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Stimulating the Pulse of
Today’s Electronics
by Tom Lecklider, Senior Technical Editor
As the lights dim, a hush falls over the hundreds of delegates anxiously
awaiting this year’s announcement. “And the product elected to the Test
Instrument Hall of Fame is…(opening the envelope)… the Pulse Generator!”
(Applause and cheering, hugs and congratulations, a few tears from the losers…)
The pulse generator won a majority of the delegate votes because it has helped
the development of so many successful products. For example, a pulse generator
simulated the encoded pulse trains at the heart of an infrared TV remote
controller. A pulse generator provided the pulse width modulated (PWM) outputs
to a prototype motor drive’s power stages. And, how would the solenoids in
electrically controlled door locks be life-tested without a pulse generator?
These and many other apparently straightforward applications are solved every
day by pulse generators. As you may expect, there are many types of generators
to suit different needs. Years ago, before such a wide range of functionality
was available, engineers mocked up small circuits with counters and decoders to
generate the specific pulse patterns they needed. Of course, that circuitry
usually was driven by a simple pulse generator serving as a clock signal.
Since then, pulse generators have developed in several dimensions. The basic
generator produces one or two pulses with variable amplitude, width, and
repetition rate. Pulse-pattern generators add programmability in the time axis.
Within limits, you can define the widths, positions, and the repetition rate of
a succession of pulses.
Arbitrary waveform generators (Arbs) are programmable in both amplitude and time
and can produce virtually any waveform with any amplitude and timing. Of course,
you can use an Arb as a pulse-pattern generator simply by restricting the shape
and height of the pulses.
Similarly, a function generator is a source of sine, triangle, and square waves.
If duty cycle and repetition rate controls are available, the square-wave output
can be altered in width and frequency to provide the required pulse signal.
Basic Pulse Generators
The Geotest Model GP1612H Programmable Pulse Generator is representative of
basic pulse generators and a direct replacement for the obsolete HP8112A Pulse
Generator. In many cases, the cost of test-procedure upgrades can be avoided by
substituting exact replacement instruments when the original no longer can be
obtained and cannot be repaired or maintained economically.
Table 1 lists the major specifications of the GP1612H. More sophisticated
products such as pulse-pattern generators or enhanced-performance units with
high power or voltage outputs are based on fundamental pulse generator
capabilities.
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Table 1. Abbreviated GP1612H Specifications
Courtesy of
Geotest-Marvin Test Systems |
To keep Table 1 a manageable size, many details have been listed by type rather
than value. For example, both the pulse width and its delay from the trigger
have a range of values, a smallest resolution applying to those values, an
accuracy, and an amount of jitter or uncertainty. Many improvements have been
made in newer instruments, but these fundamental parameters still apply.
Today’s relatively straightforward pulse generators typically are low cost and
have similar or less flexibility than the HP8112A. For example, the
single-channel Protek Model B1990 Pulse Generator provides variable pulse width
and delay, a 10-MHz repetition rate, and a 5-V maximum output into 50 Ω. The
rise time is specified to be a fixed 10 ns. Nevertheless, there are many uses
for these basic capabilities.
C&H Technologies makes the MA209 Programmable Pulse Generator in the
M-Module format for use with VXI, VME, PCI, and cPCI M-Module carrier boards.
This is a single-channel generator with similar capabilities to the HP8112A, but
the repetition rate is specified as 0.093 Hz to 100 MHz, output levels are from
-1.5-V to 6.5-V open circuit, and pulse width ranges from 5 ns to greater than
10 s. A reference oscillator input supports phase-locking the generator to a
10-MHz external clock.
A number of system-oriented features distinguish this generator from a
general-purpose benchtop instrument. They include programmable input thresholds
on the gate, trigger, and reference clock signals and a choice of front-panel or
M-Module backplane signal source/destination.
C&H President Fred Harrison said, “To date, most of our pulse generators have
been traditional with a few of our specialized designs being higher voltage or
sequencing on multiple outputs. Typical applications have included the primary
synchronization clock for a semiconductor ATE, as legacy replacements used in
combination with signal conditioning M-Modules, and as primary clocks for UUTs
on a number of test sets.”
Pulse Pattern Generators
Several test and measurement instrument manufacturers have developed new pulse
generators with digital communications in mind. This application space includes
computer I/O buses as well as wireless formats such as WiFi and, most recently,
WiMAX. These instruments feature pulse-pattern capabilities and generally run at
much higher repetition rates.
For example, the Agilent 81100 Series Pulse Pattern Generators include models
with a 660-MHz repetition rate. At this fast speed, pulse transition times are
fixed or selectable between two values. There also are models such as the 81101A
much closer in performance to the HP8112A. This is a list of the changes made to
the basic specifications:
• Minimum repetition rate reduced to 1 mHz from 100 mHz.
• Maximum width and delay ranges increased from 9.8 s to 999.5 s.
• Jitter reduced from 0.1% of setting + 50 ps to 0.01% + 15 ps.
• Output amplitude extended to 20 V open circuit from 16 V and levels set as a
combination of offset and amplitude.
• Maximum transition time extended from 100 ms to 200 ms.
• Source impedance switchable between 50 Ω and 1 kΩ.
Equally important, the newer platform supports features such as a graphical
display, autoset, and preset TTL/ECL levels. On higher-performance models in the
family, double pulse and pattern mode capabilities have been added. Pattern mode
provides a pseudorandom bit sequence (PRBS) and in some models also can be
programmed to output pulses as required within the limitation of 16-k possible
positions. The pulse output format is selected from return to zero (RZ),
non-return to zero (NRZ), or delayed NRZ (DNRZ).
Pattern mode reflects the move to serial data communications that has been in
progress for several years. It also relates to the prevalence of digital
circuitry instead of analog and the associated need for more complex digital
signal generation.
Series 3400 Pulse/Pattern Generators from Keithley Instruments also fall into
this category. The Model 3401 is a single-channel instrument while the 3402 adds
a second channel. The repetition rate and basic mode of operation are common to
both outputs, but width, pattern, transition time, and delay can be set
independently.
These instruments feature wide operating ranges such as the 1-mHz to 165-MHz
repetition rate and 3-ns to nearly 1,000-s pulse width. Patterns must fit within
the 16-k memory, and if separate patterns are programmed for each channel, they
must be the same length. NRZ and RZ formats apply to either the built-in PRBS
pattern or user-programmed patterns. The generator can be phase locked to a
10-MHz external clock.
Commenting on the importance of new technologies in his company’s products,
TEGAM’s President Adam Fleder said, “The high level of integration offered by an
FPGA-based design enabled us to combine a function generator using direct
digital synthesis, an Arb, and a pulse generator into one instrument. The 2700A
Hybrid Series Function/Arbitrary Waveform Generators also include 4 MB of
waveform memory that permits simulation of nonperiodic waveforms and pulses that
otherwise is not possible.”
The user can control the frequency, amplitude, offset, phase, duty cycle, and
rise/fall times of the standard sine, square, triangle, and pulse waveforms.
Standard pulses have repetition rates from 0.5 mHz to 25 MHz, or using the
generator’s arbitrary waveform capabilities, rise/fall times can be as low as 6
ns with a maximum 62.5-MHz repetition rate.
Fast Pulse Generators
Agilent’s Models 81130A/32A provide up to a 660-MHz repetition rate with typical
transition times of 500 ps or as low as 200 ps for ECL levels. That’s fast, but
the company classifies these models as precision pulse generators. Two-channel
versions operate in a differential mode. However, the Models 81133A, 81141A, and
81142A are the high-speed family members with repetition rates up to 13 GHz.
Beate Hoehne, product manager, explained the positioning of these instruments,
“The 81133A/81134A are 3.35-GHz pulse pattern generators targeted at cross-over
point adjustment [duty-cycle distortion] and jitter insertion requirements for
PCI Express or Serial ATA standards testing applications. The 81141A/42A provide
the extremely short pulses and fast transition times needed in physical-layer
stress tests. In addition, they are used by scientists conducting fundamental
lab research.”
The 81141A/42A have fixed 10% to 90% rise and fall times less than 25 ps, output
amplitude adjustable from 0.1 to 1.8 V, and an output voltage window from -2.0
to +3.0 V. This is another way of describing the interaction between amplitude
and offset.
Specifications include 9-ps peak-to-peak typical intrinsic data jitter and a DC
to 1-GHz delay input modulation bandwidth. One type of test deliberately injects
jitter to determine how well the clock/data receiver (CDR) rejects it. Some
communications standards specify a 500-MHz jitter bandwidth, so the generator
must have an even higher bandwidth. Another aspect involves bit error rate
testing for which very long bit sequences are needed. The 81141A/42A support
patterns up to 16-Mb in length.
As pulse speed increases, most generators offer fewer control selections. For
example, the Picosecond Pulse Labs Model 4005 5-ps Calibration Source is a
relatively low repetition rate generator with a range from 0.1 Hz to 1 MHz.
However, the pulse fall time is only 5 ps. For edges this fast, measurements are
not straightforward. No longer can you ignore the rise time of the measuring
instrument.
According to the datasheet, “The 4005 unit was measured with a 70-GHz
scope…contributors to the system rise time include the scope (4.85 ps) and the
averaged jitter (2.7 ps) for a total system rise time of 5.6 ps. The measured
fall time was 7.2 ps.”
The actual pulse fall time is calculated from:
Pulses this fast can’t easily be generated with conventional circuitry.
Picosecond Pulse Labs has developed GaAs nonlinear transmission lines (NLTL) for
this purpose. Figure 1 is a SEM image showing a few of the many varactors
arranged along the length of the line. NLTLs provide a different amount of delay
to each voltage level in a signal. Low voltages are delayed more because the
capacitance of the shunt varactors is higher at low voltages.
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Figure 1. Scanning Electron Microscope Image of Nonlinear
Transmission Line
Courtesy of
Picosecond Pulse Labs |
Because higher voltages are not delayed as much, the parts of the pulse edge
following a rising transition tend to catch up with the more slowly moving lower
amplitude parts. This process sharpens the pulse rise time. To achieve the best
results with this technology, separate pulse heads are used, each with a fixed
output level.
In addition to instrument calibration, very fast edge generators are used in
time-domain reflectometry (TDR). The faster the edge of the TDR pulse, the
higher the frequencies that can be discerned for time-domain network analysis
applications.
LeCroy has worked with Picosecond Pulse Labs to incorporate the NLTL technology
into oscilloscopes. Dr. Mike Lauterbach, LeCroy’s director-product management,
said, “We were the first company to use NLTL in the front-end sampling head of a
very wideband oscilloscope, the 100-GHz WaveExpert®. The same NLTL technology is
used to make fast edges, typically 30-ps rise/fall times for the high-speed
PPG-E135 Pulse Generator.”
The PPG-E135 is intended for use with the WaveExpert scope. It is programmed via
the scope menu system and plugs into one of the four module slots. An internal
clock drives PRBS sequences up to 231-1 in length at rates from 50 MHz to 11.5
GHz. An external clock input operates to 12.5 GHz and allows synchronization to
an external system. One of the benefits of NLTL technology in this application
is its very low jitter specified to be <1-ps rms.
Tektronix describes the DTG5000 Series of data timing generators as “combining
the power of a data generator with the capabilities of a pulse generator.” Data
generators emphasize the quantity of outputs and their timing characteristics
such as jitter. The Tektronix DTG5000 Series has a number of built-in patterns
and provides jitter-generation capabilities for as many as 100 parallel
channels.
Serial data standards require that a clock/data receiver operate correctly in
the presence of a specified amount of jitter. Generating both the right amount
as well as the correct composition of jitter is needed not only in serial data
systems, but also in more general-purpose DAC, memory, and ASIC testing.
In addition, the DTG5000 simulates the operation of a complex digital circuit.
Bob Buxton, product line marketing manager, explained. “The DTG can be used
early in the product development cycle to substitute for system components that
are not yet available. For example, it might be programmed to send interrupts
and data to a newly developed bus circuit when the processor that normally would
provide the signals doesn’t yet exist.”
Heavy-Duty Pulses
A few instruments with both source and measure capabilities also have a pulse
mode of operation. Keithley’s Series 2600 System SourceMeter products combine a
precision power supply, a true current source, a DMM, an Arb, a voltage or
current pulse generator with measurement, an electronic load, and a trigger
controller. Similarly, the Yokogawa GS610 Source Measure Unit also provides
voltage and current sources and measurement and has a pulse output mode that can
be combined with sweep capabilities.
If your application involves driving a solenoid or motor for a short period of
time, these instruments may be good choices. For example, the Yokogawa Model
GS610 can provide a continuous pulse output or various sweep modes including
linear, log, and program. The pulse width and period can be set from 100 µs to
3,600 s in 1-µs steps. The unit has a programmable voltage or current output up
to ±110 V and ±3.2 A within a 60-W source or sink maximum power limit.
You can delay the start of an output pulse from 1 µs to 3,600 s after a trigger.
Then, the start of the measurement process can be further held off to allow the
load to react to the pulse. This capability allows the steady-state current
drawn by a motor to be measured following startup, for example.
Optoelectronic device characterization is an application area in which
Keithley’s Series 2600 SourceMeter is used. Shorter pulses down to 5 µs are
provided by the company’s Model 6221 Current Source and as short as 10 ns in the
Model 4200-PG2 Pulse Generator option for the Model 4200-SCS Semiconductor
Characterization System.
Pulse I-V testing addresses two challenging problems: charge trapping and
self-heating. High dielectric constant materials such as HfO2 are being
introduced into semiconductor manufacturing. Depending on the exact method of
material application and subsequent processing, a large number of charge
trapping sites may be present. Trapped charge alters device thresholds. Very
fast pulse testing can measure the device’s intrinsic characteristics because
charge-trapping effects are time related.
Self-heating long has been associated with resistance measurement and often is
addressed by pulse testing. If the test pulse is short relative to a device’s
thermal time constant, measurements taken immediately following the pulse will
be accurate because the device’s temperature hasn’t yet begun to change. For
example, this technique is used to avoid self-heating errors in platinum
resistance temperature detectors (RTD). The RTD’s resistance is measured to
determine temperature, so self-heating must be kept to a minimum.
Precision Pulse Delay
Highland Technology calls the Model T560 an embedded delay generator. It does
provide a delay, but that’s only part of the functionality. In response to a
trigger input at up to a 20-MHz rate, the instrument outputs four TTL-level
pulses, individually programmable for pulse width and delay. Width ranges from 5
ns to 10 s and delay from zero to 10 s, both parameters with 10-ps resolution.
This mix of capabilities serves applications such as radar/lidar simulation and
laser sequencing.
Conclusion
As the many product examples show, the subject of pulse generators is very
broad. It also is deep in the sense that a large number of companies make pulse
generators of several types and package formats. In addition, a great deal of
overlap exists among products other than at the extremes of performance or
price.
Units that have specialized capabilities may have limited usefulness. For
example, high power units don’t support a high repetition rate. Very low-cost
pulse generators are limited in their capabilities. Very fast pulse generators
are not appropriate for lower speed work.
However, if you need one or more pulse outputs with only moderate speed,
higher-level considerations will influence your buying decision. For example, if
you already are using a PXI-based test system, PXI modules from National
Instruments, Pickering, and United Electrical Industries may be suitable.
Similarly, you would want to choose an instrument that best matched existing
VXI-, LXI-, GPIB-, or PCI-based systems. An Arb is a good choice if your pulse
requirements are not too stringent and you will need to generate other types of
signals as well as pulses.
As in any instrument selection process, begin by thoroughly understanding your
own requirements. For example, you may need to generate a pulse pattern that
repeats after each trigger input. That’s easy enough, but if trigger pulses
occur more closely spaced than the length of the pattern, should the pattern
immediately start over on each trigger? Should the pattern always run to
completion? Is your understanding of the term retriggerable the same as the
definition used by the generator manufacturer? This kind of detail indicates the
degree of complexity possible with modern pulse generators.
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