Field of the invention
[0001] The present invention represents a new way of packaging passive and active microwave
circuits, and in particular circuits involving microstrip transmission lines and similar
substrate bound transmission lines, in such a way that the problem associated with
cavity mode resonances are avoided.
Background
[0002] Electronic circuits are today used in almost all products, and in particular in products
related to transfer of information. Such transfer of information can be done along
wires and cables at low frequencies (e.g. wire-bound telephony), or wireless through
air at higher frequencies using radio waves both for reception of e.g. broadcasted
audio and TV, and for two-way communication such as in mobile telephony. In the latter
high frequency cases both high and low frequency transmission lines and circuits are
used to realize the needed hardware. The high frequency components are used to transmit
and receive the radio waves, whereas the low frequency circuits are used for modulating
the sound or video information on the radio waves, and for the corresponding demodulation.
Thus, both low and high frequency circuits are needed.
[0003] Electronic circuits below typically 300 MHz (i.e. wavelengths longer than 1 meter)
are easily realized in printed circuit boards (PCB) and in integrated circuits using
designs based on concentrated circuit elements such as resistors, inductors, capacitors
and transistor amplifiers. Such technology may also work at higher frequency, but
the performance degrades gradually when the size of the PCB and integrated circuit
package become comparable to a wavelength. When this happens, it is better to realize
the circuits by connecting together in various ways pieces of transmission lines or
waveguides, such as e.g. microstrip lines of coplanar waveguides. This is normally
referred to as microwave technology and is commonly in use between 300 MHz and 30
GHz, i.e. the microwave region. The corresponding electronic circuits can therefore
be referred to as microwave circuits, and they are often located on a planar dielectric
substrate with a metal ground plane. We will herein refer to such microwave circuit
on a substrate with a ground plane as a microwave circuit board.
[0004] Both passive and active electronics circuits often need to be packaged in a shielded
environment such as a closed metal cavity, e.g. a metal box, for mechanical protection,
but also to satisfy requirements to electromagnetic compatibility such as radiated
emissions and susceptibility. Such requirements are often regulatory (for commercial
devices), but can also be user-defined (such as for radio telescopes). Packaging is
difficult in the microwave region, because the volume inside the packaging cavity
may be large enough to support resonant modes within the frequency range of the microwave
circuits, and, if such modal resonances are present, they will completely destroy
the operation of the electronic circuit. Such destruction may appear not only at the
frequency of the resonance, but even below and above this band due to nonlinearity
and saturation effects in the circuits. There is therefore a need for an improved
technical solution to this packaging problem.
[0005] There are already existing solutions to the packaging problem, such as reducing the
size of the cavity volume and loading the cavity with microwave absorbers. However,
the size reduction may not always be possible due to the given size of the circuit
board. E. g., the enclosing cavity needs to have at least two dimensions (height and
width) smaller than typically 0.5 wavelengths in order to be sure to avoid cavity
resonances. The requirement may become even stronger if the substrate of the circuit
board has high permittivity or is thick. Also, absorbers inside the cavity may cause
undesired losses and therefore reduced performance of the microwave circuit. There
is therefore a need for an alternative packaging solution that is easy to apply also
when the circuit boards and correspondingly the enclosing cavities are wider than
0.5 wavelengths, and which does not require use of any absorbing material.
[0006] P.-S. Kildal disclosed in the application
WO 2010/003808 a new way of realizing microwave devices, such as electromagnetic transmission lines,
waveguides and circuits of them, that is advantageous when the frequency is so high
that existing transmission lines and waveguides have too large losses or cannot be
manufactured cost-effectively with the tolerances required. The microwave devices
are realized by a narrow gap between two parallel surfaces of conducting material,
by using a texture or multilayer structure on one of the surfaces. The fields are
mainly present inside the gap, and not in the texture or layer structure itself, so
the losses are small. The microwave device further comprises one or more conducting
elements, such as a metal ridge or a groove in one of the two surfaces, or a metal
strip located in a multilayer structure between the two surfaces. The waves propagate
along the conducting elements. No metal connections between the two metal surfaces
are needed. At least one of the surfaces is provided with means, such as a bed of
posts/nails, to prohibit the waves from propagating in other directions between them
than along the ridge, groove or strip,
[0007] WO 2010/003808 describes how at least one of two parallel metal plates can be provided with means
that stop wave propagation in the gap between the two surfaces, which may be used
as a packaging solution. In particular, the bed of posts/posts disclosed in
WO 2010/003808 has been demonstrated to be very useful, as described in "
Parallel Plate Cavity Mode Suppression in Microstrip Circuit Packages Using a Lid
of Posts", by E. Rajo-Iglesias, A. U. Zaman and P-S. Kildal, IEEE Microwave and Wireless
Components Letters, Vol. 20, No. 1, January 2010. However, when this technique is applied at frequencies typically below 10 GHz, the
packaged microwave device becomes large and bulky because the height of the posts
needs to be close to a quarter of a wavelength. Therefore, there is a need for a thinner
and more compact solution, still providing low loss and a large stopband for cavity
resonances.
[0008] Kildal describes also in
WO 2010/003808 how wave propagation between two parallel metal plates can be stopped by using a
multilayer structure on one of the surfaces, where the posts/posts are replaced by
an EBG surface in the form of metal patches. These form a periodic pattern in two
directions along the lower surface..". However, such multilayer surfaces are relatively
expensive to realize. There is therefore a need for a more cost-efficient solution.
Summary of the invention
[0009] The object of the present invention is to provide a compact and cost-efficient way
of packaging passive and active microwave circuits that removes or at least strongly
reduces problems related to resonances in the cavity inside which the circuit board
is located.
[0010] This object is achieved by means of a microwave circuit package including at least
one curved post/nail as defined in the appended claims.
[0011] According to a first aspect of the present invention there is provided passive or
active microwave circuit package, comprising: two conducting surfaces forming a gap
therebetween; and a microwave circuit located on a microwave substrate with a ground
plane and comprising one or more sections of one or more different transmission lines,
e.g. microstrip lines or coplanar waveguides;
wherein said microwave circuit is enclosed in the gap between said two conducting
surfaces, where one of the surfaces may be formed by the ground plane of the microwave
circuit board or an extension of this,
wherein at least one of the surfaces is provided with at least one conducting element
in the form of a curved post of conducting material rising from and being attached
to the conducting surface in such a way that wave propagation inside the gap between
the two surfaces is stopped, at least at the frequency of operation, and
wherein said curved post has on at least part of its extension a helical configuration.
[0012] Preferably, there is provided a plurality of curved posts having at least partly
a helical configuration.
[0013] The invention is in particular useable for packaging of microstrip circuits, but
it is not limited to this. According to the invention, there is provided a metal cavity
with conducting walls, e.g. of metal, that encloses the microwave circuit board, where
the ground plane of the microwave circuit board may form at least part of one of the
larger walls of the cavity. At least one of the two larger opposing surfaces of the
cavity is provided with conducting posts, e.g. of metal, curved in a helical configuration
and preferably located in a periodic or nearly-periodic grid over at least part of
the surface. The posts are preferably arranged in such a way that there is a narrow
or no gap between the ends of the curved posts and the microwave circuits, and in
such a way that the posts do not mechanically touch the conducting parts of the transmission
lines on the microwave circuit board. Thus, the curved posts may be connected only
to one of said surfaces each in such a way that the curved posts on either of the
surfaces are facing a smooth part of the opposite conducting surface, and preferably
the surface not carrying the transmission line. The other end of the posts may form
a gap to the other surface. Alternatively, the other end may be touching a dielectric
parts of the microwave circuit board, or be touching the ground plane of the microwave
circuit board.
[0014] The microwave circuit packaging solution of the present invention provides a very
efficient remedy to the above-discussed problem with resonant modes experienced in
many prior art packaging solutions. Further, the present invention provides an alternative
packaging solution that is easy to apply also when the circuit boards and correspondingly
the enclosing cavities are wider than 0.5 wavelengths, and it does not require use
of any absorbing material. Still further, the microwave circuit package of the present
invention is relatively simple and cost-efficient to realize, and e.g. more cost-efficient
than EBG surfaces. The present invention represents a new, way of packaging passive
and active microwave circuits, and in particular circuits involving microstrip transmission
lines and similar substrate bound transmission lines. The two surfaces may form the
bottom and lid of a cavity with conducting sidewalls. The posts, preferably arranged
in a periodic grid, creates together with the ground plane of the microwave circuit
board, or together with the smooth metal plane below the microwave circuit board,
a stopband for waves propagating between the lid of posts and the ground plane. Thereby,
cavity resonances are avoided or suppressed that otherwise create a big problem associated
with the packaging in metal boxes with smooth metal walls.
[0015] The smooth surface of the present invention may together with the conducting lines
on the microwave circuit board work as a waveguide or waveguide circuit, similar to
embodiments disclosed in
WO 2010/003808, said document hereby incorporated by reference. Therefore, the present invention
is to be considered as a specific realization of the more general invention described
in
WO 2010/003808, wherein the present invention provides a new realization which is particularly advantageous
at low frequencies, such as in an operation frequency range of 100 MHz to 30 GHz,
and preferably within the range 500 MHz to 10 GHz, and most preferably within the
range 1-10 GHz.
[0016] The curved posts of the present inventions have, at least partly, a helical configuration.
This is a way to reduce the effective length of the posts at the same time as their
electrical function that stops wave propagation between the two plates is maintained.
Therefore, they will also prevent cavity mode resonances. Compared to straight posts,
the height of curved posts having at least partly a helical configuration can be 3-10
times shorter in effective length, thus resulting in a significantly more compact
packaged microwave circuit. The period of the helical configuration can be sub-wavelength.
[0017] By curved posts in "helical configuration" is in the present application meant a
helix or coil shape, continuously forming a number of turns. Preferably, the turns
are arranged as concentric rings, but other configurations are feasible as well. For
example, the turns need not be circular, but may have other forms, such as an oval
shape, a square or hexagonal shape, an octagonal shape, or the like. Preferably, the
helical configuration forms a curve that lies on or abuts a cylinder or cone, at a
constant angle to the line segments making up the surface. However, the angle need
not necessarily be constant, and further, other shapes than cylinders and cones are
feasible, such as the shape of an hourglass or a barrel, and even flat spirals on
a dielectric substrate. Such flat spirals are preferably connected to the surface
by means of vertical posts, e.g. connected to the center of the spirals. The shape
of the helical configuration, the total height, the diameter of the turns, the thickness
of the wire, the number of turns, the angle of the turns, the distance between the
posts and the opposite surface, etc. are all parameters that may be modified for optimization
of the posts for particular use situations. For example, an increased number of turns,
corresponding to an increased electrical length, will move the frequency band inside
which cavity mode resonances are avoided to lower frequencies. Similarly, a larger
diameter shifts also the stopband down in frequency, corresponding to an increased
wire length, and the relative size of the stop band is not much affected.
[0018] In one embodiment, each curved post is ending in a turn with a smaller inclination
angle than the rest of the turns, and preferably ends in an essentially flat ring.
The end turns/rings are preferably essentially parallel to the opposing surface. In
this embodiment the ends of the curved posts define a more unique spacing to the opposite
smooth surface than a pointed open-ended curved post.
[0019] The helical configuration may also be wound around a dielectric rod or similar, or
surrounded by dielectric material e.g. foam, both of which will provide good mechanical
support.
[0020] Preferably, the number of turns of the helical configuration is within the range
2-20, and preferably 3-15 and most preferably 4-10.
[0021] Further, the curved posts preferably have conductive contact only to one of said
surfaces, leaving a gap towards the other surface inside which the microwave circuit
board is located.
[0022] The curved posts comprise a first part connected to one of said surfaces that may
have a relatively straight configuration, and a second part having the helical configuration.
The provision of such a part with a relatively straight configuration may make production
and assembly easier. Further, the height of this relatively straight part is part
of the length of the wire, and an increase/reduction will contribute to reduce/increase
the frequencies at which the stop band occurs. However, it is also feasible to use
posts entirely being of a helical configuration.
[0023] The diameter of the turns of the helical configuration is preferably within the range
10-50% of the axial height of the part having the helical configuration, and preferably
within the range 20-40%.
[0024] The gap inside which the microwave circuit board is located is preferably filled
with air, gas or vacuum. However, it is also feasible to have the gap filled with
a dielectric material. The curved posts of the present invention can also be surrounded
by dielectric material preferable in the form of a foam, or wound around dielectric
rods, both providing mechanical support for the curved posts that therefore can be
made thinner such as e.g. by the soft copper wires used to wind inductive coils for
low frequency circuits.
[0025] The two surfaces may be connected together for rigidity by a mechanical structure
defining the sidewalls of the cavity. The sidewalls may be arranged on all the sides,
thereby entirely enclosing the cavity, or along only some sides, leaving at least
one opening on the sides. Alternatively, it is possible to use an open solution without
sidewalls.
[0026] The two surfaces may be essentially planar. However, it is also feasible that at
least part of the two surfaces are curved in the same way so that the gap between
them is kept is so small that wave propagation inside the gap is stopped.
[0027] The microwave circuit boards enclosed between the two surfaces of the present invention
may comprise one or more components such as power amplifiers, low noise amplifiers,
ICs, MMICs, filters, matching networks, power dividers and combiners, couplers, antennas
and so on.
[0028] The basic geometry of a microwave circuit package in accordance with the present
invention comprises two parallel conducting surfaces forming the two larger walls
of a cavity. These surfaces can be the surfaces of two metal bulks, but they can also
be made of other types of materials having a metalized surface. They can also be made
of other materials with good electric conductivity. One of the surfaces is provided
with the curved posts. The two surfaces can be plane or curved, but they are in both
cases separated by a very small distance, a gap, between the end of the curved posts
the other plate, inside which the microwave circuits are located. The curved posts
prohibit wave propagation inside the gap, in such a way that cavity resonances are
avoided.
[0029] According to another aspect of the invention there is provided a method of packaging
a passive or active microwave circuit, comprising the steps:
enclosing a microwave circuit located on a microwave substrate with a ground plane
and comprising one or more sections of one or more different transmission lines, e.g.
microstrip lines or coplanar waveguides, in the gap between two conducting surfaces,
where one of the surfaces may be formed by the ground plane of the microwave circuit
board or an extension of this,
providing at least one of the surfaces with at least one conducting element in the
form of a curved post of conducting material rising from and being attached to the
conducting surface in such a way that wave propagation inside the gap between the
two surfaces is stopped, at least at the frequency of operation, said curved post
having on at least part of its extension a helical configuration.
Drawings
[0030]
Figure 1 shows a sketch of an example of a microwave circuit that is packaged by using
a lid of curved posts of helical forms, according to the invention. The larger upper
surface, i.e. the lid of helical posts/nails, is shown up-side-down as a bed of posts/nails
in Figure 1a, the microstrip circuit board is shown in Figure 1c, and the microstrip
circuit board is shown upside down as located on top of the bed of posts/nails in
Figure 1b, thereby together forming a metal cavity where the cavity resonances are
avoided by the curved helical posts according to the invention. Figure 1d shows a
detail of the lid of helical posts.
Figures 2 and 3 show cross sections of the example in Figure 1 when the microstrip
line is a so-called inverted or suspended microstrip line.
Figure 3 illustrates a packaged microwave circuit similar to the ones illustrated
in Figs. 1-2, but with the metal walls of the cavity removed.
Figures 4a, b, c, d, e, f, g and h show different embodiments of the curved posts
in helical configuration according to the invention. Figs 4a-d are side-views, whereas
Fig. 4e-g are top-views, Fig 4h is a perspective view, and Fig 4i is a sideview.
Detailed description of the figures
[0031] Figure 1 shows a sketch of an example of a microwave circuit that is packaged by
using a lid of curved posts of helical forms, according to the invention.
[0032] There are two metal pieces providing the upper 1 and lower 2 conducting surfaces.
The lower surface 2 is preferably essentially smooth, and formed by a ground plane
2 on which a microstrip transmission line 5 is arranged. Connectors 9 are arranged
on the side of the plane 2 opposite to said transmission line, and connected to the
microstrip transmission line through the plane 2.
[0033] The upper surface 1 is provided with a surrounding rim 3 to which the upper surface
can be mounted, and comprises a region which is lower than the rim and thereby provides
a gap 4 between the upper and lower surfaces when assembled. The gap 4 is air-filled,
but it can also be fully or partly filled with dielectric material.
[0034] The larger upper surface 1 comprises a bed of curved posts 6 comprising, at least
partly, a helical configuration. The posts are preferably equidistantly and evenly
distributed over the surface. In Fig. 1a the lid of helical posts is shown up-side-down
as a bed of posts. The posts 6 provide cut-off conditions for all waves propagating
between the lower and upper surfaces except the desired waves along the transmission
line 5. The posts work similar to a PMC within the operating frequency band.
[0035] The rim 3 is preferably arranged to extend to a height exceeding the height of the
posts.
[0036] The microstrip circuit board is shown in Figure 1c, and the microstrip circuit board
is shown upside down as located on top of the bed of posts/nails in Figure 1b, thereby
together forming a metal cavity where the cavity resonances are avoided by the curved
helical posts according to the invention. The upper and lower surfaces may be connected
to each other by bolting. To this end, screw holes 8 may be arranged at the boundary
of the lower upper metal plane, to be used to fix it to the metal rim 3 of the upper
metal piece, and there are matching screw holes 7 in this rim.
[0037] Figure 1d shows a detail of the lid of helical posts.
[0039] This package can be used for packaging many conventional circuits to form microwave
device of the type discussed above. The package component may typically have a length
of about 10 cm, a width of about 5-6 cm. The posts may typically be arranged in even
rows and columns. The distances between each row and between each column are preferably
about the same. This distance is typically, from center to center of the adjacent
posts, 7.5 mm.
[0040] Figures 2 and 3 show cross sections of the example similar to the one in Figure 1,
where the microstrip line 5 forms a so-called inverted or suspended microstrip line.
The transmission line is here supported by a thin substrate layer 10 located on the
top of the posts 6. The space 11 between the posts may e.g. be air-filled, or filled
with a dielectric, such as a foam. Such lines have the transmission line fields in
an air gap, instead of in the dielectric substrate, and the substrate is only used
to mechanically support the microstrip line 5 at distance from helical-shaped curved
posts 6 according to the invention, and at distance from the other smooth metal surface
acting as a ground plane 2 for the microstrip line. Figure 3 is used to illustrate
that the metal walls 3 of the cavity actually can be removed. The curved posts 6 will
still stop waves from propagating between the two plates according to the invention.
[0041] Figures 4 a-h show different embodiments of the curved posts in helical configuration
according to embodiments of the invention.
[0042] The posts may have an entirely helical configuration. Such a post is illustrated
in Fig 4a. This post, having an entirely helical configuration may typically have
about 10 turns, and a height of about 15 mm. However, alternative configurations are
possible as well.
[0043] Fig. 4b illustrates an example of a post having a first part having a straight configuration
with a height L, and a second part with a helical configuration. The total height
of the post is H, and a gap between the post and the opposite surface is g. The diameter
of the turns of the helical part is D. In an exemplary configuration, g is 2 mm, H
is 37 mm, D is 10.5 mm and L is 9 mm. The number of turns is in this example 6. With
these dimensions, a wide stop band is created when used between the parallel plates,
from 0.55 GHz to 1.41 GHz. However, all of these parameters may be adjusted for optimization
of the posts for the intended frequency of operation and the intended application
of the microwave device. For example, the posts can be made even more compact if required,
by increasing the number of turns.
[0044] Fig 4c-d illustrates helical configurations similar to the one discussed with relation
to Fig 4a, but where the general shape of the helical configuration is in the form
of a cone (Fig 4c), with the diameters of the turns increasing from one end to the
other, and in the form of a barrel (Fig 4d), with the diameters of the turns decreasing
from the middle towards both ends, instead of the generally cylindrical shape illustrated
in Fig 4a.
[0045] The turns of the helical configurations illustrated in Figs. 4a-d are generally circular,
as seen from above. However, alternatively the turns may be arranged in other forms.
For example, the turns may have a rectangular or square shape, as illustrated in Fig
4f, or have a hexagonal shape, as illustrated in Fig 4g, or an octagonal shape, as
illustrated in Fig 4e.
[0046] The posts having a helical configuration may also be realized as flat spirals on
a dielectric substrate, as illustrated in Fig 4h.
[0047] In yet another embodiment, illustrated in Fig 4i, the curved post is ending in a
turn with a smaller inclination angle than the rest of the turns, and preferably ends
in an essentially flat ring. The upper turn is hereby preferably essentially parallel
to the opposing surface. In this embodiment the ends of the curved posts define a
more unique spacing to the opposite smooth surface than a pointed open-ended curved
post.
[0048] The shape of the helical configuration, the total height, the diameter of the turns,
the thickness of the wire, the number of turns, the angle of the turns, the distance
between the posts and the opposite surface, etc. are all parameters that may be modified
for optimization of the posts for particular use situations.
[0049] The invention is not limited to the embodiments shown here. In particular, the invention
can be located inside the package of an IC or in the multiple layers on an IC chip.
Also, at least one of the conducting surfaces may be provided with penetrating probes,
apertures, slots or similar elements through which waves are radiated or being coupled
to exterior circuits. Further, the helical configuration of the curved posts may be
arranged in many different shapes, as discussed above. Further, the posts may be arranged
on either one of the two surfaces, or even on both surfaces. Further, the two surfaces
may be connected in various ways, and the cavity need not be closed, but may be open
at one or several sides.
1. A passive or active microwave circuit package, comprising:
two conducting surfaces forming a gap therebetween; and
a microwave circuit located on a microwave substrate with a ground plane and comprising
one or more sections of one or more different transmission lines, e.g. microstrip
lines or coplanar waveguides,
wherein said microwave circuit is enclosed in the gap between said two conducting
surfaces, where one of the surfaces may be formed by the ground plane of the microwave
circuit board or an extension of this,
wherein at least one of the surfaces is provided with at least one conducting element
in the form of a curved post of conducting material rising from and being attached
to the conducting surface in such a way that wave propagation inside the gap between
the two surfaces is stopped, at least at the frequency of operation, and
wherein said curved post has on at least part of its extension a helical configuration.
2. The microwave circuit package according to claim 1, wherein the two surfaces are connected
to each other by vertical walls so that a shielded cavity is formed inside which the
microwave circuits are located.
3. The microwave circuit package according to claim 1 or 2, wherein the curved posts
are connected only to one of said surfaces, leaving a gap towards the other surface.
4. The microwave circuit package according to claim 3, wherein the curved posts comprise
a first part connected to one of said surfaces and having a relatively straight configuration,
and a second part having the helical configuration.
5. The microwave circuit package according to claim 4, wherein the curved posts are mounted
in cavities in one of said surfaces in such a way that they are rising from the surface
with their helical configuration.
6. The microwave circuit package according to any of the previous claims, wherein the
gap is filled with air, gas or vacuum.
7. The microwave circuit package according to any of the previous claims, wherein the
gap is filled fully or partly with dielectric material.
8. The microwave circuit package according to any of the previous claims, wherein the
curved posts are wound on dielectric rods.
9. The microwave circuit package according to any of the previous claims, wherein each
of the curved posts are ending in a turn with smaller inclination angle than the rest
of the turns, and preferably ends in an essentially flat ring.
10. The microwave circuit package according to any of the previous claims, wherein the
curved posts are connected only to one of said surfaces, and the other end is touching
a dielectric parts of the microwave circuit board.
11. The microwave circuit package according to any of the previous claims, wherein the
curved posts are connected only to one of said surfaces, and the other end is touching
the ground plane of the microwave circuit board.
12. The microwave circuit package according to any of the previous claims, wherein the
curved posts are realized as flat spirals with a vertical part connected to said surface.
13. The microwave circuit package according to any of the previous claims, wherein the
curved posts are embedded in or on a dielectric material, e.g. a foam material.
14. The microwave circuit package according to any one of the preceding claims, wherein
a plurality of curved posts are provided, and arranged in a periodic or nearly-period
grid over at least part of said surface.
15. The microwave circuit package according to any one of the preceding claims, wherein
the microwave circuit has a frequency of operation within the range 100 MHz - 30 GHz,
and preferably within the range 500 MHz to 10 GHz, and most preferably within the
range 1 - 10 GHz.
16. A method of packaging a passive or active microwave circuit, comprising the steps:
enclosing a microwave circuit located on a microwave substrate with a ground plane
and comprising one or more sections of one or more different transmission lines, e.g.
microstrip lines or coplanar waveguides, in the gap between two conducting surfaces,
where one of the surfaces may be formed by the ground plane of the microwave circuit
board or an extension of this,
providing at least one of the surfaces with at least one conducting element in the
form of a curved post of conducting material rising from and being attached to the
conducting surface in such a way that wave propagation inside the gap between the
two surfaces is stopped, at least at the frequency of operation, said curved post
having on at least part of its extension a helical configuration.