The extraordinary complexity of smartphones, tablets, and other wireless-enabled products coupled with new product “sell-by” dates measured in months can make design, testing, and verification a formidable challenge at best.
The New Face of Hardware in the Loop
By Justin Panzer, Business Development Manager, Tektronix
In Defense of DARPA; Lamenting Bell Labs
By Barry Manz
A federal agency like DARPA is a sitting duck for politicians and assorted other critics. It has come up with some truly bizarre programs over years that ultimately either delivered no tangible results, were canceled before they could cause any damage, or attempted to answer questions that nobody was asking or needed answers to. Read More...
Shielded Honeycomb Vent Panels
Design and sales support are now available for MAJR Products’ ready-to-install EMI/RFI shielded honeycomb ventilation panels. Designated the 3000 series, these ventilation panels are ideal for both portable and rack mounted telecommunication systems.
Compact Broadband Amplifier
The R&S BBA150 broadband amplifier is now also available with a frequency range from 2.5 to 6.0 GHz in addition to the existing range from 0.8 to 3.0 GHz. The state-of-the-art amplifier design yields up to 200W of output power in just four height units.
Rohde & Schwarz
Low Loss 4x2 Hybrid Diplexer
The new CM-20 is an integrated 4x2 hybrid multi-band combiner with a unique design that combines two low band signals within 698 to 960 MHz with two high band signals within 1710 to 2170 MHz. It is ideal for low PIM head end DAS applications.
Coaxial Bandpass Filter
ZX75BP-1940+ is a wideband bandpass filter in a rugged connectorized package covering 1710 to 2170 MHz. It is designed for asymmetric rejection applications such as super-heterodyne receivers. Features include fast roll-off on the upper sideband, good matching and low loss in the pass band, and consistent performance over temperature.
The FSC-200 Fan Speed Controller provides precise management of fan speeds during thermal management studies of electronic devices. It comes with the company’s stageSPEED software, which allows the use of a PC to incrementally control the speed of the fan and fan trays from 0 to 100% of their maximum power.
See all products in this issue
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
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.
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MILITARY MICROWAVE DIGEST
•Directivity and VSWR Measurements
Return loss and VSWR measurements are complicated by the finite performance of the directional device used to measure the reflected power. The only accurate and convenient way to make return loss measurements is with a well matched high directivity directional coupler or bridge.
•Switch Solutions for Systems with Low PIM Requirements
Dow-Key Microwave has invested in R&D for new RF switch products designed specifically to reduce intermodulation (IM) in coaxial switches.
• How to Specify RF and Microwave Filters
Covers cavity, ceramic, LC, crystal and helical filters.
• Mounting Considerations for Medium Power Surface-Mount RF Devices
Covers all factors that must be considered when mounting SMT devices.
• Biasing MMIC Amplifiers
How to bias MMICs along with theory and techniques.