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A POWER AMPLIFIER YIELDS 10 WATTS OVER 8 14 GHZ USING -
GAAS MMlCS IN AN LTCC SERIAL COMBLNEWDIVIDER NETWORK
J.W. Gipprich, L.E. Dickens, J.A. Faulkner
Westinghouse Electric Corporation
P.O. Box 746
Baltimore, Maryland 21203
Abstract is extremely disadvantageous because the resulting
aspect ratio of the surface area deviates drastically from
The main thrust of this work was to develop a high that required for a "behind the element" location. The
efficiency N-way combining scheme consistent with the signal divider/combiner scheme just described is usually
physical geometries associated with X and Ku band active called the "corporate" structure.
apertures. A 12-way combiner utilizing stripline serial feed
networks in Low Temperature Cofired Ceramic (LTCC)
was designed, fabricated and tested over an 8-14 GHz
bandwidth. This combiner was integrated with 6 dual
channel 1 watt MMlCs to achieve 10 watts peak output 3 dB HYBRIDS
power with greater than 86% combining efficiency at
center band. The results of this work are described within
this paper.
Introduction
Electronically scanned active array antenna applications
such as weather monitoring, windsheadmicrobursts,terrain
avoidance/ground mapping and high resolution imaging
IN
radar call for very wideband performance of both the
transmit (Tx) and the receive (Rx) functions. The large
numbers of T/R channels comprising these active arrays
drives the designer to the lowest cost solutions for the Tx
power amplifier functions. Concomitant with the
requirement of low cost is the requirement for attaining
very high performance within physical constraints dictated
by operating frequency, antenna element spacing, prime Figure 1. Corporate Splitter/Combiner
power availability and system heat removal (cooling)
capacity. The simultaneous solution to low cost and high
performance power amplifiers is achieved by utilizing high
yielding low power MMlCs with low loss combining
techniques. Figure 2 presents an amplifier layout more appropriate to
the "behind the element" location because the width is
ADDroach unaffected by the number of combined MMICs. The figure
shows the associated coupler structure which uses the
The amplifier herein presented attains very broadband "serial" dividedcombiner scheme for an 8 way network
performance combining six dual channel MMlCs with a 12- comparable to the corporate scheme shown in figure 1.
way serial combiner in a configuration which supports the This configuration has somewhat lower loss than the
transmit function of an active array with element spacings corporate structure of figure 1 because the mean path
of less than 0.50 inches. Typically, these power amplifiers length for the amplifier MMlCs is less resulting in lower 12R
have used several transistors combined in parallel by losses. In addition, the serial structure is not forced to
several binary levels of microstrip Lange couplers to meander into a central feed point as the corporate
achieve the required output power. This arrangement is structure. This meander often leads to radiating
shown in figure 1. This "pyramid" structure which results discontinuities which result in additional losses.
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CH3277-1/93/0000-1369$01.CK)0 1993 IEEE 1993 IEEE MlT-S Digest
multi-layered laminations. The microwave signal layers
are well shielded and isolated within the LTCC substrate.
The GaAs MMlCs are mounted in pockets (cutouts) in the
RF LTCC. The use of the LTCC provides exceptional
OUT isolation in critical areas and results in absolute stability of
the module (no tendency to oscillate under any condition
of termination at any operational temperature).
The LTCC microwave signal distribution circuitry is shown
in Figure 3. The LTCC material chosen for this design is
the DuPont type 845 which has a dielectric constant of 4.8
and a loss tangent of .004. The dividerkombiner networks
are 12 way power split circuits using six-way serial
distribution and two-way parallel combining. The six-way
networks are five overlay stripline directional couplers
arranged in a serial fashion to provide six equal amplitude
signals at the outputs. The couplers were designed for a
.042 inch ground plane separation and a .0039 inch
l l k k k l f
9.03 8.45 7.78 6.99 6.02 4.77 3
conductor spacing. The amount of coupling for each
coupler was controlled by the amount of offset from the
dB dB dB dB dB dB dB main coupler line.
Figure 2. Serial Splitter/Combiner
A wide band power amplifier was designed, fabricated and
tested using this series feed technique in conjunction with
GaAs MMlC power amplifiers. The GaAs MMlC chosen
for the wideband power amplifier was the Raytheon RMM
2060 Power Amplifier Chip. The 2060 is a broadband (6-
18 GHz) fully matched dual channel amplifier integrated on Figure 3. LTCC Stripline Splitter/Combiner
a single GaAs substrate. Each channel is comprised of 4
stages of amplification with a gain of 18 dB and an output
power of +29 dBm at the 2 dB compression level. Over
the 8-14 GHz bandwidth, each channel typically provides
an output power greater than 1 watt. To achieve 10 watts Figure 4 shows the multilayer LTCC assembly. The
of power output, six dual channel chips are combined substrate contains 15 layers of ceramic, each layer is
using the serial dividerkombiner network. The high gain .0039 inch thick. The bottom of the substrate is metallized
nature of the power amplifier minimizes the effect of the and serves the bottom stripline ground plane. The top of
splitter losses on the power added efficiency of the overall the fifth layer contains the main line feeding the coupler
1OW power amplifier. The distributed nature of the first network. The coupler circuitry is located above or1 the top
two stages in the 2060 provide an excellent input VSWR of the sixth layer. The top stripline ground plane is located
to the chip. The low input VSWR coupled with the on the eleventh layer. The next four layers contain the
uniformity of S parameters, in particular an insertion phase gate lines, a ground plane for isolation, the drain lines, and
uniformity of +lo degrees among the 6 chips, reduces the a top ground plane respectively. Cofired vias connect the
interaction of the chips with the splitterkombiner and buried DC bias and power lines to the top surface where
enhances the overall combining efficiency. connections to the GaAs chips are made. Decoupling
capacitors are mounted to the top surface. The amplifier
The substrate selected for microwave signal, as well as chips which are mounted on CM-15 carriers for thermal
DC bias and drain power distribution was a Low spreading, are mounted onto chassis ground through a
Temperature Cofired Ceramic (LTCC) substrate made of cutout in the LTCC substrate.
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Figure 4. LTCC Multilayer Assembly
Results Figures 6 and 7 show the amplifier assembly mounted on
a test fixture and mounted in a conventional X-ku band
The dividedcombiner network was tested prior to insertion module package. The test fixture includes in addition to
of the amplifier chips to determine the combining the power amplifier, a driver stage, gate regulators, drain
efficiency. 50 ohm microstrip lines were placed in the regulator/modulators, and storage capacitors for pulse
locations of the amplifier chips. The insertion loss was operation. The tests were run at 25°C for peak input
measured for the "back to back" dividerkombiner network levels of -5 dbm to +8 dbm. The RF signals were pulsed
as a function of frequency. The measured insertion loss at a duty cycle of 1% and a pulse width of 10
was less than 1.3 dB of the band center and about 1.8 dB microseconds. The performance shown in Figure 8 was
at the edges of the 8-14 GHZ band. The combiner loss achieved with no RF tuning. Greater than 10 watts (+40
(assumed to half of the total loss) is .65 dB and .90 dB at dbm) was achieved over most of the band from 9 to 13
the center and band edges respectively. The Measured GHz dropping slightly below 10 watts at the 8-14 GHz
losses agree well with the predicted losses modelled using band edges. The measured power at 8 and 14 GHz was
Touchstone. The measured results are shown in figure 5. 39.8 dbm and 39.6 dbm respectively.
52 1
REF 0 . 0 dE
2 0 . S dE/
Predicted
Measured
START 6.000000000 W z
STOP 16.080000000 ClHz
Figure 5. Measured Stripline
Back to Back Insertion Loss Figure 6. l o w Power Amplifier Assembly
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Figure 7. l o w Power Assembly
in X-KU T/R Package
WIDE BAND HIGH POWEH AMPLIFIER
Figure 8. Measured 1Ow Power Output
Summarv
A 10 watt power amplifier with greater than 81%
combining efficiency was demonstratedover the 8-14 GHz
bandwidth. The amplifier required no RF tuning and met
the physical dimensions required for an X-KU band active
array module.
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