 Two Sides of the DoD Coin: Budgets Slashed, UAV Market Soars
By Fred Ortiz, President
dB Control
As we embark on a new year, imminent cuts to the U.S. Department of Defense (DoD) budget are top of mind for those of us in the military electronics market. At a recent House Armed Services Committee hearing, the nation’s military chiefs cited a $600 billion defense cut as “catastrophic to the military” and having a “severe and irreversible impact.”
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LightSquared:
The Show’s Over
…Or Should Be
By Barry Manz
There are a lot of very technically astute people at the Federal Communications Commission. Many have decades of experience at every level of RF and microwave technology. How then might LightSquared’s proposal for a satellite/terrestrial LTE network have ever gotten past its first hurdle? Even a cursory inspection of the plan, in which the company's network would operate extremely close to GPS frequencies at L-band, makes interference to GPS devices almost a certainty. Read More...
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 Parallel Gap Welder
Model SMAPRO180 parallel gap welding machine can weld gold and silver ribbons as well as enameled wires without additional coating layer stripping steps. It eliminates the expensive and difficult stripping process and results in more reliable joints.
SW Tech Equipment
 Signal and Spectrum Analyzer
The R&S FSW signal and spectrum analyzer comes in three models that cover the frequency ranges from 2 kHz to 8 GHz, 13 GHz and 26.5 GHz. The analyzer outperforms all other high-end instruments on the market, with phase noise values that are up to 10 dB lower.
Rohde & Schwarz
 Externally Biased Balanced Mixer
Model SFB-15-N2 is a V-band, externally biased balanced mixer. The mixer employs high performance GaAs Schottky beamlead diodes, balanced configuration and proprietary bias circuitry to produce superior RF performance with very LO pumping level.
Sage Millimeter
See all products in this issue
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The
Doherty Amplifier: New After 70 Years
By Freescale Semiconductor, RF Division
The Doherty amplifier architecture has in
less than 5 years become the “amplifier of choice”
for new wireless transmitters after essentially laying dormant
since W.H. Doherty first described it in 1936. The Doherty’s
obscurity is directly attributable to the predominant modulation
schemes (AM and FM) employed in communication systems over
the years, which do not possess high peak-to-average ratios
(PARs). The resurgence of interest in the concept is based
on its very high power-added efficiency when amplifying
input signals with high PARs – precisely the type
exhibited by WCDMA, CDMA2000, and systems employing Orthogonal
Frequency Division Multiplexing (OFDM), such as WiMAX and
the upcoming Long-Term Evolution (LTE) enhancement to the
UMTS wireless standard.
In fact, when properly designed, a Doherty
amplifier can produce increases in efficiency of 11% to
14% when compared to standard parallel Class AB amplifiers
that have traditionally been employed in wireless base station
transmitters. Since the transmitter accounts for a high
percentage of overall system power consumption, the cost
savings delivered by the Doherty amplifier’s efficiency
can reduce base station annual electricity costs. Thus its
appeal for wireless base station manufacturers and wireless
service providers.
While the intrinsic high efficiency of the Doherty architecture
makes it desirable for current and next-generation wireless
systems, it presents unique challenges from a design perspective.
The linearity and output power of the Doherty architecture
are slightly less than exhibited by a dual Class AB amplifier,
and it can produce higher distortion as well. Fortunately,
the advancements in analog and digital predistortion and
feed-forward linearization techniques can dramatically reduce
the Doherty’s distortion. In addition, careful amplifier
design can mitigate its inherently lower linearity. The
remaining challenge is to create RF power transistors that
can accommodate the requirements of the two types of amplifiers
employed by the Doherty architecture and produce optimum
RF output power over a wide array of signal conditions.
A Doherty overview
A “classic” Doherty amplifier (Figure 1) employs
two amplifiers. The carrier amplifier is biased to operate
in Class AB mode and the peaking amplifier is biased to
operate in Class C mode. The input signal is split by a
power divider equally to each amplifier with a 90-deg. difference
in phase. After the signals are amplified, the signals are
recombined with a power combiner. Both amplifiers operate
when the input signal peaks, and are each presented with
the load impedance that enables maximum power output. However,
as the input signal decreases in power, the Class C peaking
amplifier turns off and only the Class AB carrier operates.
At these lower power levels, the Class AB carrier amplifier
is presented with a modulated load impedance that enables
higher efficiency and gain. The result is an extremely efficient
solution for amplifying the complex modulation schemes employed
in current and emerging wireless systems.
FREESCALE
SEMICONDUCTOR
www.freescale.com
TXTLINX.COM 105
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