. Nyquist’s Theorem provides an upper limit. In practice the sampling rate should be higher.

Measurement Equipment EE 564 © H. Heck 2008 Section Sampling Rate: Nyquist’s Theorem As the sampling rate nears the Nyquist limit, the measurement depends strongly on the signal reconstruction algorithm and where the signal is sampled. 1/10f Rule of thumb: sampling rate  10 f max. 1/2f 1/f

Measurement Equipment EE 564 © H. Heck 2008 Section Probes: Bandwidth Probe bandwidth is the maximum –3dB frequency a user can expect with a scope/probe system, where. [2.6.3]

Measurement Equipment EE 564 © H. Heck 2008 Section Probes: Loading Input resistance and capacitance determine the loading effect of a probe. Probe loading is frequency dependent.  At DC and low frequencies, resistive loading is most important.  As f increases ( t r  ), capacitive loading becomes dominant.

Measurement Equipment EE 564 © H. Heck 2008 Section Probes: Loading #2 Loading causes probes to affect the measured rise time and delay. The time required to charge the capacitance from 10% to 90% of the signal level is [2.6.4] Probe w/ highest impedance (lowest C probe ) gives the least circuit loading.

Measurement Equipment EE 564 © H. Heck 2008 Section Probes: Loading #3 Which we can use to determine the actual signal rise time: [2.6.5] [2.6.6] Probe rise time is determined when the probe is driven from a terminated 50  source. Use it to calculate the rise time of the probe/scope system:

Measurement Equipment EE 564 © H. Heck 2008 Section Probes: Attenuation Attenuation = ratio of output signal to input signal. Goal is to maintain constant attenuation over a wide range of frequencies, limiting it to 3dB as the frequency increases to the rated bandwidth. [2.6.7]

Measurement Equipment EE 564 © H. Heck 2008 Section Probe Types: Hand Held Easiest, most common. Use for quick checks. Lacking for accuracy & repeatability. Common problem: large ground loops.

Measurement Equipment EE 564 © H. Heck 2008 Section Probe Types: SMA Connectors Better repeatability than handheld. Adds cost due to soldering of SMA. Still adds noticeable discontinuity at fast edge rates.

Measurement Equipment EE 564 © H. Heck 2008 Section Probe Types: Microprobes Most accurate – the only one suitable for measuring small variations. Expensive & time consuming to use.

Measurement Equipment EE 564 © H. Heck 2008 Section Probe Types: Signal Quality & Edge Rate Effects Ringing: SMA vs. Handheld Edge Rate: SMA vs. Microprobe

Measurement Equipment EE 564 © H. Heck 2008 Section Probe Selection Guidelines Be sure the probe will match the input resistance and capacitance of the scope being used.  i.e. 50  scope inputs require 50  probes. Select a probe with adequate rise time and bandwidth for the oscilloscope and DUT. Minimize probe loading effects by selecting low impedance test points. Remember that probe impedance varies inversely with frequency.  e.g. probe with 50 MHz bandwidth and Z i = 10 M  at DC has a Z i = 1.5 k  at 50 MHz.  Choose a probe with lowest possible C i and highest R i for best overall signal fidelity.

Measurement Equipment EE 564 © H. Heck 2008 Section Probe Grounding Inductance in the ground leads causes ringing: [2.6.7] Signal GND PCB Signal GND PCB Large Ground Loop SmallGroundLoop

Measurement Equipment EE 564 © H. Heck 2008 Section System Validation & Margin Testing Use oscilloscope & probes to measure signals in your design. Probe clock and data to measure setup and hold margin.  Under extreme temperature and voltage.  With “skewed” hardware: e.g. slow/fast Si process, low/high Z 0 PCB.  For selected signals: e.g. worst case lengths and/or spacing. Characterize noise margins by varying R TT and/or V REF. Sometimes we can vary line lengths to find out where the system fails.  e.g. Pentium® II Processor FSB with “riser cards” of various lengths.

Measurement Equipment EE 564 © H. Heck 2008 Section Summary Measurements are necessary to prove that your design works and are required to build and correlate accurate models. Bandwidth and sampling rate are the key metrics for characterizing oscilloscope performance. Probes have strong impacts on measurement accuracy. Bandwidth, loading, and attenuation are key parameters for describing probe performance. Probing techniques (handheld, SMA, microprobe, as well as ground loops) also have strong influence on the fidelity of measurements.

Measurement Equipment EE 564 © H. Heck 2008 Section References General S. Hall, G. Hall, and J. McCall, High Speed Digital System Design, John Wiley & Sons, Inc. (Wiley Interscience), 2000, 1 st edition. W. Dally and J. Poulton, Digital Systems Engineering, Chapters 4.3 & 11, Cambridge University Press, H. Johnson and M. Graham, High Speed Digital Design: A Handbook of Black Magic, PTR Prentice Hall, R. Poon, Computer Circuits Electrical Design, Prentice Hall, 1 st edition, 1995.

Measurement Equipment EE 564 © H. Heck 2008 Section References Oscilloscopes Tektronix, Inc., “Effects of Bandwidth on Transient Information,” Application Note 55W , April Tektronix, Inc., “Sampling Oscilloscope Techniques,” Application Note 47W , October Hewlett Packard Corp., “Bandwidth and Sampling Rate in Digitizing Oscilloscopes,” Application Note 344, April Lecroy, “DSO Applications in High Speed Electronics,” Lecroy, “Troubleshooting High-Speed Digital Signals with DSOs,” Lecroy, “Debug and Characterization of High-Speed Digital Electronics,” sp. sp

Measurement Equipment EE 564 © H. Heck 2008 Section References Probes Tektronix, Inc., “High Speed Probing,” Application Note 55W , June Tektronix, Inc., “Primer: ABCs of Probes,” Application Note 60W , July Lecroy, “Probes and Probing,” Cascade Microtech, “High Speed Digital Microprobing: Principles and Applications,” Application Note HSDM-391, Cascade Microtech, “Microprobing with the HP 5412T Oscilloscope,” D. Carlton, et. al., “Accurate Measurement of High-Speed Package and Interconnect Parasitics,” Journal of Microwave Technology, July 1988, pp