FILTER THAT IS VARIABLE BY MEANS OF A CAPACITOR THAT IS
SWITCHED USING MEMS COMPONENTS
The subject matter of the present invention relates to a variable
filter using capacitors switched by means of microelectromechanical systems
(MEMS) components
In the description, the expression "tunable filter" will be understood
to mean a filter belonging to the family of passband filters produced from
coupled oscillating (inductorlcapacitor) circuits, said filters being configurable
and adjustable.
Likewise, the term "jamming" is used to denote signals that disrupt
the useful signal. The term "resonator" is used in the present invention to
refer to resonant circuits, also called oscillator circuits. The term
"interdigitated" is known in the art.
MEMS technology has allowed advances in microelectronics to be
used to produce deformable microsystems the mechanical behavior of which
modulates electrical behavior.
Various actuators and switches are described in the prior art.
Thus, the prior art discloses MEMS structures that use a movable conductive
element and a number of fixed current carrying contact terminals
advantageousk allowing a larger current to be carried relative to prior
devices in which the current was made to flow through movable conductive
elements. The fields of radio communication and radio navigation make use
of low power transceiver filter systems for highly linear co-site filtering. Cosite
or proximity operation is in particular obtained when a receiver capturing
a weak signal is located nearby a high-power emitter.
Co-site filters have a very substantial impact on power
consumption and bulk. Most of the systems currently used have the following
problems:
unsatisfactory filtering system linearity when received and filtered
signal power varies;
unsatisfactory band coverage depending on the application;
unsatisfactory selectivity, which is necessarily improved by increasing
quality factor;
parasitic amplitudelphase modulation, which may appear depending
5 on the amount of jamming at reception. This parasitic amplitudelphase
modulation affects the measurement error, known by the acronym
EVM (error vector magnitude), used to quantify the performance of a
digital radio emitter or receiver; and
substantial bulk and a substantial power consumption when live to RF
10 power.
To solve these various problems it is known to use a varactor
diode based filter, however varactor diodes are nonlinear components that
have a low immunity to jamming. It is also known to use capacitors switched
by relay or p-i-n (positive-intrinsic-negative) diode. However, switching times
15 are too long for this type of switching. In the case where p-i-n diodes are
used power consumption is high.
The various solutions known in the art do not adequately solve the
following problems:
the presence of an intentional or unintentional jamming signal at a
20 frequency relatively near the frequency of the useful signal;
the linearity of the filtering system whatever the frequency used.
The technical teachings of patent application US 2005/0017824
relate to a filter comprising two elements 8, 9 placed in parallel with each
other and connected via a coupling element 18 that is a capacitor. The first
25 conductor 8 and the second conductor 9 are rectangular features placed in
parallel and spaced apart by a given distance. An element 10, which is a third
conductor, is located between the first and the second elements 8,9. The
coupling capacitor 18 is connected to the two elements 8 and 9.
Document KR 2001 0094509, a summary of which is available in
30 the Espacenet patent database, describes a microstrip capacitor.
The document entitled "adjustable bandwidth filter design based
on interdigital capacitors" IEEE microwave and wireless components letters,
pages 16-1 8, XPOl1199157 relates to microstrip filters.
The document entitled "a microstrip bandpass filter with ultra-wide
stopband" IEEE transactions on microwave theory and techniques, pages
1468-1472, XP011215082 also describes a microstrip technology. Figure 1-
shows a filter structure that comprises a number of "open stubs" and
"interdigital" capacitors.
The document entitled "corrugated microstrip coupled lines for
constant absolute bandwidth tunable filters" IEEE transactions on microwave
theory and techniques, Vol. 58, No. 4, (2010-04-01), pages 956-963,
XP011305950 for example shows a three-pole filter in figure 6. The idea in
this paper is to demonstrate that "corrugated" microstrip lines can also be
used to control coupling coefficient, enabling constant absolute bandwidth
filtering. Figure 8 shows a model of a miniaturized two-pole electrical circuit.
Patent EP 1 953 914 relates to a multiplexer and a diplexer.
Patent application US 200210149448 relates to a device allowing
losses in ferromagnetic components to be characterized.
The subject matter of the present invention notably relates to a
novel approach using MEMS components to switch between various
capacitance values in a tunable filter. This advantageously enables
adjustable or reconfigurable filtering or filtering with a filter the constant pass
band of which can be tuned to a frequency using variable MEMS RF
capacitors.
25 The invention relates to a tunable filter comprising at least two
resonator circuits placed between a first matching network connected to a
first inputloutput and a second matching network connected to a second
inputloutput, said matching networks consisting of an inductor (L4, L5) and a
capacitor (C4, C5) connected in parallel, and:
one resonator is connected at a first of its ends on one side to the
ground plane M of the filter by metallized holes and at a second end to
a MEMS network;
the distance d between the 2 resonators forms an inter-resonator
5 inductive coupling circuit;
an inter-resonator coupling capacitor is formed by 2 etched lines that
are connected to the first and second resonators, respectively; -
the MEMS networks are distributed around the ends of the resonators;
the MEMS networks are connected between the first and second
10 resonator and the ground plane M by virtue of vias or metallized holes;
and
a number of independent electrical control voltages Vi designed to
actuate the MEMS or MEMS network are employed.
The tunable filter according to the invention is, for example,
15 produced in microstrip technology.
The filter having a feature at least described above is used in a
receiver chain, said tunable filter being placed closest a receiving antenna
and just before a low noise amplifier.
According to another embodiment, the tunable filter according to
20 the invention is used in a receiver chain, said tunable filter being placed
downstream of a low noise amplifier and of a high-field protection device and
of an antenna.
The tunable filter may also be placed between a power driver or
controller and an amplifier.
25 Other features and advantages of the device according to the
invention will become more clearly apparent on reading the following
description of an embodiment given by way of non-limiting example and
illustrated by the appended figures, in which:
figure I is a basic diagram of the filter according to the invention;
figure 2 is an example configuration of the MEMS filter according to
the invention;
figure 3 is an embodiment of the filter according to the invention;
figure 4 is an example of a 3 bit network of MEMS switched
capacitors;
figure 5 is an example of a network of capacitors switched by ohmic
MEMS, an 8 bit network;
figure 6 shows an example transfer function for the filter at a frequency
of 950 MHz;
figure 7 shows an example transfer function for the filter at a frequency
of 1454 MHz;
figure 8 shows an example transfer function for the filter at a frequency
of 2300 MHz;
figure 9 is an example type 1 receiver architecture;
figure 10 is an example type 2 receiver architecture; and
figure 11 is an example emitter:
The filter studied, an example of which is given by way of
illustration to clarify the subject matter of the present invention, is a working 2
pole filter covering an octave with a constant passband over the entire
tunable range.
Figure 1 shows a basic diagram of a passband filter according to
the invention having inputs/outputs denoted INIOUT. The principle
implemented is the use of coupled oscillating circuits.
The core of the device consists of two oscillating circuits 1, 2
having a self-inductance Lo and a variable capacitance Co. These two
oscillating circuits I, 2 or resonator circuits or resonators are coupled by a
coupling circuit 3 produced by placing a coupling inductor LC,,, and a
capacitor CC,,, in parallel in this embodiment.
The assembly consisting of the first coupling circuit 1 and the
coupling circuit is connected to the input IN of the filter by a matching network
4, which converts an impedance of 50 ohms to the impedance required to
achieve filtering with a constant bandpass, this being one of the advantages
of the present device.
Symmetrically, an identical matching network 5 allows the output
of the filter to be coupled to the second resonant circuit.
A matching network 4, 5 may consist of a fixed inductor wifn a
capacitor in parallel coupled to the inductance Lo of the resonant circuit via
-5 an intermediate connection, according to a model known to those skilled in
the art, these elements not being shown in the figure for the sake of
simplicity.
Correct matching to 50 ohms and the constancy of the passband
at -3 dB depend on mathematical relationships between the elements of the
10 matching circuit Lo, CO,L coupa nd .,C, ,,
Figure 2 shows an example filter configuration and schematically
shows an example architecture using MEMS switched capacitors placed at
the ends of the resonator or resonant circuit used in the diagram in figure 1.
The configuration chosen in this example is a coupled line configuration
15 employing inter-resonator inductive coupling 10, i.e. coupling between two
resonators 14a and 14b, and a capacitive coupling 11 achieved via an
interdigitated capacitor placed between the two resonators, this low
capacitance valu e,,C,, of the coupling circuit 11 is formed from two coupled
lines I la, I I b that are narrower and closely spaced over a distance of a few
20 mm and connected to the resonators 14a and 14b. The sum of these two
types of coupling 10, 11 makes it possible to achieve a filter with an almost
constant bandwidth.
Figure 3 shows a physical representation of the MEMS filter
produced in microstrip technology. The lines of the circuit are etched on a
substrate S. The other side of the circuit is a ground plane M (not shown in
figure for the sake of simplicity) known to those skilled in the art. InputJoutput
access INIOUT is provided by lines etched on the substrate, of 50 R
impedance, for example. The matching network 4, 5 is produced from a
discreet inductor L4, L5 and capacitor C4, C5 soldered in parallel. The
matching network 4, 5 is connected in series with the inputloutput line
INIOUT. These two components (inductor and capacitor) match the
input/output access of the filter and the resonator 14a, 14b and thus allow
harmonious and simple impedance conversion 13 between the 50 n access
and the impedance of the resonator as there is a large impedance mismatch
at this line intersection (13, INIOUT). A resonator 14a, 14b is connected at a
first of its ends 14al, 14bl, respectively, on one side to a ground plane by
metallized holes or vias 16 and at a second end 14a2, 14b2, respectively, to a
MEMS network 12. The distance d between the two resonators (14a, 14b)
creates the inter-resonator inductive coupling circuit 10. This distance is for
example set using a simulation employing methods known to those skilled in
the art, in order to obtain the desired conversion function for a given
application of the tunable filter. The inter-resonator coupling capacitor 11 is
formed by two small etched lines I la, I I b that are connected to the
resonator 14a, 14b. The widths of the lines I l a , I l b are for example set
depending on the frequency of the filter and therefore on the intended
application. The MEMS networks 12 are distributed around the ends of the
resonators 14a, 14b in this embodiment, the distribution being chosen in
order to group the various elements as much as possible. The MEW
networks 12 are connected between the resonator 14a, 14b and the ground
plane M via the vias or metallized holes 16. 8 independent electrical control
voltages V1 to V8 17 allow the MEMS 12 to be actuated. The actuation
voltages are delivered to the MEMS by high-impedance lines. The filter
schematically shown in figure 3 is symmetric about an axis A.
The radio-frequency or RF MEMS network 12 may consist of an
array of capacitive MEMS the capacitances of which can be set to a number
of values (as shown in figure 4) or indeed of ohmic MEMS used to switch a
network of fixed capacitors (as shown in figure 5). The capacitances Ci are
calculated in order to obtain a constant step frequency. The number of
capacitors gives the value of the frequency increment. The MEMS
components are represented by switches. The capacitors may be placed in
parallel and connected to one or more MEMS. The filter is designed to have
a constant passband width. This particular structure has a better power
withstand than was possible in the prior art. The impedance R1 is given by
the foilowing formula: R1 = J z * Q * L ~ * t~h~is , impedance is not shown in
figures 4 and 5. It is the impedance of the entire oscillating circuit (Lo, Co).
Where oo = 2*n*fO and Q=fO/Af, fO being the working frequency and Af being
5 the passband of the filter at -3 dB.
The power passing through the filter obeys the following
relationship at the terminats; of the oscillating circuit: P=Z.V,,~/R~ were Veff is
the RMS voltage. Peak voltage = 4 2 V.3, Vpesk= J(R~.P).T he impedance
seen by the MEMS is (R1)12 when the filter is tuned.
10 For a power P the peak voltage across the terminals of the MEMS
is a maximum at the maximum frequency with an amplitude equal to
d(~q~.1 2 ) .
To maximize the power withstand of the filter, it is necessary to
decrease the value of R1 and therefore alter the design of the filter, the value
15 of the capacitance of the MEMS increases as R1 decreases. The MEMS filter
thus defined may accept high powers. To further increase the admissible
power of the filter it is possible to place a number of MEMS in parallel.
In figures 3 and 5, the control signal actuating the MEMS is
modeled by the voltages V1 to V8, which select the capacitance to apply in
20 the filter to obtain the desired frequency.
In figure 4, the actuating control signal is modeled by bits Bitl,
Bit2, Bit3. The variable capacitance C = 2"Cs, where Cs is the capacitance of
a basic element that can take 2 values. The total capacitance is then the sum
of the capacitances of the arrays in the low state Co and the capacitances of
25 the arrays in the high state C1. In the case of a 3 bit capacitance, the
capacitance corresponding to the binary value "101" is equal to C =
In figure 5, the actuating control signal has an 8 bit capacity. The
ohmic MEMS network allows the c'.a pacitors C1 to Cs to be selected, which
30 capacitors have 8 different capacitances. In this case the frequency
increment value of the filter is 28 = 256.
Numerical example of a device according to the invention
For a filter with a passband of 50 MHz, the frequency range to
cover is from 950 to 2300 MHz.
Attenuation at FO +/00-MIH z >20 dB
5 Attenuation at FO +I-200MHz >35 dB
With an 8 position MEMS switch it is thus possible to obtain
28 = 256 steps.
Since the frequency range is 1.35 GHz, with such a step it is
possible to increment frequency in steps of about 5.3 MHz. This step is
10 compatible with the desired passband.
The minimum crossover must be such that the central frequency
of the first step corresponds to 2300 MHz - 25 MHz for optimal crossover in
the band at 0.5 dB for the filter. Thus, a "box of weights" comprising 8
elements is used for an overestimate of 20% and to provide some latitude in
15 the band covered by the device i.e. for the following steps:
1 I I I 1 I I I I I
the passband of the filter is almost constant. The desired
passband of about 50 MHz is obtained at the minimum and maximum
frequencies, the filter width being larger (64 MHz) midband:
Step
CapacitancepF
8
0.04
1
5
6
0.156
Combination
of steps
Frequency
(MHz)
Loss dB
Passband
MHz
7
0.08
2
2.5
0
2300
2.843
47-
3
1.25
P8
2273
2.719
48
4
0.625
P7
2248
2.616
48
5
0.312
P8+P7
2223
2.518
51
P6
2202
2.463
50
P5
2115
2.254
51
P4
1967
2.064
60
The filter covers a frequency band ranging from 861 MHz when ail
the capacitors are activated (activation of all the MEMS) to 2300-MHz when
5 all the MEMS are deactivated.
Figures 6, 7 and 8 show the transfer function dB(S(2,l)) and
matching dB(S(1 ,I)) obtained for the filter at three central frequencies: 950
MHz, 1454 MHz and 2300 MHz (s parameters spl and sll well known in the
art). These figures show that a degree of performance is obtained over a very
I 0 wide frequency band.
Figure 9 shows a first embodiment in which a tunable filter 30
according to the invention is placed furthest upstream in a receiver chain
closest to a receiving antenna 31 and just before a low noise amplifier (LNA)
32 between the filter and antenna. The tunable MEMS filter has the
15 advantage of minimizing losses, thereby guaranteeing that the receiver has a
low noise factor and a power level sufficient to protect the receiver from highpower
out-of-band jamming signals. Only part of the high-field protection,
voltages higher than 30 volts, will remain at the antenna 31, lightning
,
protection device 34, the other part of the field protection will be installed just
20 in front of the LNA amplifier 32 with a power limiting device 33 protecting the
LNA but only in the passband of the filter.
Figure 10 is another embodiment of the filter 40 according to the
invention. This example architecture is used when the sources of high-field
are a little further away and the fields in question will not be as high; such an
25 architecture allows immunity to medium distance jamming (1 to 5 MHz) to be
improved via use of a more selective tunable filter and therefore a smaller
frequency' separation, for instance in the case where a number of
transceivers are used on the same site. The tunable filter 40 according to the
invention will be placed downstream of an LNA 41 and of a high-field
protection device 43 and of an antenna 42.
Regarding use in an emitter (figure 1 I), inserting the tunable filter
50 according to the invention in a medium-power emitter chain will allow
wide-band noise \o be improved beyond the passband of the filter, which is
placed between the power driver or controller 51 and the amplifier 52
connected to the antenna 53. Another important point is that maximum
applicable RF power must be taken into account when used in an emitter.
This allows waveforms with nonconstant envelopes, such as those used in
OFDM, to pass through these filters without EVM degradation, because the
advantage of the filter is that it is very linear. In a transceiver station, the
same filter may be used both for transmission and reception.
A notable advantage of the invention is that it provides a device
having a capacitance that can be made to vary over a very wide range by
switching capacitors, thereby allowing a wide band to be covered.
It also provides devices having the following improvements:
a lower filter loss;
an improved filter selectivity;
a wide band coverage;
very rapid frequency changes (agility);
an almost constant passband;
a better linearity than in the prior art;
a negligible current consumption; and
a much higher admissible power than varactor filters, which are limited
in voltage by the value of their control voltage; which, because of the
large frequency variation of the device, may have low values of 1 to 2 ,
volts.
The voltage applied to the MEMS component may be quite high
and, as intermodulation performance is excellent, a rather substantial
improvement in jamming immunity may be obtained and the filter according
to the invention may be used for medium-power emissions (about 5 to 10
watts).
In this power field, adjustable filters using fixed capacitors
actuated by p-i-n diodes in the tuning system have admissible powers similar
to the present device, but at the cost of the high electrical power consumption
required to keep the p-i-n diodes on.
In the device presented, very little power is required to turn a
MEMS on, which is one of the advantages of this device.

Claims
1 - A tunable filter comprising at least two resonator circuits (14a1 14b)
placed between a first matching network (4) connected to a first inputloutput
and a second matching network (5) connected to a second inputloutput,
characterized in that said matching networks (4, 5) consist of an inductor (L4,
L5) and a capacitor (C4! C5) connected in parallel, and in that:
one resonator (14a1 14b) is connected at a first of its ends (14all 14bl)
on one side to the ground plane M of the filter by metallized holes (16)
and at a second end (14a2, 14b2) to a MEMS network (12);
the distance d between the 2 resonators (14a, 14b) forms an interresonator
inductive coupling circuit (1 0);
an inter-resonator coupling capacitor (1 1) is formed by 2 etched lines
( I 1 a, I I b) that are connected to the first and second resonators (14a,
14b), respectively;
the MEMS networks (12) are distributed around the ends of the
resonators (1 4a1 14b);
the MEMS networks (12) are connected between the first and second
resonator (14a1 14b) and the ground plane M by virtue of vias or
metallized holes (16); and
the filter comprises a number of independent electrical control
voltages Vi (17) designed to actuate the MEMS (12).
2 - The tunable filter as claimed in claim 1, characterized in that microstrip
technology is used.
3 - The use of a tunable filter as claimed in either of claims 1 and 2 in a
receiver chain, said tunable filter being placed closest a receiving antenna
(31) and just before a low noise amplifier (32).
4 - The use of a tunable filter (40) as claimed in either of claims 1 qnd 2 in a
receiver chain, said tunable filter being placed downstream of a low noise
amplifier (41) and of a high-field protection device and of an' antenna (42).
5 5 - The use of a tunable filter (50) as claimed in either of claims 1 and 2, said
tunable filter being placed between a power driver or controller (51) and an
amplifier (52).
Dated this 01.07.2013
ATTORNEY FOR THE APPLICANT[S]