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High-Speed DigitalTest & Measurement

Chris Allen (callen@eecs.ku.edu)

Course website URL people.eecs.ku.edu/~callen/713/EECS713.htm

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TopicsTest equipment

Oscilloscope characteristicsOscilloscope probes

• Passive probes• Active probes• Probe inductance from ground lead

Design for test

Test proceduresMeasuring noise marginMeasuring timing marginSensitivity to supply voltage variationsSensitivity to temperature variationsTest vectors

Future trends

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Test equipmentOscilloscope characteristics

Vertical amplifier bandwidth (typically specified as 3-dB BW, BW3dB)

Limits the observed risetime of the measured signal

The risetime of the vertical amplifier adds in root-sum-square (RSS) fashion to the circuit’s actual Tr to yield a measured Tr

amplifierverticalTcircuitTmeasuredT 2r

2rr

Note: to relate BW3dB to T10-90 T10-90 = 0.338 / BW3dB

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Test equipmentScope probe characteristics

The probe connecting the oscilloscope to the circuit under test also has a frequency response, characterized by Tr(probe), that affects the measurement.

Including the contribution of the scope probe yields

Consequently, the circuit’s actual risetime may be shorter than what measurements indicate.

Example: Using a 500-MHz oscilloscope with a 1-GHz probe an 820-ps risetime is measured. What is the circuit’s actual risetime?

500 MHz Tr(vert amp) = 676 ps, 1 GHz Tr(probe) = 338 ps

probeTamplifierverticalTcircuitTmeasuredT 2r

2r

2rr

ps318ps338ps676ps820circuitT 222r

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Test equipmentScope probe characteristics

The model for the probe includes an input capacitance in parallel with a high-value resistance, R1 (9 MΩ) and an inductive ground lead.

The scope is modeled as a high-value resistor, R2, in parallel with an input capacitance, C2.

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Test equipmentScope probe characteristics

At DC this arrangement produces a 10x attenuation by the voltage divider R2/(R1 + R2).

At DC the impedance of the probe/scope is|Zmeas| = 10 MΩ.

At 100 MHz the impedance is(ignoring XLGround Lead

)

C1 (12 pF) |XC1| = 133

C2 (20 pF) |XC2| = 80

C3 (55 pF) |XC3| = 29

|Zmeas| |XC1 + (XC2

// XC3)| = 154

At higher frequencies, the probe/scope impedance decreases further.

Simplified equivalent circuitSimplified equivalent circuit

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Test equipmentOther measurement considerations

The relatively low probe/scope impedance at high frequencies can load the circuit under test causing:

• changes in circuit performance

• corrupt measurements

To avoid the loadingproblem we can useprobes with lesscapacitance.

Another option is touse active probesthat use FETamplifiers to isolatemeasurement capacitance from thecircuit under test.

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Test equipmentActive probes

The FET in the active probe acts as a signal buffer.The probes’s input impedance is now the FET’s input impedance

• typical FET input capacitance < 1 pF

• high input resistance

Consequently, active probes

• reduce circuit loading

• have a wide operating bandwidth(Tektronix active probe 500 MHz to 4 GHz)

• requires a bias voltage to power the FET amplifier

Active probes can present a bias voltage to the probe tip

• useful for probing unterminated outputs

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Test equipmentProbe inductance from ground lead

Oscilloscope measurements generally require a ground connection as a voltage reference.

Typical scope probes have a ground lead wire for signal reference.

This ground lead has significant inductance (100s of nH) and can act as an antenna (both radiating signals as well as coupling ambient RF signals into the measurement).

A long ground lead presents a number of measurement problems

• increased Tr(measured)

• crosstalk

• electromagnetic interference (EMI)

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Test equipmentProbe inductance from ground lead

To avoid these problems we must reduce Lgnd

Special probe tip attachments provide a ground connection with reduced lead inductance (few nH)

These can improve the measurement-induced errors affecting Tr,

crosstalk, and EMI

However to effectively use these tip attachments allsignals to be probedmust have a nearby ground pad for the probe.

Therefore generous use of ground metal on the boardsurface facilitates probing.

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Test equipmentSpecial probing fixtures

The author discusses a low-cost, shop-built 21:1 probebuilt from a length of coaxial cableand a leaded 1-kΩ resistor.

Benefits of this approach include

• high DC resistance of 1050 Ω (vs. 50 Ω of cable alone)

• low circuit loading, fast rise time (Tr)minimal capacitance (~ 0.5 pF)1-kΩ series resistance

• low cost

Assumptionsscope is terminated with 50 Ω (to reduce reflections)

ground pad is available near the signal to be probed (short lead length)

21:1048.0ratioDivision

100050

50ratioDivision

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Test equipmentSpecial probing fixtures

Low-cost, shop-built 21:1 probe (photo essay)from http://paulorenato.com/joomla/index.php?option=com_content&view=article&id=93&Itemid=4

circuit under test: 125-MHz oscillatorcircuit under test: 125-MHz oscillator

testing with passive probestesting with passive probes

testing with shop-built, wide bandwidth probetesting with shop-built, wide bandwidth probe

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Design for testSpecial probing fixtures

Special test points can also be incorporated into the board design to ease the probing of high-speed signals.

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Test proceduresOnce a digital circuit is functioning, it is useful to determine

its operating margin

Consider the variety of factors that contribute to this margin

• noise margin

• reflections

• timing margin

• temperature effects

• supply voltage effect

Several tests are recommended to ensure reliable operation

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Test proceduresNoise margin / reflection testing

By introducing additional noise at various nodes in the system, signals with noise sensitivity can be identified.

Useful in locating the source of intermittent errors.

The Noise Source is composed of resistor, R, that develops thermal noise (broadband, random) which is then amplified to the desired level.

Other noise generations approaches are available (e.g., communication noise sources).

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Test proceduresTiming testing

By varying the relative timing of the clock and data signals, the timing margin can be estimated.

Use of a coaxial delay lineselect a clock orremove a portion insert a short

data line to of the line and segment ofbe tested expose the copper coaxial line

The opened trace can be readily repaired after testing

Another method for making this measurement is to use an independent clock source, synchronized with variable phase control.

Demonstrates usefulness of ground area on surface

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Test proceduresTesting tolerance to varying supply voltages, VEE, VTT

Test to find out how sensitive your design is to voltage variations,to establish the supply voltage tolerances.

This test demonstrates another advantage of separate VEE and VTT supplies, as opposed to

These voltage variations can change threshold voltage and signal DC bias levels resulting in small timing changes.

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Test proceduresTesting tolerance to varying temperature

Variations in ambient temperature can change the device temperature which may result in significant propagation delay changes.

The effect may be localized by selective temperature adjustment by applying local heat/cooling at the chip level to isolate the effect.

Heat gun

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Test proceduresTest vectors

At the device level, board level, or system level comprehensive testing requires exercising all circuit functions at speed, simultaneously.

Testing functions independently is useful but not conclusive.

Such testing requires that all inputs and expected outputs be specified.These are known as test vectors

The inputs are used to stimulate the circuit and the responses are observed.

The test vectors bring the circuit or system to a known state (e.g., reset, or all 0s) and then steps through all states of interest.

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Test proceduresTest vectors

Such testing brings together all factors affecting performance including timing, crosstalk, EMI, noise on the power and ground.

Such testing is often expensive and limited.

It typically involves an extensive variety of test equipment, programming, and planning.

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Future trendsApplication-driven advances

Evolutionary and revolutionary advances require ever-increasing data rates and computational throughput in smaller packages while consuming less power.

Leading applications include:• computing (gaming consoles, super computers)• sensors (fine resolution video, radar, RFID)• wireless communication

Emerging technologies• energy harvesting (power derived from local environment)• optical interconnects die-to-die (optical transmitters/receivers integrated into die)

– may involve holographic reflectors or optically transparent substrates