Transcript HSD Test & Measurement

High-Speed Digital
Test & Measurement
Chris Allen ([email protected])
Course website URL
people.eecs.ku.edu/~callen/713/EECS713.htm
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Topics
Test equipment
Oscilloscope characteristics
Oscilloscope probes
• Passive probes
• Active probes
• Probe inductance from ground lead
Design for test
Test procedures
Measuring noise margin
Measuring timing margin
Sensitivity to supply voltage variations
Sensitivity to temperature variations
Test vectors
Future trends
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Test equipment
Oscilloscope characteristics
Note: to relate BW3dB to T10-90
T10-90 = 0.338 / BW3dB
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
Tr measured   Tr2 circuit   Tr2 vertical amplifier 
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Test equipment
Scope 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
Tr measured   Tr2 circuit   Tr2 vertical amplifier   Tr2 probe
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
Tr circuit  
820 ps2  676 ps2  338 ps2
 318 ps
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Test equipment
Scope 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 equipment
Scope 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.
Simplified equivalent circuit
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.
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Test equipment
Other 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 loading
problem we can use
probes with less
capacitance.
Another option is to
use active probes
that use FET
amplifiers to isolate
measurement
capacitance from the
circuit under test.
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Test equipment
Active 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 equipment
Probe 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 equipment
Probe 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 all
signals to be probed
must have a nearby
ground pad for the probe.
Therefore generous use of
ground metal on the board
surface facilitates probing.
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Test equipment
Special probing fixtures
The author discusses a low-cost, shop-built 21:1 probe
built from a length of coaxial cable
and a leaded 1-kΩ resistor.
Division ratio 
50
50  1000
Division ratio  0.048  1 : 21
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
Assumptions
scope is terminated with 50 Ω (to reduce reflections)
ground pad is available near the signal to be probed (short lead length)
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Test equipment
Special 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
testing with passive probes
circuit under test: 125-MHz oscillator
testing with shop-built, wide bandwidth probe
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Design for test
Special 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 procedures
Once 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 procedures
Noise 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 procedures
Timing testing
By varying the relative timing of the clock and data signals, the timing
margin can be estimated.
Use of a coaxial delay line
select a clock or
remove a portion
data line to
of the line and
be tested
expose the copper
insert a short
segment of
coaxial line
Demonstrates
usefulness of
ground area
on surface
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.
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Test procedures
Testing 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 procedures
Testing 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 procedures
Test 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 procedures
Test 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 trends
Application-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
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