Interference: The Creeping Menace
By Anatech Electronics
Interference has been the bane of communication system designers since the earliest days of radio. At least back then, there were fewer sources to contend with but also far fewer remedies at their disposal.
|MILITARY MICROWAVE DIGEST
Band Reject Filter Series
Higher frequency band reject (notch) filters are designed to operate over the frequency range of .01 to 28 GHz. These filters are characterized by having the reverse properties of band pass filters and are offered in multiple topologies. Available in compact sizes.
SP6T RF Switch
JSW6-33DR+ is a medium power reflective SP6T RF switch, with reflective short on output ports in the off condition. Made using Silicon-on-Insulator process, it has very high IP3, a built-in CMOS driver and negative voltage generator.
Group Delay Equalized Bandpass Filter
Part number 2903 is a group delayed equalized elliptic type bandpass filter that has a typical 1 dB bandwidth of 94 MHz and a typical 60 dB bandwidth of 171 MHz. Insertion loss is <2 dB and group delay variation from 110 to 170 MHz is <3nsec.
Absorptive Low Pass Filter
Model AF9350 is a UHF, low pass filter that covers the 10 to 500 MHz band and has an average power rating of 400W CW. It incurs a rejection of 45 dB minimum at the 750 to 3000 MHz band, and power rating of 25W CW from 501 to 5000 MHz.
LTE Band 14 Ceramic Duplexer
This high performance LTE ceramic duplexer was designed and built for use in public safety communication and commercial cellular applications. It operates in Band 14 and offers low insertion loss and high isolation to enable clear communications in the LTE network.
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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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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...