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

© H. Heck 2008 Section 2.6 1

Module 2: Transmission Line BasicsTopic 6: Measurement Equipment

OGI EE564

Howard Heck


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Where Are We?

1. Introduction

2. Transmission Line Basics1. 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


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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


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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.


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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)


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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


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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 bemissed.


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Oscilloscope Bandwidth Rule of Thumb: The minimum scope bandwidth should

be at least 5 times the highest frequency component being sampled.

max5 fBWscope [2.6.1]

rdB tff 35.0

3max [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 onoscilloscope


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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.


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Oscilloscope Sampling – Types of Sampling

Real Time: “Single shot”, on the fly acquisition. All data acquired in one cycle.

1 1 1 1 11

1

1

1

1

1

11

1

1111

1 1

1 2 3 4 56

7

8

9

18

16

1514

17

13121110

19 20

1

1

1

1

22

2

2

3

3 3

3

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.


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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.


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Sampling Rate: Nyquist’s Theorem

Nyquist’s Theorem: If a signal is sampled at a frequency 2f, 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 2f 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.


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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 10fmax.

1/2f

1/f

1/f


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Probes: Bandwidth

Probe bandwidth is the maximum –3dB frequency a user can expect with a scope/probe system, where .

rdB t

f35.0

3

probesourcesource CCRBW

2

1[2.6.3]


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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 (tr ), capacitive loading becomes dominant.


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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

probesourcesourceprober CCRt 2.2, [2.6.4]

Probe w/ highest impedance (lowest Cprobe) gives the

least circuit loading.


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Probes: Loading #3

Which we can use to determine the actual signal rise time:

2,

2,

2, proberscopersystemr ttt [2.6.5]

2,

2,

2, systemrsignalrmeasuredr ttt [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:


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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.

221

1

probesoucesource CCfRnAttenuatio

[2.6.7]


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Probe Types: Hand Held

Easiest, most common.Use for quick checks.Lacking for accuracy & repeatability.Common problem: large ground loops.


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Probe Types: SMA Connectors

Better repeatability than handheld.Adds cost due to soldering of SMA.Still adds noticeable discontinuity at fast edge

rates.


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Probe Types: Microprobes

Most accurate – the only one suitable for measuring small variations.

Expensive & time consuming to use.


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Probe Types: Signal Quality & Edge Rate Effects

Ringing: SMA vs. Handheld

Edge Rate: SMA vs. Microprobe


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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 Zi = 10M at DC has a Zi

= 1.5k at 50 MHz.

Choose a probe with lowest possible Ci and highest Ri for best

overall signal fidelity.


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Probe Grounding Inductance in the ground leads causes ringing:

Lgnd_lead

Rsource

Vsource

R1= 9M

C1=8-12pF

7-50 pF

500 R21M

C1220 pF

Probe Cable8 - 10 pF/ft

1.5 ns/ft

6 ft

Probe Tip

DUT Probe Scope

[2.6.7]LCf ring 2

1

SignalSignal

GNDGND

PCBPCB

SignalSignal

GNDGND

PCBPCB

LargeLargeGroundGround

LoopLoop SmallSmallGroundGround

LoopLoop

25

35

45

55

65

75

85

95

105

115

125

0.0 0.5 1.0 1.5 2.0

Time [ns]

Z0

[]

Short

Long


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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 Z0

PCB. For selected signals: e.g. worst case lengths and/or spacing.

Characterize noise margins by varying RTT and/or VREF.

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.

Vref

Vref

Vnoise


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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.


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References

General S. Hall, G. Hall, and J. McCall, High Speed Digital

System Design, John Wiley & Sons, Inc. (Wiley Interscience), 2000, 1st edition.

W. Dally and J. Poulton, Digital Systems Engineering, Chapters 4.3 & 11, Cambridge University Press, 1998.

H. Johnson and M. Graham, High Speed Digital Design: A Handbook of Black Magic, PTR Prentice Hall, 1993.

R. Poon, Computer Circuits Electrical Design, Prentice Hall, 1st edition, 1995.


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ReferencesOscilloscopes Tektronix, Inc., “Effects of Bandwidth on Transient

Information,” Application Note 55W-12047-0, April 1998. Tektronix, Inc., “Sampling Oscilloscope Techniques,”

Application Note 47W-7209-0, October 1989. Hewlett Packard Corp., “Bandwidth and Sampling Rate in

Digitizing Oscilloscopes,” Application Note 344, April 1986.

Lecroy, “DSO Applications in High Speed Electronics,” http://www.lecroy.com/applications/HighSpeedElectronics/HighSpeedElectronics.asp

. Lecroy, “Troubleshooting High-Speed Digital Signals with

DSOs,” http://www.lecroy.com/applications/Digital/default.asp.

Lecroy, “Debug and Characterization of High-Speed Digital Electronics,” http://www.lecroy.com/applications/DigitalDebug/default.asp.


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References

Probes Tektronix, Inc., “High Speed Probing,” Application Note 55W-

12107-0, June 1998. Tektronix, Inc., “Primer: ABCs of Probes,” Application Note

60W-6053-7, July 1998. Lecroy, “Probes and Probing,” http://

www.lecroy.com/applications/ProbesProbing/default.asp. Cascade Microtech, “High Speed Digital Microprobing:

Principles and Applications,” Application Note HSDM-391, 1992.

Cascade Microtech, “Microprobing with the HP 5412T Oscilloscope,” 1991.

D. Carlton, et. al., “Accurate Measurement of High-Speed Package and Interconnect Parasitics,” Journal of Microwave Technology, July 1988, pp. 8-15.

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

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