amplifiers
3.7 Watt Ka-band power module for local
multi-point distribution systems applications
Crowding of lower frequencies promise to open a host of Ka-band multipoint
distribution applications and products.
By Carlton T. Creamer,
James J. Komiak,
Wendell Kong, P.C. Chao,
Kirby Nichols,
Christopher O’Neil,
Scott MacKelvey,
Paul D. Cooper
and Dana Wheeler
A
s demand for broadband data distribution increases and as lower
frequency bands become saturated,
communications providers are increasingly focusing on Ka-band for
next generation terrestrial systems. A
number of terrestrial wireless systems currently exist including local,
multi-point distribution systems
(LMDS) at frequencies of 27 to 30
GHz, and digital radio at 28 and 38
GHz. Considerable growth in these
areas is projected. Designers of these
systems require low cost solid state
power amplifiers (SSPAs) as the
output stages for radio transmitters.
The moderate power requirements
range from 1 to 5 W.
This article describes the design, fabrication and test of a 3.7 W, Ka-band
high power module. These results were
achieved using gallium arsenide
pseudomorphic high-electron mobility
transistors (GaAs pHEMT), monolithic
microwave integrated circuit based
(MMIC) power amplifiers designed and
fabricated using 0.15 µm fully selective
gate recess technology .
major elements of the power module
put power is > 34 dBm (> 33 dBm at 1
design are the MMIC amplifiers, a twodB compression) with a dB associated
way signal combiner and a package to
power gain and a 25% power added efficiency at 30 GHz. From
28 to 33 GHz, the 3 dB
compressed output power
is > 32 dBm. The driver
MMIC is a similar twostage Class AB design
with a 3.33:1 aspect ratio
(600 µm driving 2 mm).
Saturated 3 dB gain compression chip power averaged 28.0 dBm and 13 dB
associated power gain at
30 GHz. From 28 to 33
GHz, the 3 dB compressed
output power is > 25 dBm.
The drain bias for the chip Figure 1. Two-stage output amplifier.
set is +4.5 volts (See figures 1 and 2.)
The third MMIC amplifier used as a simple gain
block is a commercially
available 26 to 40 GHz
balanced LNA design (not
shown).
LMDS module design
Figure 3 presents the
module block diagram.
The projected performance
is summarized in the table
below the figure. The
Figure 2. Two-stage driver amplifier.
Approach
The LMDS module design exploits
recent advances in Ka-band power
MMIC amplifier design and fabrication.
The newly developed chips include the
power Amplifier MMIC and the driver
Amplifier MMIC. [1].
The output periphery is 6.4 mm
with a 3.2:1 2nd to 1st stage ratio (2 mm
1st stage).
The measured 3 dB compressed out-
48
Description Transition
RF output power –8.9
Gain
–0.3
DC power
Device
DA
6.1
15
0.3
22.6
16.5
4.2
Divider
19.1
–0.5
PA
34.1
15
20
Combiner Transition
36.6
–0.5
36.3
–0.3
Total
36.3
44.9
24.5
Figure 3. Block diagram of the Ka-band power amplifier and value table.
www.rfdesign.com
April 2000
isolate the electronics from the operating environment and complete the thermal path. Solid state Ka-band amplifiers, that incorporate combining
schemes, require low loss solutions in
order to achieve the highest power levels while preserving DC to RF efficiency. A simple single section Wilkinson
combiner realized as an MIC circuit
fabricated on .010” Al2O3, was selected
for this application. It provides efficient
signal combining with adequate bandwidth. The design was initially analyzed using Libra and completed by
performing a 2-dimensional EM simulation of the layout. The predicted
insertion loss is < 0.5 dB.
Package design
A properly designed millimeter wave
power amplifier package includes a
light weight, hermetic mode free enclosure, a low loss transition from the
microstrip environment inside the
package to the user interface (in this
case, coax), and DC bias and distribution circuitry. Materials are carefully
selected to maintain low thermal
impedance, and CTEs that match closely to the hybrid and MMIC components. We chose a light weight aluminum package and copper moly carriers, that screw into the housing floor, to
provide low thermal impedance. The
transition is a simple air-line design
that mates to a glass to metal seal and
V-connector. Low and high pass DC filtering circuits are incorporated as part
of the power distribution. Resistive
dividers are used to reduce gate bias
supply to quiescent levels. Smaller
resistance values are favored at the
cost of slightly higher power consumption from the minus supply. This is
done to minimize gate voltage variations under RF drive and helps to preserve overall module efficiency.
Integration and test results
Two modules were assembled and
Figure 4. Small signal gain and input.
tested. The results were very similar
and only the results of the first module
are reported here. The module operates
from a +5.0 volt drain supply and a 1.0
V gate supply. It consumes 24.5 W of
power at quiescent bias.
The measurements included gain,
input return loss, output power,
power added efficiency (PAE) and
50
April 2000
Figure 5. Output power and P.A.E. vs. input power of the LMDS module.
intercept point. Overall performance
was excellent.
Small signal gain averaged 42.5 dB
and was centered at 29.5 GHz. The 1 .0
dB bandwidth is 1.7 GHz. This was
achieved with minimal tuning and is
close to the projected performance. The
input match was better than 1.62:1
across the full 1B bandwidth. The
results are shown in Figure 4.
Measured saturated output power
was 3.72 W (35.7 dBm) at 29.5 GHz. 1.0
dB gain compression power was 32.8
dBm (1.9 W). The power added efficiency was 15.8% at saturation. Transfer
curves are shown in Figure 5.
The results matched the projected
results within 0.6 dB (note that the
MMIC data used for purposes of the
performance projection is from pulsed
on-wafer measurement at 25% duty
cycle). This is primarily due to higher
channel temperatures as a result of
packaging and CW operation. Even
small thermal resistances of the attachment and mounting surface materials
result in substantial temperature
increases when handling large power
dissipations.
Intercept point
Two tone measurements of the module were made at four power levels
ranging from 100 mW to 1.0 W output
power at 29.5 GHz. Upper and lower
third order intercept point (TOI) values
are shown in Figure 6. Average magnitude of the upper and lower TOI is 38.6
dBm at an output power of 1.0 W.
Conclusion and recommendations
A high gain, multi-watt Ka-band
power amplifier module for LMDS
applications has been successfully
demonstrated using high power commercially available power chips fabricated in the Sanders foundry.
Producing the lowest cost final product
will require implementation of alternative packaging technologies that are in
development.
References:
[1] J. J. Komiak, W. Kong, P.C.
Chao, K. Nichols, “3 Watt Ka-Band
MMIC HPA and Driver Amplifier
Implemented in a Fully Selective 0.15
um Power PHEMT Process,” 1998
IEEE GaAs IC Symposium.
About the authors
Carlton T. Creamer, James J.
Komiak, Wendell Kong, P.C. Chao,
Kirby Nichols, Scott MacKelvey,
work in the Microwave Space and
Mission Electronics of Sanders, A
Lockheed Martin Company, Nashua,
NH. 03061.
Figure 6. Photo of LMDS Ka-band amplifier.
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April 2000