TITLE
Microwave filter
TECHNICAL FIELD
The present invention relates to a microwave filter and a printed circuit board.
BACKGROUND
Microwave filters are today often realized as microstrip filters integrated in the
layout of Printed Circuit Boards (PCB). The PCB is in the form of a layered
structure with a ground plane on one side of a dielectric substrate and the
printed circuit is in the form of microstrips on the other side of the substrate.
The PCB comprises a number of components and filters that together gives a
desired performance of the PCB. A drawback with this solution is that when
the filter characteristics have to be changed, the complete PCB layout must
be changed in order to match the filter and the PCB to avoid discontinuities.
Hence, in prior art the PCB is dependent on filter specifics.
There is thus a need for an improved PCB and microwave filter unit in a strip
line configuration allowing the PCB to be non filter specific and where a
standard PCB without special treatment consequently can be used for
different filter properties.
SUMMARY
The object of the invention is to reduce at least some of the mentioned
deficiencies with the prior art solutions and to provide an improved
microwave filter and a corresponding method where the microwave filter unit
is realized in a strip line configuration not being dependent on a ground plane
of the PCB to which the filter is mounted, allowing the PCB to be non filter
specific and where a standard PCB without special treatment can be used.
2
The invention refers to a microwave filter unit according to claim 1 and a
printed circuit board according to claim 2.
In the coming multifunction radar systems with capability of beam steering
(AESA=Active Electrical Steered Antenna), the invention finds its place
specifically. In general the invention is suitable for implementation on printed
circuit boards for microwave frequencies.
The present invention has the benefit of solution comprising a printed circuit
board that can be used with different filter units with different filter
characteristics, which means that the same printed circuit board can be used
for different purposes by choosing suitable filter units. The filter units can thus
be designed operating at different frequencies and requiring exactly the same
area on the circuit board. The circuit board thus becomes non filter specific.
Additional benefits are that the invention gives a low-loss and broadbanddesign
of coupling RF microstrip mode up to stripline mode, and vice versa,
at RF ports, and that frequency selectivity is done at stripline level.
Yet further advantages are that in-house design using regular tools is
possible and that a low cost component easily can be mounted on a circuit
board, only requiring so called sight marks.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will below be described in connection to a number of drawings
in which:
Figure 1 schematically shows a top view of a printed circuit board and a filter
according to the invention;
Figure 2 schematically shows a side view along line A-A in figure 1;
3
Figure 3a schematically shows a side view of a filter unit along line A-A in
figure 1;
Figure 3b schematically shows a cross-sectional side view of a printed circuit
board along line A-A in figure 1;
Figure 4a schematically shows a top view of a printed circuit board according
to the invention;
Figure 4b schematically shows an enlarged portion of the top view of the
printed circuit board in figure 4a;
Figure 5a schematically shows a bottom view of a filter unit according to the
invention;
Figure 5b schematically shows an enlarged portion of a bottom view of the
filter unit in figure 5a, and in which;
Figure 6 schematically teaches a general coplanar waveguide geometry with
lower ground plane (CPWG)
DETAILED DESCRIPTION
In the drawings an orthogonal system has been depicted with arrows X, Y
and Z for facilitating the description of the invention. The three directions
referred to are; a longitudinal direction X (length), a lateral direction Y (width)
and a thickness direction Z.
Common reference numbers are recurring in figures 1-5.
The phnted circuit board 2 has an extension in the X-Y-plane and is layered
in the thickness direction Z. The filter unit 1 has an extension in the X-Yplane
and is layered in the thickness direction Z.
Figure 1 schematically shows a top view of a printed circuit board and a filter
according to the invention. Figure 1 shows a stripline microwave filter unit 1
4
attached galvanic to a printed circuit board 2 comprising a microstrip
structure, followed by a transition to co-planar waveguide structure with lower
ground plane, illustrated more clearly in Fig 4a and Fig 4b. The filter unit 1
comprises a layered structure comprising a first ground plane 3, a second
ground plane 4 and a dielectric first substrate 5 therebetween. The filter unit
1 also comprises a first conductor structure 6 embedded in the first substrate
5. The first conductor structure 6 has a first end portion 7 and a second end
portion 8. The first end portion 7 is connected to a bottom outside 9 of the
filter unit 1 by a first connector 10 and the second end portion 8 is connected
to the bottom outside 9 of the filter unit 1 by a second connector 11. The first
ground plane 3 is connected to the second ground plane 4 by a third
connector 12.
Figure 1 shows that the second ground plane 4 is positioned on the bottom
outside 9 of the filter unit 1 and that the second ground plane 4 has a first
notch 13 in connection to the first connector 10 revealing the first substrate 5
and that the second ground plane 4 has a second notch 14 in connection to
the second connector 11 revealing the first substrate 5. The first connector
10 is connected, via the first connector 10, to a first connector pad 15
positioned in the first notch 13 on the bottom outside 9 of the first substrate 5.
The second connector 11 is connected, via the second connector 11, to a
second connector pad 16 positioned in the second notch 14 on the bottom
outside of the first substrate 5. The third connector 12 comprises fourth
connectors 17 electromagnetic coupled to the first connector 10 and fifth
connectors 18 electromagnetic coupled to the second connector 11.
The first end portion 7, the first connector 10, the first connector pad 15, the
fourth connectors 17 and the first notch 13 are positioned in relation to each
other such that a predetermined impedance is essentially obtained for the
transmission of a signal from the first connector pad 15 to the first end portion
7.
5
The second end portion 8, the second connector 11, the second connector
pad 16, the fifth connectors 18 and the second notch 14 are positioned in
relation to each other such that a predetermined impedance is essentially
obtained for the transmission of a signal from the second end portion 8 to the
second connector pad 16.
Figure 1 shows that the printed circuit board 2 comprises a third ground
plane 19, a second conductor structure 20 and a dielectric second substrate
21 therebetween. The second conductor structure 20 comprises a third end
portion 22 and a fourth end portion 23. The third end portion 22 and the
fourth end portion 23 are positioned relative each other such that the first
connector pad 15 of the filter unit 1 can be attached to the third end portion
22 and such that the second connector pad 16 can be attached to the fourth
end portion 23. The printed circuit board 2 comprises a first ground portion 24
positioned on the same side of the second substrate 21 as the second
conductor structure 20 and is connected to the third ground plane 19 by a
first ground connector 25. The first ground portion 24 comprises a third notch
26 positioned such that the third end portion 22 is positioned within the third
notch 26.
The printed circuit board 2 comprises a second ground portion 27 positioned
on the same side of the second substrate 21 as the second conductor
structure 20 and is connected to the third ground plane 19 by a second
ground connector 28. The second ground portion 27 comprises a fourth
notch 29 positioned such that the fourth end portion 23 is positioned within
the fourth notch 29.
The first ground portion 24, the third notch 26, the third end portion 22 and
the first ground connector 25 are being positioned in relation to each other
such that a predetermined impedance is essentially obtained in the third end
portion 22 for the transmission of a signal from the second conductor
structure 20 to the filter unit 1.
6
The second ground portion 27, the fourth notch 29, the fourth end portion 23
and the second ground connector 28 are being positioned in relation to each
other such that a predetermined impedance Is essentially obtained in the
fourth end portion 23 for the transmission of a signal from the filter unit 1 to
the second conductor structure 20.
When the filter unit 1 is attached to the printed circuit board 2, the first ground
portion 24 and the second ground portion 27 is galvanic connected to the
second ground plane 4 of the filter unit 1 and the first connector pad 15 of the
filter unit 1 is galvanic connected to the third end portion 22 and the second
connector pad 16 is galvanic connected to the fourth end portion 23. Here,
"galvanic connected" could be achieved by soldering or any other suitable
attachment means for galvanic connection.
The ground planes, the conductor structures, the connectors, connector pads
and ground portions are all made of electrically conducting materials such as
metals.
In another example, the first ground portion 24 and/or the second ground
portion 27 may comprise two or more parts being arranged in relation to each
other in such a way that a good galvanic contact is established with the
second ground plane 4 of the filter unit 1 and in such a way that the a
predetermined impedance is essentially obtained in the third end portion 22
for the transmission of a signal from the second conductor structure 20 to the
filter unit and in such a way that a predetermined impedance is essentially
obtained in the fourth end portion 23 for the transmission of a signal from the
filter unit 1 to the second conductor structure 20.
Figure 2 schematically shows a side view along line A-A in figure 1. Figure 2
shows the filter unit 1, the first and second ground portions 24, 27 and the
printed circuit board 2 separated from each other, i.e. before assembly. It
should be noted that the first and second ground portions 24, 27
advantageously is a part of the printed circuit board 2 and not separate units.
7
The benefit lies in that the first and second ground portions 24, 27 can be
made during nnanufacturing of the printed circuit board, for example by
etching.
The first and second ground connectors 25, 28 and the third, fourth and fifth
connectors 12, 17, 18 could all be so called vias, i.e. plated holes that
provide electrical connections.
Figure 3a schematically shows that the first conductive structure comprises a
flat strip of metal which is embedded in an insulating material and
sandwiched between two parallel ground planes. The insulating material
forms the dielectric substrate. The width w8 of the strip, the thickness b of the
substrate and the relative permittivity of the substrate determine the
characteristic impedance of the strip which is a transmission line, in the filter
unit, the first conductive structure comprises a number of strips being
electromagnetically connected. The interrelationship between these parts
forms the filter characteristics. The first conductive structure does not have to
be equally spaced between the ground planes. In the general case, the
dielectric material may be of different characteristics and thickness above
and below the first conductive structure.
In one example of the invention, the manufacture of the filter unit is done by
putting together two parts, each part comprising a ground plane and a
dielectric substrate. One of the parts comprises the first conductive structure
and when the two parts are put together, the above described sandwich
structure of the filter unit is achieved. The first conductive structure can be
etched on the surface on one of the parts or may be a separate structure that
is sandwiched between the two substrates. The method described has been
proven to be fast and cheap.
In another example, both parts may each comprise a first conductive
structure which are matched to each other when the parts are put together. In
both examples, the parts can be attached to each other by attachment
8
means such as glue, but may also be attached to each other by the surfaces
of the substrates bonding to each other.
The microstrip in the printed circuit board is a type of electrical transmission
line which can be fabricated using printed circuit board technology, and is
used to convey microwave-frequency signals. It comprises the second
conducting strip separated from the third ground plane by the dielectric layer
of the second substrate. Microwave components are used in radars,
antennas, couplers, filters, power dividers etc. and can be formed from a
microstrip. The microstrip comprises a pattern of metallization on the
substrate. Microstrip is thus much less expensive than traditional waveguide
technology, as well as being far lighter and more compact.
Figure 3a schematically shows a side view of a filter unit along line A-A in
figure 1. Figure 3b schematically shows a cross-sectional side view of a
printed circuit board along line A-A in figure 1.
Figure 3a is identical to the filter unit in figure 2 and figure 3b is identical to
the printed circuit board shown in figure 2 but with the first and second
ground portions 24, 27 being part of the printed circuit board 2. In addition to
the reference numbers in figure 2, figures 3a, 3b, 4b and 5b show a number
of reference numbers regarding dimensions of various parts of the filter unit
1.
Figure 4a schematically shows a top view of a printed circuit board according
to the invention. In figure 4a a support portion 30 is positioned between the
first ground portion 24 and the second ground portion 27 for support of the
filter unit 1 on the printed circuit board 2. The support portion could also be
connected to the third ground plane 19 via connectors for additional
conduction between the third ground plane 19 and the filter unit 1 via
galvanic contact with the second ground plane 4.
Figure 4b schematically shows an enlarged portion of the top view of the
printed circuit board in figure 4a.
9
Figure 5a schematically shows a bottom view of a filter unit according to the
invention.
Figure 5b schematically shows an enlarged portion of a bottom view of the
filter unit in figure 5a.
The invention makes use of two well defined structures, a printed circuit
board 2 and a filter unit 1. As soon as a microwave material is selected, its
dielectric constant ^> and thickness h, dictates certain dimensions as e.g.
conductor widths and gaps, it is therefore advisable, in cases where it is
possible, to show closed form expressions for the impedance Z of a
transmission line or conductor. It must be understood that there does not
exist closed form expressions for all dimensions needed in this invention, so
numerical tools are used in such cases.
The printed circuit board 2 and the first ground portion 24 are seamless
integrated to one unit, shown in Figs. 3b, 4a and 4b. This results in a
microstrip line structure followed by a transition to a variation of a coplanar
waveguide geometry with lower ground plane, hereinafter called CPWG.
Microstrip: The microstrip line geometry is partly illustrated in Figures 4a and
4b and its cross section is illustrated in figure 3b. In general we assume a
substrate thickness of d and a strip conductor of width w4 and thickness t1
and a dielectric constant ^r_ Thus, the characteristic impedance can be
calculated as
z,=^\-j^\n —-+—7 , >'"--r-^
(1)
7 I 12071 ^ H'4
ZQ = \ , • r .— / . fl, for. > 1
V^ r.eff r% + 1 -393 + 0.667 In (>^'4/^ + 1.444)] " d
10
_ £ ^ + l e^-1 1
2 2 ^l + 12^/w4
The effective dielectric constant '-'^ can be interpreted as the dielectric
constant of a honnogeneous medium that replaces the air above the
conductor of width w4.
CPWG:
After the microstrip line there is a transition to a structure with a geometry
that is a variation of a CPWG. In the invention there is a galvanic connection
from the first ground portion 24 and the second ground portion 27 to the third
ground plane 19 via the connectors 25 and 28, respectively. In the classical
CPWG structure the grounding of 24 and 27 is arranged by other means.
Above the CPWG-structure, the filter unit 1 is mounted. Such a stacked
structure does, to our best of knowledge, have not yet any closed form
expressions for the resulting geometries of conductor widths and gaps that
will give a desired characteristic impedance ZO, say close to 50 Ohm.
However, the CPWG structure have been analyzed separately as a stand
7 e
alone structure. The expressions for " and '"^ are given below, assuming
G=G1=G2, 2b=2a+G and W5=2a, dielectric constant ^^ and a substrate
thickness d, see Figs. 3b and 6
6071 1
K{k') K{k:) (3)
K{k')K{k,)
^ ' K{k)m)
K{k)K{k;) (4)
11
where
k=alb^—^
k. =imh{%al2d)li&n\\{%bl2d)
k' = ^|\.0-kk:
= yjl.O-k;
2a = L2
2b = G + L2 (5)
and K(k) is the complete elliptic integral of the first kind.
With reference to figures 1, 2, 3a, 5a and 5b: Stripline is a planar-type of
transnnission line that lends itself to microwave design. The geometry of a
stripline consists of a thin conducting strip of width w8 and thickness t4, and
is centred between two wide conducting ground planes, defining the
boundary of a dielectric substrate of thickness b with a dielectric constant ^r.
The expression for the characteristic impedance ZO is
2 ^ 3071
I—( J e w^/_b ^ + Cr L— ]
^ '^[l-tA/b 0.0885e,^
where
^ 0.08558 7 2 , f 1 A ( I A ( 1 ill ,-r,
Cr= In +1 1 In —-1 (7)
' 71 \^l-t4/h [l-t4/h j [l-t4/h j [{l-t4/bf J J
Equations (6) and (7) are valid for W8/(&-M)> 0.35 , with a maximum error of
1.2% at the lower limit of w8.
The first and second ground portions 24, 27 have a thickness t3 that
corresponds to the thickness t1 of the second conductor structure 20 in such
12
a way that the second ground portions 24, 27 can be in galvanic contact with
the second ground plane 4 when assembled. The third and fourth end
portions 22, 23 also have a thickness that allows for the second ground plane
4 of the filter unit 1 to be attached to the ground portions 24, 27 and at the
same time for the first and second connector pads 15, 16 to be galvanic
connected to the third and fourth end portions 22, 23 respectively.
For the same reasons, the second ground plane 4 have a thickness t2 that
correspond to the thickness of the first and second connector pads 15, 16.
A numerical example of the invention will now be described with reference to
figures 4b and 5b. The example has experimentally been proven to show
good results for characteristic impedance ZO close to 50 ohm with very low
signal losses. This example is valid for both ends of the filter unit and both
ends of corresponding portions of the printed circuit board described in
connection to figures 1-6.
Fig. 4b shows a detailed top view of the layout of the PCB 2. In figure 4b, the
first ground portion 24 is shown together with the second conductor structure
20. The plated via holes connecting the first ground portion to the third
ground plane 19 are shown by dashed circles. The second ground portion 27
is constructed in the same way as the first ground portion, and with the same
dimensions.
The length of the first ground portion 24 in the x-direction, called Li, is in our
example 3 mm. The minimum width of the first ground portion, Wi, is 5 mm.
The width of the ground portion can be made greater to match the filter that is
needed. The diameter of each plated via hole is 0.3 mm. The second
conductor structure 20 is the structure that guides the signal from the PCB
into the microstrip to stripline transition. Depending on the dielectric constant
and the thickness of the substrate 21, the width of this conductor is chosen
so to create the characteristic impedance that is desired. In our example the
13
thickness d of the second substrate 21 is 0.254 mnn and the dielectric
constant Er is 3.66, which gives the width W4= 0.524 mm.
In the first ground portion is cut a notch 14. Into this notch the second
conductor structure 20 is laid out. The conductor 20 is centred in the slot
making the gaps Gi and G2 equal in size, however this is not strictly
necessary if for some purpose one would like to have an asymmetric
structure. The second conductor structure, which creates an end portion
labelled the third end portion 22, has a width W5 (in our example 0.4 mm).
This width can be chosen in a certain range depending on the size of the gap
Gi and G2 (which in our example is 0.22 mm). The width W5 and the gap size
Gi=G2 are chosen as to (together with the thickness and the dielectric
constant of the second substrate 21) create a coplanar waveguide structure
with a certain specified characteristic impedance (in our example this
impedance is 50 Q).
In order to reduce unwanted coupling from the second conductor 20 to the
first ground portion 24, the corners of the first ground portion are cut at a 45°
angle (giving that the lengths Le and We are equal). This angle is not
specifically important and can be chosen in a certain range if some other
angle is more convenient. The size of the cut corner We can be chosen in a
range of values (in our example it is 0.55 mm). The length of the transition of
the second conductor structure 20 from width W4 to width W5 should not be
too short (to reduce the impedance mismatch) and is in our example chosen
to be 0.3 mm.
As discussed above, the third end portion together with the first ground
portion creates a coplanar waveguide structure. The dimensions of this
waveguide structure are chosen in order to create a specific characteristic
impedance (in our specific example chosen to be 50 Q). Depending on the
dimensions of the width W5 of the third end portion 22 and the gap size
Gi=G2 the width of the third notch 26 will have a certain value (in our
example 0.84 mm). The length of the third notch 26 should be chosen in
14
conjunction with the length of the third end portion to create a smooth
transition from microstrip mode to coplanar waveguide mode for the
microwave signal. A trade-off must be made between the length Li of the
microstrip to stripline transition and the performance of the transition. In our
case it is seen that a length Li of 3 mm is sufficient to give good
performance.
The third end portion ends in a semi-circle (for convenience chosen to have a
radius Ri equal to 0.2 mm). The end of the third notch 26 also ends in a
semi-circle (for convenience chosen to have a radius R2 equal to 0.42 mm in
our example). The length of the third end portion L3 is in our example 1 mm.
The length of the third notch L7 is in our example 1.25 mm. The length of the
gap L4 between the third end portion and the first ground portion is in our
example 0.82 mm. This length can be chosen in a certain range to achieve
desired performance.
The spacing Si between the centre line of the transition and the plated via
holes connecting the first ground portion to the third ground plane 19 should
not be too small. Otherwise this would interfere with the microstrip mode of
the second conductor structure. In our example this length has been chosen
to be 1.25 mm. The distance between the edges of the first ground portion
and the centre of the closest via holes S2 and S3 can be different (for
convenience it is chosen to be equal to 0.55 mm for both S2 and S3 in our
example). The separation between the centres of the via holes S4 and S5 can
also be chosen to be different (in our example they are equal and of size 0.7
mm). All spacings between the via holes are of less importance and can be
chosen rather freely.
Fig. 5b shows a detailed bottom view of the second ground plane 4. Note that
it is only the part of the ground plane around the first microstrip to stripline
transition that is shown. The part of the ground plane around the second
transition is designed in the same way. A view of the whole ground plane is
shown in Fig. 5a.
15
In Fig. 5b is shown the second ground plane 4, the first connector pad 15,
and the first notch 13. Shown by dashed lines are also the third connectors
12 (connecting the first ground plane 3 to the second ground plane 4), the
first connector 10 (connecting the first connector pad to the first end portion 7
of the first conductor structure 6), and the first end portion 7 of the first
conductor structure 6.
In Fig. 5b, the diameters of the third connectors are all equal to 0.3 mm. The
diameter of the first connector 10 is also 0.3 mm. The spacing between the
third connectors (S4 and S5 are the same as in Fig. 4b and is 0.7 mm). The
distances between the third connectors and the edge of the second ground
plane Se and S7 are both equal to 0.35 mm.
The length La and the width W7 of the transition part of the second ground
plane is 2.8 mm and 4.6 mm, respectively. The width of the first connector
pad W5 0.4 mm (as in Fig. 4b). The lengths W2, W3, Gi, G2, L5, L3, Ri, R2, Si,
and L7 are also the same as in Fig. 4b.
The lengths of the cut corners of the second ground plane Lg and Wg are
both equal to 0.35 mm.
The width Ws of the first end portion 7 of the first conductor structure 6 is
chosen to result In a specific impedance of the stripline transmission line.
Given that the thickness b of the first dielectric substrate 5 is in the range of
1.5 to 1.6 mm and the dielectric constant £r is 3.66 in our example, this gives
a width Ws of 0.8 mm for a 50 Q impedance. The radius of the end of the first
end portion is 0.4 mm.
The first ground portion 24 and the second ground portion 27 are designed to
match the second ground plane 4. The dimensions of the first and second
ground portions are however made 0.2 mm larger so that the soldering of the
filter unit 1 to the printed circuit board 2 will be open for inspection.
16
With reference to figures, 1-6 it should be clear for a person skilled in the art
that not only the described exannples are part of the invention, but that
additional arrangements of the first and second ground portions 24, 27 can
be contemplated as long as the predetermined impedance matching is met.
For example, the first and second ground portions may extend over the entire
filter unit area as long as the above described .
The invention is not limited to the embodiments and examples described
above, but may vary freely within the scope of the amended claims.

AMENDED CLAIMS
received by the International Bureau on 9 November 2010 (09.11.2010).
1. A stripline microv^ave filter unit (1) for a printed circuit board (2), the filter
unit (1) being in ttie form of a layered structure comprising a first ground
plane (3), a second ground plane (4) and a dielectric first substrate (5)
therebetween, the filter unit (1) also comprising a first conductor structure (6)
embedded in the first substrate (5), the first conductor structure (6) having a
first end portion (7) and a second end portion (8), the first end portion (7)
being connected to a bottom outside (9) of the filter unit (1) by a first
connector (10) and the second end portion (8) being connected to the bottom
outside (9) of the filter unit (1) by a second connector (11), the first ground
plane (3) being connected to the second ground plane (4) by a third
connector (12),
characterized in that the second ground plane (4) is positioned on the bottom
outside (9) of the filter unit (1) and that the second ground plane (4) has a
first notch (13) in connection to the first connector (10) revealing the first
substrate (5) and that the second ground plane (4) has a second notch (14)
in connection to the second connector (11) revealing the first substrate (5),
the first connector (10) being connected, via the first connector (10), to a first
connector pad (15) positioned in the first notch (13) on the bottom outside (9)
of the first substrate (5), the second connector (11) being connected, via the
second connector (11), to a second connector pad (16) positioned in the
second notch (14) on the bottom outside of the first substrate (5), the third
connector (12) comprising fourth connectors (17) electromagnetic coupled to
the first connector (10) and fifth connectors (18) electromagnetic coupled to
the second connector (11), wherein the first end portion (7), the first
connector (10), the first connector pad (15), the fourth connectors (17) and
the first notch (13) being positioned in relation to each other such that a
predetermined impedance is essentially obtained for the transmission of a
signal from the first connector pad (15) to the first end portion (7), and
wherein the second end portion (8), the second connector (11), the second
connector pad (16), the fifth connectors (18) and the second notch (14) being
positioned in relation to each other such that a predetermined impedance is
essentially obtained for the transmission of a signal from the second end
portion (8) to the second connector pad (16).
2. A filter unit (1) according to claim 1, wherein the first ground plane (3), the
second ground plane (4), the first conductor structure (6), the first end
portion (7), the second end portion (8), the bottom outside (9), the first
connector (10), the second end portion (8), the second connector (11), the
third connector (12), the first connector pad (15), the second connector pad
(16), the fourth connectors (17) and the fifth connectors (18) are made of
electrically conducting materials.
3. A filter unit (1) according to claim 1 or 2, wherein the first conductor
structure (6) comprises a flat strip of metal which is embedded in the
dielectric first substrate (5) and sandwiched between the first ground plane
(3) and a second ground plane (4) being parallel to each other.
4. A filter unit (1) according to any one of claims 1-3, wherein first conductor
structure (6) comprises a number of strips being electromagnetically
connected, wherein the interrelationship between these parts forms the filter
characteristics.
5. A filter unit (1) according to any one of claims 1-4, wherein the first
conductor structure (6) is equally spaced between the ground planes or the
dielectric first substrate (5) has different characteristics and thicl