September 2004
Arbitrary waveform generators (AWG) are signal
sources that derive their analog outputs directly from digital
information. Those AWGs capable of generating instantaneous
bandwidths more than a few hundred MHz are generally considered
wideband AWGs, compared to more general purpose AWGs offering
analog bandwidths only in the tens of MHz. The wideband AWG's
ability to generate arbitrary signals is important in aerospace
and defense and emerging communications where a wide range of
signal formats are used. Many of these are either proprietary
or do not comply with any accepted commercial standard.
The Wideband Challenge: Bandwidth and Dynamic
Range
Wideband AWGs are used increasingly to simulate signal scenarios
for receiver testing and transmitter upconversion blocks. The
fundamental challenge designers face, however, has been the
choice of the AWG's bandwidth or dynamic range. They simply
are not able to achieve both, simultaneously, because digital
to analog converters (DAC) used inside most commercial AWGs
offer either high sample rates (bandwidth) or high bit resolution
(dynamic range). This issue forces designers to add extra signal
conditioning elements to their wideband designs to remove the
distortion products caused by the AWG. Alternately, designers
can purchase higher performing DACs and build-up AWG capabilities
needed for their specific design. This latter approach is frequently
used in the evaluation of high performance military systems
like radar, electronic warfare (EW) and secure communications,
but requires the designer to allocate extra resources building
test equipment instead of focusing on the more value-added design
elements of the project. Developing custom AWGs is also counter
to the military's move toward greater reliance on commercial
off-the-shelf (COTS) equipment.
The N6030A is a high-performance AWG that
provides engineers with both wide bandwidth and wide dynamic
range, simultaneously. Its dual, differential output channels
operate at 1.25 gigasamples per second and offer 15 bits of
resolution giving engineers 500 MHz of instantaneous analog
bandwidth and more than 65 dB of spurious-free dynamic range
in each channel. When combined with a high-performance vector
signal generator, IQ bandwidths of 1000 MHz can be achieved
at microwave frequencies.
Radar Signal Simulation
Modern radar and EW systems utilize pulse compression techniques
to maximize both radar range and target resolution. The benefit
of pulse compression is that it allows the radar to identify
and classify enemy threats sooner compared to legacy systems.
Bandwidths for such pulse compressed or "chirp" radars
routinely exceed 300 MHz and can extend into the GHz range.
AWGs used to test these radar receivers must not only have the
required wideband capability, but also be capable of generating
clean, low-distortion signals so as not to introduce false targets
(spurs) into the radar receiver.
Wideband AWGs are also used to test upconversion
blocks of the radar/EW transmitter. Especially important is
that the AWG possesses a two-channel architecture when used
as the system's baseband generator (exciter) to create the in-phase
(I) and quadrature (Q) signal components.
Frequency Agile Radios
Many military and commercial radios employ complex modulation
schemes that hop their carrier frequencies. Where commercial
radios often hop to improve spectral efficiency, military radios
utilize frequency agility to improve communication security
(low probability of intercept). In both cases, test waveforms
are required to modulate and/or hop the modulated signal across
a wide bandwidth. One such example is the Link-16 tactical radios
used by the United States Navy, the Joint Services, and forces
of the North Atlantic Treaty Organization (NATO). Though the
actual modulated carrier is only a few MHz wide, it is dynamically
hopped across 255 MHz of frequency spectrum. New radio designs
based on the Wideband Networking Waveform (WNW) format have
even wider hop bandwidths. Another example where test signals
are required to test frequency agile radios is the IEEE 802.15.3a
Ultra Wideband (UWB) format. One implementation includes hopping
a 528 MHz wide signal across three adjacent channels.
In either case, engineers use wideband AWGs
to encode, modulate and hop waveforms during the test and evaluation
of their new transceiver designs. As in the case of radar and
EW, signal distortions in the AWG output make it challenging
for the designer to discriminate between distortions in the
design and those generated by the AWG.
Consolidated Assets
R&D labs looking to maximize usage of their capital equipment
will often buy test equipment for a variety of design tasks.
Wideband AWGs, by their very nature, have the ability to generate
custom waveforms for both analog and digital designs. Wideband
AWGs can generate data patterns for high-speed digital bus and
signal integrity tests. They can also be used to generate standard
waveforms like those found in traditional function generators.
With a wideband AWG, one general purpose asset can be used to
create all these signals.
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Contacts:
Janet Smith, Agilent
+1 970 679 5397
janet_smith@agilent.com
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