WE4C-2
A
43-GHz
AlInAs/GaInAs/InP
HEMT
Grid
Oscillator
Polly Preventza,' Mehran Matloubian,'
and
David
B.
Rutledge,'
'Department
of
Electrical
Engineering,
California
Institute
of
Technology
,
Pasadena
CA
91
125
'Hughes
Research
Laboratories,
Malibu CA
90265
Abstract-A
36-element hybrid grid oscillator
has
been
fabricated.
The
active
devices
are
InP-based High Electron Mobility
Transistors
(HEMT's). The
grid oscillates at
43GHz
with
an effective radiated
power
of
200mW.
Mea-
surements show
the
E
and H-plane radiation
patterns have
side lobes
10dB
below
the
main
beam.
These results
are
a significant
improve-
ment over
a previous millimeter-wave grid
os-
cillator, which had
a divided beam because of
substrate
modes.
I. INTRODUCTION
In
order
to
produce high
power
at
microwave
and
millimeter-wave
frequencies
the
output
powers
of
many
solid-state
devices
are
combined. Quasi-optical
free-
space
power
combiners
eliminate
losses
associated
with
waveguides
and
transmission
lines.
Quasi-optical
grid
oscillators
are
periodic,
strongly
coupled, oscillating
structures
based
on
integrating
active
devices
directly
into
a
planar
array. Fig.
1
shows
the
approach.
The
first
grid oscillator
was
a
5-GHz
100-MESFET
grid
demonstrated
by
Popovic
e2
al.
[l].
A
100-MESFET
os-
cillator grid
produced
10
W
at
10
GHz
[2].
A
monolithic
grid
was
demonstrated
that
oscillated
at
35GHz
[3].
The
Eplane
radiation pattern
of
this
grid
is shown
in
mirror
active
grid
tuning
slab
1"
output
Beam
Eplane
angle,
degrees
Fig.
2.
E-plane radiation pattern
of
a 36-element
mono-
lithic
grid
oscillator operating at
35
GHz
[3,4].
Fig.
2.
The
side-lobes
have
a
peak value
only
2dB
less
than the
main-lobe peak. D.W.
Griffin
[5]
pointed
out
that
the
unsatisfactory
Eplane
pattern
is
due
to
substrate-mode excitation
that
contributes
to
the
radi-
ation
pattern
through
the
edges
of
the
grid.
In
this
paper
the
design
and
performance
of
a 36-
element
InP
HEMT
grid oscillator is described.
A
pho-
tograph
of
the
grid
is shown
in
Fig.
3.
The
period
of
the
grid
and
the
electrical thickness
of
the substrate,
con-
trol the
substrate-mode excitation.
To
minimize
the
power
in the substrate
modes
we
use
a
thin (254pm)
Duroid
substrate
with
a dielectric
constant
of
2.2.
This
gives
an
electrical thickness
at
the
oscillation
frequency
of
only
19'.
We
also
chose
the
vertical
and
horizontal
period
of
the
grid
to
minimize
substrate-mode
excita-
tion.
Fig.
1.
A
grid
oscillator.
The
mirror
and
the dielectric slab
provide
tuning
and
help
the
devices
to
lock
together.
1057
0-7803-3814-6/97/$5.00
0
IEEE
1997
IEEE
M'IT-S
Digest
Fig.
3.
Photograph
of
the
36-element
hybrid
HEMT
grid
oscillator.
The
horizontal
lines
are
bias
lines.
Ferrite slabs
are
placed
on
each side
of
the
grid
underneath
the
bond
wires
to
suppress
oscillations
at
low
frequencies.
Ferrite
beads are
also
added
along
the
leads
to
suppress
bias line
oscillations.
2.25
mm
1
Fig.
4.
The
equivalent
waveguide
unit
cell.
Electric
walls
(solid
lines)
and
magnetic
walls
(dashed lines)
are
imposed
by
symmetry
on
the
boundaries
of
the
unit
cell.
The
drain
and
source
of
the
devices
are
wire-bonded
to
the
vertical
leads.
The
gate
is wire-bonded
to
the
horizontal
leads.
11.
DESIGN
The
devices
are
AlInAs/GaInAs
on InP
HEMT's
fabricated
at
Hughes
Research
Laboratories.
The
HEMT's
have
a
total gate
width
of
75pm
[6,7].
The
devices
have
a
peak transconductance
of
848
mS/mm
and
a
full
channel
current
of
750
mA/mm
measured
at
a
gate
bias
of
0.2V
and
a
drain
bias
of
1.5V.
In
order
to
analyze
the
grid
we
assume
that
all
de-
vices
are
identical.
Each
device
lies
in
an
equivalent
waveguide
unit
cell
which
is
defined by
the
symmetry
of
the
grid.
This
equivalent
unit
cell
has
magnetic
walls
on
the
sides
and
electric
walls
on
the top
and
the
bot-
tom
as
shown
in
Fig.
4.
The
vertical
spacing
deter-
mines
the
excitation
of
TM
modes
and
the
horizontal
spacing
determines
the
TE
mode
power. We
calcu-
lated the substrate-mode
power
for
a
6x6
array
of
uni-
formly excited dipoles
and
another array with
alternat-
ing
180"
phase shifts.
We selected spacings
that
gave
low
substrate-mode
power levels.
The
transmission-line
equivalent
circuit is
shown
in
Fig. 5.
The
inductances
and
capacitances
shown
are
calculated using
the
EMF
method
[8].
Free
space is represented
by
3774
scaled
by
the
aspect
ratio
b/a
of
the
unit
cell.
The
circula-
tor
is
inserted
at
the
drain terminal
to
calculate
the
saturated
circular
function.
This
approach
is
described
in
[9].
The
locus
of
the
function
crosses
the
zero-phase,
unity-magnitude
point
at
44.3
GHz,
indicating
an
oscil-
lation
at
this
frequency.
111.
PERFORMANCE
An
HP8563A
spectrum
analyzer
with
an
HP11974-
series
preselected mixer
was
used
to
measure
the
oscil-
lation
spectrum
of
the
grid
(Fig.7).
An
output
tuner
was placed
in
front
of
the
grid
that
stabilized
the
signal
and
maximized
the
output
power.
The
tuner
used
is
a
X,/4-thick
dielectric
slab with
a
dielectric
constant
of
10.5
placed
6.5
mm
in
front
of
the
grid.
All
devices
in
a
single
row
are
biased
in
parallel.
Four
separabe
dc
sup-
plies
are
used
(one
for
each
of
the top and
bottom
rows,
one
for
rows
2
and
3,
and
another
for
rows
4
and
5).
The
separate
supplies
allowed
us
to
control
the
angle
of
the
Fig.
5.
Transmission-line
equivalent
circuit.
The
reflection
coefficient
at
the
circulator terminal
calculates
the
circular
function.
The
reactance
of
the
mirror
is
determined
by
the
thickness
of
the substrate
and
the
spacing from
the
grid.
1058
1
-j
Circular
Function
-
Unit
Circle
------
-90
-60
-30
0
30
60
90
Angle,
degrees
Fig.
6.
Saturated
circular
function,
C,
of
the
grid.
the
oscilation
criterion
is
satisfied
at
44.3
GHz,
where
C=l.
beam.
The
oscillation frequency
is 43
GHz,
about
3%
lower
than
the
design
frequency,
44.3
GHz.
The
highest
effective
radiated
power
(ERP)
is
200mW.
The
effec-
tive
transmitter
power
(ETP)
defined
by
Gouker
[lo]
is
low,
only
5mW.
The
total
dc
power
supplied
was
272
mW.
The
far-field
radiation
patterns
of
the
grid
were
mea-
sured
and
are
shown
in
Fig.
8.
The
theoretical
patterns
are for
a
uniformly excited
array
of
36
short
dipoles
spaced
2.75
mm apart
in
the
H-plane
and
2.25
mm apart
in
the
Eplane
and
placed
5.8"
in front
of
a mir-
ror.
Both
E
and
H-plane
patterns
have
side
lobes
about lOdB
lower
than
the
main
beam.
No
evidence
of
substarte-modes
is
seen.
By
changing
the
position
of
the
mirror
and
the
tuning slab
the
oscillator
can
20
d
g
10
ra
0
-20
-10
0
10
20
Frequency
offset,
h5Hz
Fig.
7.
Oscillation spectrum.
The
center
frequency
is
42.94
GHz.
-301""""1""'"'~~'1~''~~
I
I
-90
-60
-30
0
30
60
90
Angle,
degrees
(b)
Fig.
8.
Measured E-plane
(a)
and
H-plane
(b)
patterns
(solid
lines) with
6mm
mirror
spacing
and
theoretical pat-
terns
(dashed lines) with
5.8
mm
mirror
spacing.
be tuned
to
operate
at
other
frequencies
and
different
power levels.
These
tuning
curves
are
shown
in Fig.
9.
The
ERP
and
frequency
repeat
at
half-wavelength in-
tervals.
IV.
CONCLUSIONS
A
43-GHz
36-element
grid oscillator has
been
demon-
strated
with
an
ERP
of
200mW.
The
grid
was
de-
signed
to
reduce
substrate-mode
excitation.
This
grid
also
demonstrated
that
hybrid circuit
techniques
can
be
used for
quasi-optical grids
at
millimeter-wave frequen-
cies.
The
electrical thickness
and
the
geometry
of
the
array
(spacing
of
the
devices) can
control
the
excitation
-
.-
1059
43.0
;?
3
$
42.8
t
4
I(,,
,
,
,
I’I’I‘
I’I
I
I
I
I
I
I
I
I
I
I
I
3
4
5
6
7
8
9
10
Mirror
distance
from
the
grid,
mm
Fig.
9.
Frequency
and
power
tuning
of
the
grid
as
a
func-
tion
of
mirror position.
For
these
measurements
a
dielectric
slab
was
placed
6.5”
in
front
of
the
grid.
of
substrate
modes.
V.
ACKNOWLEDGMENTS
We
appreciate
the support
of
the
Army
Research
Office
and the
Physical
Optics Corporation.
We would
like
to
thank
Freddie Williams
for
his help
on mounting
and
wire-bonding
the
devices
to
the
grid.
VI.
REFERENCES
[l]
Z.B.
Popovic,
R.M. Weikle,
M.Kim,
D.B.
Rutledge,
“A
100-MESFET
Planar Grid
Oscillator,”
IEEE
Trans. Microwave
Theory
Tech.,
MTT-39.,
pp.
193-
200, Feb. 1991.
[2]
J.B.
Hacker,
M.P.
DeLisio, M.
Kim,
C.-M.
Liu,
S.-
J.
Li,
S.W.
Wedge,
D.B. Rutledge,
“A
10-Watt
X-
Band Grid
Oscillator,’’
1994
IEEE MTT-S
Int. Mi-
crowave
Symp.
Dig.,
pp.
823-826,
1994.
M.P.De
Lisio, D.B.
Rutledge,
“A 35 GHz
HBT
Mono-
lithic Grid Oscillator”,
Proc.
SPIE,
17th
Int.
Conf.
on
Infrared
and
Millimeter Waves,
R.J.
Temkin,
[4]
M.Kim,
“Grid
Amplifiers,”
Ph.D.
Thesis, California
Institute
of
Technology,
Pasadena,
CA,
1993
[5]
D.W. Griffin,
“Monolithic
Active
Array
Limitation
Due
to
Substrate
Modes,”
1995
IEEE
AP-S Int.
Symp.
Dig.,
pp.
1300-1303, 1995.
L.E.
Larson,
M.J.
Delaney,
M.A.
Thompson,
R.A.
[3]
M.
Kim,
E.A.
Sovero,
R.M.
Weikle,
J.B.
Hacker,
Ed.,
pp.
402-403,
1992.
[6]
M.
Matloubian,
L.
Jelloian,
AS.
Brown, L.D.
Nguyen,
Rhodes,
J
.E.
Pence, “V-Band
High-Efficiency High-
Power
AlInAs/GaInAs/InP
HEMT’s,”
IEEE
Trans.
Microwave
Theory
Tech.,
MTT-dl.,
pp.
2206-2210,
Dec. 1993.
[7]
M.
Matloubian,
L.D.
Nguyen,
A.S.
Brown,
L.E.
Larson, M.A.
Melendes,
M.A.
Thompson,
“High
Power
and
High
Efficiency
AlInAs/GaInAs
on
InP
HEMTs,”
1991
IEEE MTT-S
Int.
Microwave
Symp.
Dig.
Vol.
2,
pp.
721-724,
1991.
[8]
R.M.
Weikle,
“Quasi-optical
planar
grids
for
Mi-
crowave
and
Millimeter-wave Power
Combining,”
Ph.D.
Thesis, California
Institute
of
Technology,
Pasadena,
CA,
1992
[9]
R.D.
Martinez, R.C.
Compton,
“A
General
Ap-
proach
for
the
S-Parameter
Design
of
Oscillators
with
1
and
2-Port
Active
Devices,”
IEEE
Trans.
Microwave
Theory
Tech.,
MTT-do.,
pp.
569-574,
March
1992.
[lo]
M.Gouker,
‘(Toward
Standard
Figures-of-Merit
for
Spatial
and
Quasi-Optical
Power-Combined
Arrays,”
IEEE
Trans,
Microwave
Theory
Tech.,
MTT-43.,
pp.
1614-1617,
July
1995.
1060