Title: Electromagnetic band gap microwave filter
Abstract: A microwave filter is formed from an electromagnetic band gap structure. The electromagnetic band gap structure includes a periodic array of metal features (16, 42, 44, 50) formed within a dielectric matrix (14, 52). A defect feature (17, 48) is formed within the periodic array of metal features (16, 42, 44, 50) in order to create a pass band within a stop band region.
Patent Number: 6,943,650 Issued on 09/13/2005 to Ramprasad, et al.
| Inventors:
|
Ramprasad; Ramamurthy (Phoenix, AZ);
Petras; Michael F. (Phoenix, AZ)
|
| Assignee:
|
Freescale Semiconductor, Inc. (Austin, TX)
|
| Appl. No.:
|
447448 |
| Filed:
|
May 29, 2003 |
| Current U.S. Class: |
333/202; 333/195 |
| Intern'l Class: |
H01P 001/20 |
| Field of Search: |
333/202,195,204,219,203
505/210
|
References Cited [Referenced By]
U.S. Patent Documents
| 5115216 | May., 1992 | Hikita et al.
| |
| 5818309 | Oct., 1998 | De Los Santos.
| |
| 6825741 | Nov., 2004 | Chappell et al.
| |
| Foreign Patent Documents |
| WO 01/8466/3 | Nov., 2001 | WO.
| |
Primary Examiner: Mai; Lam T.
Attorney, Agent or Firm: Clingan, Jr.; James L.
Claims
1. An electromagnetic band gap filter, comprised of:
a dielectric matrix;
a lattice formed within said dielectric matrix, that has a stop
band response that includes zero frequency; and
a defect feature formed in said lattice, thereby forming a pass band within said
stop band at a microwave frequency;
wherein said defect feature is comprised of a capacitor placed within said periodic
lattice.
2. An electromagnetic band gap filter, comprised of:
a dielectric matrix;
a lattice formed within said dielectric matrix that has a stop
band response that includes zero frequency; and
a defect feature formed in said lattice, thereby forming a pass band within said
band at a microwave frequency;
wherein said periodic lattice is comprised of a row of metallic rods
3. An electromagnetic band gap filter, comprised of:
a dielectric matrix;
a lattice formed within said dielectric matrix that has a stop
band response that includes zero frequency; and
a defect feature formed in said lattice, thereby forming a pass band within said
band at a microwave frequency;
wherein said periodic lattice is comprised of a row of capacitors.
4. The filter of claim 3, wherein said defect feature is comprised of a defect
capacitor having a capacitance different from said capacitors forming the periodic lattice.
5. An electromagnetic band gap filter, comprised of:
a dielectric matrix;
a lattice formed within said dielectric matrix that has a stop
band response that includes zero frequency; and
a defect feature formed in said lattice, thereby forming a pass band within said
band at a microwave frequency;
wherein said lattice is comprised of:
a row of metallic posts having alternating heights; and
a plurality of metallic plates placed proximal to the ends of each metal post
in said row of metal posts, wherein said metal plates overlap thereby forming a
periodic array of capacitors.
6. An electromagnetic band gap filter, comprised of:
a dielectric matrix;
a lattice formed within said dielectric matrix that has a stop
band response that includes zero frequency; and
a defect feature formed in said lattice, thereby forming a pass band within said
band at a microwave frequency;
wherein said defect is comprised of an oversized metal plate formed in the periodic
lattice.
7. An electromagnetic band gap filter, comprised of:
a dielectric matrix;
a lattice formed within said dielectric matrix that has a stop
band response that includes zero frequency; and
a defect feature formed in said lattice, thereby forming a pass band within said
band at a microwave frequency;
wherein the periodic lattice couples a signal line to a ground line, thereby
selectively rejecting signals propagating through said signal line and said ground
line.
8. A microwave filter, comprised of:
lattice means to form an electromagnetic band gap structure within a dielectric
matrix, thereby creating a transmission spectrum having a stop band that includes
zero frequency; and
defect means to form a defect in said periodic metal lattice, thereby forming
a pass band in said stop band at a microwave frequency;
wherein said lattice means is comprised of vias formed in said dielectric matrix.
Description
FIELD OF THE INVENTION
The present invention relates to the field of electronic filters, and more particularly
to filters constructed from materials having an electromagnetic band gap.
BACKGROUND OF THE INVENTION
Filters are widely employed to modify the frequency response of electronic
circuits. Filters typically use pass or stop bands to modify the frequency response
of a circuit by selectively transmitting or attenuating one or more frequencies
within a spectrum. Filters may exhibit low pass, high pass, band pass, and band
rejection attributes.
Wireless communications has greatly crowded the electromagnetic spectrum.
Such signal congestion has increased the demand for high performance electronic
filters. In particular, filters that function at microwave frequencies are particularly
important in wireless communications. Designing passive RF components in the microwave
region is, however, particularly challenging. For wireless applications, notch
filters that have a narrow band pass region are particularly useful. Competent
notch filters exhibit accurate frequency selectivity and low insertion losses.
The issue of insertion losses is of particular concern in portable wireless devices
that rely on batteries for power.
Electromagnetic Band Gap ("EBG") structures offer an expedient solution
to the wireless communications demand for high performance notch filters. EBG structures
function as a filter by exploiting their inherent electromagnetic band gap. At
frequencies outside of the band gap, EBG structures pass signals. However, at frequencies
that are within the band gap, EBG structures block the transmission of the signal.
The band gap behavior of EBG structures, also commonly referred to as photonic
crystals, arises from the periodicity of the crystal lattice that forms the EBG
structure. There are a variety of ways to fabricate the lattices that form EBG
structures. One such method includes forming periodic inclusions in a dielectric
matrix. Another method is to form a lattice of metal balls within a dielectric matrix.
Whatever method is used to create the EBG structure, the formation of the
electromagnetic band gap arises in much of the same way as it does in semiconductor
materials. When electromagnetic waves propagate through a periodic structure or
array, Bragg diffraction creates destructive interference between the waves at
particular frequencies. This Bragg diffraction gives rise to the band gap of the
structure or material. EBG structures exhibit a characteristic band gap that has
a center frequency related to the lattice constant of the periodic array. Specifically,
the center frequency of this characteristic band gap is proportional to the speed
of light divided by twice the lattice constant multiplied by the square root of
the dielectric constant of the embedding medium.
EBG structures are commonly used in optical communications devices. For optical
applications, EBG structures are sized to enable on-chip integration. For the characteristic
band gap to occur at optical frequencies, the overall EBG structure lattice should
have a size on the order of microns. The high demands placed on filters for wireless
communications applications has generated interest in the use of EBG structures
at the microwave portion of the electromagnetic spectrum. However, for the characteristic
band gap to occur at microwave frequencies such as 10 GHz, for example, the overall
EBG structure lattice needs to have a relatively large size, on the order of centimeters.
This relatively large size makes the use of EBG structures for microwave communications,
particularly with portable wireless devices, problematic.
Even so, the use of EBG structures for wireless communications filters has numerous
advantages. First, EBG filters have low losses, making them ideal for high quality
radio frequency (RF) applications. In addition, the low cost of manufacturing EBG
structures makes them highly competitive with other components. Consequently, there
is a need to develop smaller EBG structures that are useful in microwave communications applications.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a one dimensional periodic structure that exhibits an electromagnetic
band gap as understood by one of ordinary skill in the art.
FIG. 2 illustrates a transmission spectrum of the one dimensional periodic structure
illustrated in FIG. 1 also as understood by one of ordinary skill in the art.
FIG. 3 illustrates a structure devised in accordance with an embodiment of the invention.
FIG. 4 illustrates a transmission spectrum exhibited by the structure of FIG. 3.
FIG. 5 illustrates a top view of the structure of FIG. 3.
FIG. 6 illustrates a one dimensional periodic structure that produces an electromagnetic
band gap according to another embodiment of the invention.
FIG. 7 illustrates a transmission spectrum of the one dimensional periodic structure
illustrated in FIG. 6.
FIG. 8 illustrates a variation to the structure of FIG. 6.
FIG. 9 illustrates a transmission spectrum of the structure of FIG. 8.
FIG. 10 illustrates a top view of the structure of FIG. 8.
FIG. 11 illustrates a perspective view of a two dimensional embodiment of the invention.
FIG. 12 illustrates a top view of another two dimensional embodiment of the invention.
FIG. 13 illustrates a two-pass-band communications filtering device devised
in accordance with any of embodiments of the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Electromagnetic Band Gap (EBG) or Photonic Band Gap (PBG) crystals
offer a powerful method of controlling the propagation of Electromagnetic (EM)
waves. To understand the basic concepts involved, we draw an analogy with electronic
semiconductor materials. Crystalline semiconductors are composed of a periodic
arrangement of a basic building block of atoms. Therefore, a crystal in this context
can be considered a periodic potential that a propagating electron encounters.
Thus, the geometry and symmetry of the crystal dictates many of the conduction
properties of the crystal. In particular, due to Bragg-like diffraction from the
periodic potential, electrons are forbidden to propagate with certain energies
in certain directions. If the periodic potential is strong enough, the gap might
extend to all possible directions resulting in a complete band gap. For instance,
an insulator has a complete band gap between the valence and conduction energy bands.
In EBG or PBG crystals, the periodic "potential" is due to a lattice of macroscopic
inclusions that have different EM properties compared to the background medium
in which the inclusions are embedded. EM properties include the permittivity (i.e.,
the dielectric constant), permeability, and conductivity. If the EM properties
of the inclusions are sufficiently different from those of the background medium,
and the absorption of the EM waves by these materials is minimal (i.e., if the
materials are low loss), then scattering of EM waves by the periodic array of inclusions
results in many phenomena analogous to those in periodic semiconductors. In particular,
band gaps are produced that forbid the propagation of EM waves with certain frequencies
along certain directions creating what are called EBGs or PBGs in the associated
"band structures".
An example of a widely used device that uses the EBG concept is the dielectric
mirror, which is essentially a quarter-wave stack of alternating layers of materials
each with a different dielectric constant. EM waves of the right wavelength incident
normally to this stack are completely reflected due to destructive interference
of multiple scattered waves at the various internal interfaces. The dielectric
mirror is thus an example of a one-dimensional dielectric EBG lattice, and so displays
a band gap for only normally incident waves. Two- and three-dimensional lattices
of the proper type are required to ensure band gaps along more than one (or all)
directions of wave propagation.
An analogy between electronic crystals and EBG crystals can be taken further
in
order to understand the origins of "defect states" in the band gap. Just as defects
such as vacancies, impurities, dislocations, etc., create defect states in the
band gap of semiconductors, analogous defects in EBG lattices create passband features
in their band gaps enabling EM waves to propagate at those specific frequencies.
It turns out that some "metallodielectric" EBG lattices display a zeroth order
band gap. Metallodielectric lattices are structures that contain a periodic arrangement
of metallic inclusions embedded in a dielectric matrix. Most metals exhibit large
losses at optical frequencies, the frequency range relevant for opto-electronic
applications. However, at microwave frequencies most metals display negligible
losses. Therefore, metallodielectric structures offer the opportunity of designing
low loss microwave devices which could also be small enough to enable practical
applications owing to the existence of the zeroth order band gap.
Referring to the Figures by characters of reference, prior art FIG. 1 illustrates
a one dimensional periodic structure that exhibits an electromagnetic band gap
as understood by one of ordinary skill in the art. In FIG. 1, the EBG structure
is embedded in a microstrip transmission line. This EBG structure is realized as
an array of vias
2 connecting a signal line
4 to a ground line
6
(shunt). Signal line
4 and ground line
6 form the micro-strip transmission
line. Vias
2 are formed in a dielectric matrix
8 that lies between
signal line
4 and ground line
6. Together, vias
2 and dielectric
matrix
8 form the EBG structure. The periodicity of vias
2 within
dielectric matrix
8 gives rise to an electromagnetic band structure that
can be exploited for filtering applications.
It is possible to significantly shrink EBG structures for microwave applications
through introducing metallic vias
2 into dielectric matrix
8, resulting
in a metallo-dielectric lattice. The proximity of vias
2 creates periodic
capacitive coupling between metallic vias
2. This capacitive coupling between
vias
2 decreases a lower band edge of the band gap of EBG structure to frequencies
that are lower than those achievable without capacitive coupling. While metallic
vias
2 create significant losses at optical frequencies, vias
2 show
negligible losses at microwave frequencies. As a result, for microwave applications,
the EBG structure is nearly lossless.
Prior art FIG. 2 illustrates the transmission spectrum of the one dimensional
EBG structure illustrated in FIG. 1 as understood by one of ordinary skill in the
art. EBG structures exhibit a frequency response which has a center frequency (f
1)
related to the lattice constant of the periodic array. Signals with frequencies
that lie within the band gaps are prevented from propagating between the signal
line
4 and ground line
6. Referring to FIG. 1, the lattice constant
is the distance between vias
2. Specifically, the relation of this center
frequency (f
1) of the characteristic band gap is given by Equation 1 below:
f1≈c/(2
a√(∈μ)) Equation 1
where "c" is the speed of light in a vacuum, "a" is the lattice constant of
EBG structure, and "∈" and "μ" are the relative permittivity and permeability,
respectively, of dielectric matrix
8. The EBG structure formed by vias
2
and dielectric matrix
8 exhibits a band gap in the transmission spectrum
illustrated in FIG. 2 at frequencies near this center frequency. Frequencies that
lie within the band gap are blocked from transmission through the EBG structure.
Referring to FIG. 1, frequencies that lie within characteristic band gap are blocked
from transmission between signal line and ground line by the EBG structure. As
a result, the EBG structure filters the frequencies from the micro-strip transmission
line that lie within the characteristic band gap.
The EBG structure also exhibits a second band gap lower in frequency than the
characteristic band gap. This second band gap, referred to as zeroth order band
gap, extends from DC to a cut-off frequency f
0. This zeroth order band
gap exists in addition to the characteristic band gap. Two-dimensional arrays of
metallic posts
2 formed in a dielectric matrix
8 also give rise to
the zeroth order band gap. This stop band from zero frequency to f
0
is created from the interaction with vias
2 of electromagnetic waves with
electric field vectors that are parallel to vias
2. The value of f
0
is determined by the fractional value that metallic vias
2 have in
overall EBG structure volume.
For filtering applications, the EBG structure exhibits two stop bands, the zeroth
band gap and the characteristic band gap. The first stop band, which is the zeroth
band gap, ranges from zero frequency to f
0. The second stop band, which
is the characteristic band gap, is centered over center frequency f
1,
and ranges from f
2 to f
3. Between f
0 and f
2
exists a pass band. Between f
2 and frequency f
3 there
is a second pass band. Utilizing the characteristic band gap ranging from frequencies
f
2 to f
3 for microwave filtering applications is feasible.
Operating within this characteristic band gap for microwave applications, however,
requires that the EBG structure have a size on the order of centimeters. Being
this big is highly undesirable for portable wireless devices. As a result, operating
in the zeroth band gap for microwave applications is desirable.
FIG. 3 illustrates an embodiment of the invention. A preferred micro-strip transmission
line includes signal line
10 and ground line
12. Between signal line
10 and ground line
12 is a filter formed in a dielectric matrix
14.
The filter is created with a series of metal posts
16 that are identically
spaced in dielectric matrix
14 to create a periodic lattice. Metal posts
16 may be preferably formed from vias
16 that are produced in dielectric
matrix
14. Metal posts
16 form a periodic metal lattice within dielectric
matrix
14. For an example of printed circuit board applications, vias
16
may be spaced at a distance of 1.5 mm, thereby giving the lattice of filter a lattice
constant of 1.5 mm. For a typical case, there is a 1 mm distance between signal
line and ground line within circuit boards. Note that these dimensions are merely
exemplary for illustrative printed circuit board embodiment. Other dimensions may
exist for other printed circuit board applications and other transmission line
embodiments such as in a system on a chip applications. Such dimensions may be
greater or smaller than these described for this example.
FIG. 4 illustrates a transmission spectrum of the structure of FIG.
3.
The periodic lattice formed by vias
16 generates the transmission spectrum
illustrated in FIG.
4. The spectrum illustrated in FIG. 4 exhibits a zeroth
stop band beginning at DC and extending to frequency f
0. The frequency
f
0, at which zeroth band gap stops, is determined by the fractional
volume of the filter that is consumed by vias
16. The center frequency of
the characteristic band gap extends from f
2 to f
3, with the
center frequency f
1 as defined above in terms of the lattice constant
of the filter.
A highly selective pass band is created in the zeroth stop band through the inclusion
of a defect feature
17 illustrated in FIG.
3. Thus, all signals with
frequencies between 0 and f
0 propagating between the signal line
10
and ground line
12 are blocked from transmission except signals with frequencies
corresponding to the pass band created by the defect feature
17. Referring
to FIG. 3, defect feature
17 is formed from dividing one via into two separate
posts
18 and
20. At the ends of posts
18 and
20 are
metal plates
22 and
24, respectively. Together, posts
18 and
20 and plates
22 and
23 form defect feature
17. The
presence of defect feature
17 has the effect of creating a highly selective
zeroth stop band as illustrated in FIG.
4.
The presence of a single defect feature
17 is merely exemplary. Depending
upon the width and location of the desired pass band, multiple defect features
17 can be introduced into the EBG lattice. For example, if the EBG filter
has a lattice constant of 1.5 mm, a distance of 1 mm between signal line
10
and ground line and 12, and vias
16 with an inductance of 0.4 nH, it would
have a characteristic frequency (f
1) of approximately 100 GHz and a
zeroth band gap ranging from zero frequency to a stop band cutoff frequency f
0
of 25 GHz. Introducing defect feature
17 having an exemplary 0.7 pF capacitance
across plates
22 and
24 creates a pass band at approximately 8 GHz
within zeroth band gap. Through varying the inductance of posts
16,
18,
and
20 and capacitance of plates
22 and
24, it is possible
to move the frequency of pass band within the zeroth band gap. Several methods
known in the art exist for varying the inductance of the posts and the capacitance
of the defects. For instance, the capacitance can be varied by changing the size
of the plates
22 and
24 and/or by varying the distance between the
plates
22 and
24.
FIG. 5 illustrates a top view of the structure of FIG.
3. Signal line
10 is visible. Along the length of signal line are vias
16 formed
through dielectric matrix
14 that lies below signal line
10. Vias
16 electrically couple signal line
10 to ground line
12 as
illustrated in FIG.
3.
FIG. 6 illustrates another embodiment of the invention that is a periodic structure
that produces an electromagnetic band gap. A filter is made from a periodic array
of metal posts
26 and capacitive-like elements
28 formed in a dielectric
matrix
30. At the top of each metal post
26 is a metal plate
32.
This filter is coupled to a microstrip transmission line that has a ground line
34 and a signal line
36. Plates
32 that are at adjacent lattice
sites are at different heights relative to ground line
34 and couple capacitively
with each other. Plates
32 are coupled to metal posts
26 that shunt
the capacitive elements to ground. Plates
32 form a break in signal line
36. Together, plates
32, metal posts
26, and dielectric matrix
30 form an electromagnetic band gap filter. Note that in this embodiment,
plates
32 form a periodic array of capacitors in series, with periodic inductive
shunt elements
26. The values of the capacitances and inductances can be
varied by appropriate variations of the geometry of the individual elements. For
instance, the capacitance value can be changed by altering the size of plates
32
and/or by altering the distance between adjacent plates
32.
FIG. 7 illustrates a transmission spectrum of a one dimensional version of the
periodic structure illustrated in FIG.
6. The periodic structure of capacitive-like
elements
28 illustrated in FIG. 6 which couple capacitively with their neighbors
produces a zeroth order band gap that begins at zero frequency and ends at frequency
f
0. The transmission characteristics of the structure illustrated in
FIG. 6 show that it is a high pass filter. The capacitor-like structure of FIG.
6 blocks frequencies below f
0, and passes frequencies above f
0.
As with the embodiment of FIG. 1, the value of f
0 is a function of the
electromagnetic properties of the materials used to construct the device. By varying
the size of metal posts
26, metal plates
32, and dielectric matrix
30, it is possible to control the bandwidth of zeroth order band gap. Consequently,
it is possible to manufacture filters having a particular frequency response for
a desired application.
FIG. 8 illustrates a variation on the EBG structure illustrated in FIG. 6, but
which contains a defect feature. Referring to FIG. 8, there is shown a micro-strip
transmission line with a signal line
36 and a ground line
38. Between
signal line
36 and ground line
38 is an electromagnetic band gap
filter. The filter is formed by a periodic array of capacitive-like elements
40
composed of plates
48 or
50 attached to vias
46 or
44,
respectively. Vias
42,
44, and
46 are metal posts that have
an inductance, and are shunted to ground line
38. Adjacent posts are of
different heights to enable capacitive coupling between adjacent plates. Plates
50 and
48 form a periodic array of capacitors within dielectric matrix
52. The periodic series of capacitances and the inductive shunts
42,
44 and
46 create an electromagnetic band gap filter having a zeroth
band gap. The defect feature in this embodiment takes the form of an oversized
capacitor plate
48. Oversized capacitor plate
48 creates a different
level of capacitance in the otherwise periodic series of capacitors
40.
As a result of this differing level of capacitance, a band pass region is created
in zeroth order band gap.
FIG. 9 illustrates a transmission spectrum of the structure of FIG.
8.
As with the transmission spectrum illustrated in FIG. 7, the transmission spectrum
illustrated in FIG. 9 has a zeroth order band gap extending from zero frequency
to f
0. The defect feature formed from oversized capacitor plate
48
creates a defect band pass region within the zeroth order band gap. The band width
and location of the defect band pass region as well as the frequency selectivity
of the band pass region are determined by the features of oversized capacitor plate
48 and the number of defect features in the filter. By altering the size
of oversized capacitor plate
48, it is possible to control the bandwidth
and amplitude of the band pass region. In FIG. 8, a single exemplary defect feature
is illustrated. The filter can have numerous defect features formed from using
oversized capacitor plate
48 periodically positioned within the original
capacitor lattice. Note that the use of an oversized capacitor plate
48
to form defect feature is also exemplary. Other methods of creating a defect structure
include (but are not limited to) varying the height of one of the metal posts
46
to either increase or decrease the distance between capacitor plates
36,
48, and
50, or using an undersized capacitor plate
48 rather
than an oversized plate.
FIG. 10 illustrates the top view of the structure of FIG.
8. In this
top view, signal line
36 of a micro-strip transmission line is illustrated
as having a series of plates
48 and
50 (plate
50 is not seen
in FIG. 10 but is illustrated in FIG.
8). Plates
48 and
50
are capacitively coupled with each other (shown as component
40 in FIG.
8) and are shunted to ground line
38 by metal posts
42,
46,
and
44 (ground line
38 and post
44 are not seen in FIG. 10
but illustrated in FIG. 8) at varying heights. A dielectric matrix
52 is
formed surrounding plates
48 and
50 and posts
42,
44
and
46 between signal line
36 and ground line
38. This periodic
array of capacitors (component
40) and inductive shunts (components
42,
44 and
46) forms an electromagnetic zeroth order band gap, as illustrated
in FIG.
9.
FIG. 11 illustrates a perspective view of a two dimensional analogue of the
structures illustrated in FIGS. 3 and 5. Fabricating the one dimensional embodiment
of the filter illustrated in FIGS. 3 and 5 as a single row of vias
16 formed
within a dielectric matrix
14 with a single defect feature
17 is
merely exemplary. It is possible to fabricate two dimensional electromagnetic band
gap filters using multiple rows of vias
16 formed within a dielectric matrix
14. Further, within each row of vias
16, multiple defect features
17 can be formed by replacing a via
16 by two smaller vias
18
and
20 and two plates
22 and
24. Such a periodic structure
with defect features
17 also has a transmission spectrum similar to the
one illustrated in FIG. 4, which shows a zeroth order band gap extending from zero
frequency to f
0. The presence of the defect features
17 results
in the defect induced pass band in the zeroth order band gap. Characteristics of
this pass band, such as its location and bandwidth, can be controlled by the number
of the capacitive defects, and the values of the post or via inductance and defect
capacitances. Several methods exist for varying the inductance of the posts and
the capacitance of the defects. For example, the capacitance can be varied by changing
the size of the plates
22 and
24 and/or by varying the distance between
the plates
22 and
24.
In the absence of defect features, the transmission spectrum would be similar
to the illustration in FIG.
2. An important implication of two (or three)
dimensional periodic lattices is that the band gaps for EM waves traveling along
different directions could be different, as the EM waves see lattices with different
periodicities along different directions. This attribute is expected to provide
additional design flexibility.
FIG. 12 illustrates a perspective view of a two dimensional analogue of the
structures of FIGS. 8 and 10. Fabricating the one dimensional embodiment of the
filter illustrated in FIGS. 3 and 5 as a single row of vias
42,
44,
and
46 formed within a dielectric matrix
52 with a single defect
feature
48 is merely exemplary. It is possible to fabricate two dimensional
electromagnetic band gap filters using multiple rows of vias
42,
44,
and
46 formed within a dielectric matrix
52. Further, within each
row of vias
42,
44, and
46, multiple defect features
48
can be formed at periodic spacings. Such a periodic structure with defect features
would have a transmission spectrum similar to the one illustrated in FIG. 9, which
shows a zeroth order band gap extending from zero frequency to f
0. The
defect feature formed from oversized capacitor plate
48 creates a defect
band pass region within the zeroth order band gap. As with the one dimensional
case, the band width and location of the defect band pass region as well as the
frequency selectivity of the band pass region are determined by the features of
oversized capacitor plate
48 and the number of defect features in filter.
By altering the size of oversized capacitor plate
48, it is possible to
control the bandwidth and amplitude of the band pass region. In FIG. 8, a single
exemplary defect feature is illustrated. In practice, the filter can have numerous
defect features formed from using oversized capacitor plate
48 periodically
positioned within the original capacitor lattice. Note that the use of an oversized
capacitor plate
48 to form defect feature is also exemplary. Other methods
of creating a defect structure include (but are not limited to) varying the height
of one of the metal posts
46 to either increase or decrease the distance
between capacitor plates
48 and
50, or using an undersized capacitor
plate
48 rather than an oversized plate.
In the absence of defect features, the transmission spectrum would be similar
to the illustration in FIG. 7, with no defect induced pass band in the zeroth band
gap. Here again, an important implication of two (or three) dimensional periodic
lattices is that the band gaps for EM waves traveling along different directions
could be different, as the EM waves see lattices with different periodicities along
different directions. This attribute is expected to provide additional design flexibility.
Several multi-port devices can be constructed using EBG filters as building
blocks, or using the concept of selective wave propagation along certain directions
to result in desired multi-port characteristics. FIG. 13 illustrates a two-pass-band
communications filtering device of the former type made in accordance with any
embodiments of the invention. Filters
54 and
56 are implemented on
either the design of FIGS. 3 and 5 or FIGS. 8 and 10. Filters
54 and
56
are provided with a defect pass band that is located at different frequencies.
These different pass band frequencies are achieved through altering the size and
number of defect features
17 or
48. In this manner, signals can be
received at port two
60 through port one
58 at one frequency and
then transmitted at a different frequency at port one
58 through port three
at
62. Choosing the one dimensional embodiments for the components
54
and
56 of FIG. 13 is merely exemplary. Components
54 and
56
can be combined into a single two dimensional embodiment that has the defects chosen
and placed appropriately so as to result in the desired three-port characteristics.
Although the present invention has been described in detail, it will be
apparent to those of skill in the art that the invention may be embodied in a variety
of specific forms and that various changes, substitutions, and alterations can
be made without departing from the spirit and scope of the invention. The described
embodiments are only illustrative and not restrictive and the scope of the invention
is, therefore, indicated by the following claims.
*
