Microwave Characterization and Modeling of Multilayered

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Microwave Characterization and Modeling of Multilayered Cofired Ceramic Waveguides
Microwave Characterization and Modeling of
Multilayered Cofired Ceramic Waveguides
Daniel Stevens and John Gipprich
Northrop Grumman Corporation
Electronic Sensors and Systems Division
P.O. Box 1521, MS 3K11
Baltimore, Maryland 21203
Phone: 410-765-2832
Fax: 410-765-2116
e-mail: Stevens.Dan@postal.essd.northgrum.com
Abstract
This paper examines, with the aid of an Electromagnetic (EM) Field Solver (High Frequency Structure Simulator, HFSS1) the performance
of via sidewall rectangular waveguide structures in a cofired ceramic substrate, and compares the modeled results to the modeled performance of a conventional solid conductor waveguide. The comparisons are made on the basis of insertion loss, reflection loss, and waveguide
cutoff frequency. In addition, HFSS simulations were performed to determine the crosstalk between two adjacent waveguides that share a
common metal via fence sidewall, as well as two adjacent waveguides with separate, closely spaced, via sidewalls. In order to facilitate
testing, a transition from stripline to cofired ceramic waveguide was developed. Finally, the authors present measured results of a via
sidewall rectangular waveguide structure fabricated as a Low Temperature Cofired Ceramic (LTCC) substrate, which demonstrate very
good agreement with the modeled performance.
Key words:
Waveguide, W/G resonators, X-band, LTCC, and EM Simulation.
1. Introduction
nar waveguides, and buried striplines. However, the RF losses of
these strip transmission lines, while reaching tolerable levels, are
still much higher than that of most traditional microwave substrates
(such as Duroid2 and ceramics).
In this paper, the researchers investigate the feasibility of using
waveguide structures in cofired ceramics since such structures, due
to their wider conductors, offer lower RF losses than strip transmission lines, particularly at higher microwave and millimeter wave
frequencies. This difference in loss becomes even more significant
for applications requiring a small ground plane spacing. Figure 1
shows a comparison of rectangular waveguide and 50 ohm stripline
loss for various ground plane spacings.
Cofired ceramics have found increasing acceptance in the packaging of various microwave integrated circuits. One reason for this
increased usage is that the electrical properties of cofired ceramics
have reached the point that microwave transmission lines and other
planar microwave structures (such as couplers and filters) can be
fabricated with reasonably low Radio Frequency (RF) losses. Due
to the way cofired ceramics are processed and fabricated, these microwave transmission line circuits and structures have been limited
primarily to planar configurations. These structures are typically
realized as strip transmission lines, such as microstrip lines, coplaThe International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 1, First Quarter 1999 (ISSN 1063-1674)
8 International Microelectronics And Packaging Society
43
Intl. Journal of Microcircuits and Electronic Packaging
, Tand=0.002,
Rho=3
Freq=10 GHz, gEr=6,
Tand=0.002,
Rho=3
r=6
1.2
1.0
0.8
0.6
Stripline
0.4
0.2
Waveguide
0.0
10
20
30
40
50
60
70
80
90
100
Figure 1. Loss (dB/in) vs ground plane spacing (mils).
2. Conventional Rectangular
Waveguide
Conventional rectangular waveguide3 consists of four solid conductor walls, a top and a bottom conductor, and two vertical sidewall
conductors. A typical rectangular waveguide, of horizontal dimension A, vertical dimension B, and length L, is shown in Figure 2A.
A ceramic filled waveguide of gr = 6.1, A = 0.25”, B = 0.10”, and L
= 0.70” has a cutoff frequency for the dominant TE10 mode of 9.6
GHz, allowing propagation of Ku Band frequencies (12-18 GHz)
with minimal RF losses. The next higher order mode, the TE20, is
cutoff for frequencies below 19.3 GHz. Figure 2B shows an EM
simulation of the frequency response of a solid conductor waveguide
with these dimensions.
B
Figure 2B. Simulation of solid conductor W/G.
3. Cofired Ceramic Rectangular
Waveguide
A rectangular waveguide may be constructed in cofired ceramic
with two parallel planar conductors serving as the top and the bottom waveguide conductors, connected together with two metal filled
“via fences” that serve as the sidewalls of the waveguide. Figure 3A
shows this construction. If the spacing of the vias within the via
fence is less than the 1/10 of the guide wavelength, then a negligible
amount of the RF signal escapes the guide structure, resulting in low
RF transmission losses. If the via spacings are too large, then a significant potential difference can develop across adjacent vias resulting in radiation outside the guide structure. Figure 3B shows an EM
simulation of the frequency response of the via sidewall waveguide
with gr = 6.1, A = 0.25”, B = 0.10”, and L = 0.70”. The via diameter
is 0.006” and the via spacing within the via fence is 0.03” (center to
center), or approximately 1/10 the guide wavelength. The frequency
response of the via sidewall waveguide agrees closely with that of
the solid sidewall waveguide of Figure 2B. It was found that a spacing of 0.25" between the inside edges of the sidewall via fences
produced the same cutoff frequency as the solid conductor waveguide.
L
A
Figure 2A. Solid conductor waveguide model.
Figure 3A. Via sidewall waveguide model.
The International Journal of Microcircuits and Electronic Packaging, Volume 22, Number 1, First Quarter 1999 (ISSN 1063-1674)
44
The simulation for Figure 3A used radiation boundaries placed
at the outer sides of the ceramic substrate in order to absorb any
signal energy escaping the waveguide structure. Figure 3C shows a
plot of the magnitude of the electric field as the wave propagates
along the guide. It can be seen that the fields are well contained by
the via fence sidewalls.
copper. Figure 4A shows the model of the solid sidewall waveguide
resonator, and Figure 4B shows the resonant response.
Figure 4A. Solid wall W/G resonator model.
Figure 3B. Simulation of via sidewall W/G.
Figure 4B. Solid wall W/G resonator simulation.
Figures 5A and 5B show the model and response of the via
sidewall waveguide resonator using 0.006” diameter vias spaced
0.02” apart. An additional set of simulations were also performed to
the above parameters except with a reduced waveguide height, B, of
0.02".
Figure 3C. E of via sidewall waveguide.
4. Via Sidewall Rectangular
Waveguide RF Losses
In order to quantify the effect of a via sidewalls on the RF losses
of a cofired ceramic waveguide, a half-wavelength resonator4 was
characterized for a solid conductor waveguide and the via sidewall
waveguide. The resonators were designed for a resonant frequency
of 10 GHz using A = 0.36", B = 0.18", gr = 6.1, and an iris spacing of
0.32". The iris opening selected was 0.04". The simulation used a
dielectric loss of 0.002 and a metal resistivity three times that of
Figure 5A. Via sidewall W/G resonator model.
45
5. Crosstalk Between Two Adjacent
Waveguides
Figure 5B. Via sidewall W/G resonator simulation.
Table 1 summarizes the resonator simulation results, including
the resonant frequency, the 3dB bandwidth, the loss, the unloaded
Q, and the attenuation in dB/inch. Equations (1) and (2) were used
for the calculation of unloaded Q and attenuation. From Table 1, it
can be seen that the via sidewalls add little, if any, to the RF losses
for vias spaced less than one-tenth the guide wavelength. In this
Table, Tand = 0.002, and Rho = 3 × copper.
Crosstalk between adjacent transmission lines is an important
issue for RF assemblies packaged in cofired ceramics. The ability
to obtain at least 40 dB of isolation is frequently necessary for many
applications. For strip transmission lines, it is common practice to
enclose RF signal conductors within via fences, or to place a via
fence between two adjacent conductors, in order to provide the needed
isolation. For even higher isolation, a double via fence can be used,
as well as separating adjacent conductors electrically far apart to
reduce coupling.
The HFSS model shown in Figure 6A was examined to determine the crosstalk between adjacent waveguide structures using a
common via fence sidewall. This waveguide has dimensions of A =
0.25", B = 0.10", a length of 0.50" and an gr = 6.1. The via diameter
is 0.006" and via spacings of 0.02”, 0.04” and 0.06” were simulated.
Table 1. Summary of waveguide resonator simulation results.
Wall
type
Solid
Via
Solid
Via
B
dim.
0.18”
0.18”
0.02”
0.02”
FRES,
GHz
9.95
9.92
9.93
9.91
B3dB,
MHz
23.5
23.9
43.4
43.7
Loss
@ Fc
32.7
28.4
29.9
30.0
Atten
QU
433
432
237
233
(dB/in)
0.197
0.197
0.360
0.364
Figure 6A. Common via sidewall crosstalk model.
QU=QL/(1-|S21|)
(1)
where QL=FRES/B3dB
and
Attenuation = 8.686p / QU·L , (in dB per unit length) (2)
Figure 5C shows the loss of the waveguide for various spacing
of the vias used to construct the waveguide sidewalls for an X-band
structure. It can be readily seen that a via spacing up to 30 mils
(center to center) provides similar loss to a solid sidewall waveguide
(that is a spacing = 0 mils).
Figure 6B. Common via fence crosstalk simulation.
Figure 7A shows a similar model except it uses two separate via
fences, separated 0.02" apart, between the adjacent waveguides.
Simulation results are shown in Figures 6B and 7B, for the common
via sidewall and the separate via sidewall models, respectively. For
a 0.02” via spacing, the common via fence sidewall provides apFigure 5C. W/G loss vs sidewall via spacing.
proximately
dB isolation
between
adjacent
waveguides
while
The International Journal of Microcircuits and Electronic Packaging,
Volume 22,50Number
1, First
Quarter
1999 (ISSN
1063-1674)
46
the separate via fence sidewall provides approximately 80 dB isolation5.
Figure 8A. Stripline to W/G transition model.
Figure 7A. Separate via sidewall crosstalk model.
The height of the top quarterwave waveguide was subsequently
increased to B = 0.05" in order to raise its impedance and thereby
minimize its effect on the frequency response of the matched transition. Figure 8B shows the response of the matched transition.
Figure 7B. Separate via fence crosstalk simulation.
Figure 8B. Stripline to W/G transition model.
6. Cofired Ceramic Waveguide
Prototype
A stripline to waveguide transition was designed to facilitate testing of a via sidewall waveguide structure. A waveguide height of B
= 0.011" was chosen since it would provide a sufficiently high loss
that could be accurately measured. An E-plane probe transition was
attempted, however this proved very unefficient in coupling to the
reduced height waveguide. Instead, an end-feed transition from
stripline to waveguide was developed. In this transition, the stripline
conductor is connected directly to the common center conductor of
two stacked waveguides. Figure 8A shows the stripline to waveguide
transition.
If the stripline ground plane spacing is made equal to twice the
waveguide height, then the ground plane step discontinuity at the
stripline to waveguide junction is eliminated. In order to couple the
signal into the lower waveguide, a short was placed in the top
waveguide a quarter of the guide wavelength from the stripline to
waveguide junction.
A prototype via sidewall rectangular waveguide was fabricated
in Low Temperature Cofired Ceramic (LTCC) in order to validate
the performance of the proposed waveguide structure. A rectangular waveguide with dimensions A = 0.36" and B = 0.011" was built
using 0.006" diameter vias spaced 0.02" apart for the sidewalls.
Figure 9A shows the layout of the LTCC test substrate used for the
prototype waveguide. The test circuit is a length of reduced height
waveguide 2.0 inches long with two transitions to stripline at each
end.
Figure 9A. Prototype LTCC W/G w/transitions.
47
Figure 9B shows the simulated performance and Figure 9C shows
the measured response. The measured insertion loss is less than 1.5 dB over an 8.0 to 12.0 GHz frequency band and measured -1.2
dB at 10 GHz. This value agrees closely to the simulated value of 1.1 dB based on the material properties of the LTCC material.
References
1. High Frequency Structure Simulator (HFSS), Hewlett Packard
Company, Westlake, California.
2. Duroid, Rogers Corp., Chandler, Arizona.
3. Samuel Laio, “Microwave Devices and Circuits”, pp. 102119, Prentice Hall, 1990.
4. G. L. Mathaei, L. Young, and E.M.T Jones, “Microwave Filters, Impedance Matching Networks, and Coupling Structures”,
pg. 243, Artech House, Dedham, Massachusetts, 1985.
5. H. Uchimura, T. Takenoshita, and M. Fuji, “Development of
the Laminated Waveguide”, IEEE MTT-S Digest, pp. 18111814, 1998.
About the authors
Figure 9B. Prototype LTCC W/G simulation.
S21
S11, S22
Figure 9C. Measured data for prototype W/G.
7. Summary and Conclusion
Daniel Stevens received his B.S. Degree from the Georgia Institute of Technology in 1983. He joined the Westinghouse Electric
Corporation in Baltimore, Maryland in 1983, where he has worked
in the area of high power solid state microwave transmitter design
for airborne radar applications. In 1996, he joined Northrop
Grumman’s Electronic Sensors and Systems Division in Baltimore
as a Senior engineer and is currently involved in the active aperture
T/R module development Group.
John Gipprich joined Westinghouse Electric Corporation in Baltimore, Maryland in 1959, as a participant in the Westinghouse/Johns
Hopkins Work Study Program. He received his B. S. and M. S.
Degrees in Electrical Engineering from Johns Hopkins University,
in 1965 and 1971, respectively. Since 1963, Mr. Gipprich has worked
in the antenna and microwave areas and has been involved in microwave circuit and subsystems design. In 1996, he joined the Northrop
Grumman Electronic Sensors and Systems Division in Baltimore.
Currently, he is an Advisory engineer in the active aperture module
engineering Department and is responsible for T/R module development and microwave multilayer cicuit designs. Mr. Gipprich is a
member of IMAPS, IMAPS National Technical Committee, and
IEEE/MTT-S. In 1987, he served as Chairman of the Baltimore
IEEE AP-MT Chapter.
Based on the results of the modeled and measured performance,
the researchers have concluded that the waveguide structure with
via fences serving as sidewalls is an acceptable alternate transmission line structure to strip transmission lines for cofired ceramic substrates. These waveguide structures can be embedded into multilayer cofired ceramic assemblies without significant crosstalk, and
efficiently transitioned to other transmission line structures. In addition, these waveguide structures, due to relatively wider conductor
widths, result in lower RF losses when compared to strip transmission lines with similar ground plane spacing. Similar results have
been recently reported5 for higher frequency waveguide structures.
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Microwave Characterization and Modeling of Multilayered

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