WO2010054904A1

WO2010054904A1 - Low-cost miniature active filter

Info

Publication number: WO2010054904A1
Application number: PCT/EP2009/063338
Authority: WO
Inventors: Philippe Dueme; François BERGERAS; Jean-Philippe Plaze
Original assignee: Thales
Priority date: 2008-11-14
Filing date: 2009-10-13
Publication date: 2010-05-20
Also published as: FR2938706A1; FR2938706B1

Abstract

The invention relates to microwave active filters consisting of p filtering cells CI1, CI2,..CIi-1,.. CIi,.. CIp in cascade for filtering a microwave signal passing through the filter, propagating in the direction from the first cell CI1 to the last cell CIp, p being an integer equal to or greater than 1, each filtering cell comprising a resonator Re1, Re2,...Rei-1,Rei.. Rep each of n1, n2.., ni-1.., ni,... np poles respectively, the number of poles ni of a resonator Re of index i being an integer equal to or greater than 1, a resonator Rei of a cell C1i being isolated from a resonator Rei-1 of a preceding cell CIi-1, at least in the direction opposite to the propagation of the microwave signal passing through the filter, by an isolating circuit (A1, A2, A3 A4, Att1, Att2) that attenuates a return microwave signal going from said cell CIi to the preceding cell CIi-1. The isolating circuit for at least one of the cells of the filter is an attenuator (Att1, Att2) of value AT that attenuates the return signal by said attenuation value AT, the isolating circuits for the other cells being active isolating circuits comprising at least one amplifying transistor. Applications: low-cost high-performance microwave active filters.

Description

MINIATURE ACTIVE FILTER WITH LOW COST

The present invention relates to low cost miniature active filters having high performance in the microwave field.

Most electronic devices and in particular in the field of radiocommunications include filtering functions.

The filter function is provided in some applications by passive filters. Nevertheless, the objectives of miniaturization, cost reduction and increased performance of microwave systems are less and less compatible with the passive component-based solutions that are currently used for filtering functions. Indeed, the use of passive components for the realization of the filters strongly limit the possibilities of miniaturization and integration and weigh on the architecture of the microwave systems (emission or reception) as well as on their performances. These limitations are particularly sensitive for applications with a wide operating frequency band, for example, in the case of preselection microwave filter banks, filters characterized by a medium or wide relative band with very steep sides, but also in case of very selective intermediate frequency filters F1 whose characteristics are determined by the frequency plane of the receivers and transmitters.

In the case of microwave receivers, these types of passive filters can not be integrated in the receiving antenna blocks because of their size and must be deported away from the antennas which leads to a limitation of the performance of the receiver, in particular by a increase in reception noise. The elimination of disturbers (disturbing frequencies) can only be performed later in the reception chain with a risk of saturation thereof or loss of the useful signal. In the case of transmitters, the passive filters cause a drop in the radiated power. For transmit / receive modules of future multifunctional active antennas, it is necessary for the filters to adapt to the mode used

(guidance, tracking, monitoring of one or more targets simultaneously ...), filtering solutions with passive elements, by nature unadaptable, are even more limiting.

The use of active filter technologies reduces the size and cost of radio frequency filters. In addition, the use of active filters could, for example, consider the creation of 'universal' radio frequency or microwave systems based on a single internal architecture but which can be reconfigured as needed. This is indeed the trend that has emerged in recent years with the concepts of 'reconfigurable systems' or 'tunable functions'. These concepts are of interest to the civil telecommunications industry, but also to the military industry. The technologies and topologies used for the design of active filters are numerous. The filters can be made on the same chip (monolithic filters) or combine structures made in MMIC technology with passive structures generally planar (hybrid filters). Depending on each application, a compromise must be sought to lead on a case-by-case basis to a balance between passive and active functions. The different topologies of active filters present in the literature can be grouped into two broad categories:

A) The active filters resulting from the modification of a conventional passive filter structure, namely:

- The dielectric resonator filters whose losses are compensated by an active loop;

planar filters whose losses are compensated by active circuits such as amplifiers, circuits simulating negative resistance, etc. ;

planar filters which are tuned in frequency by the use of varactor diodes (which may be associated with circuits simulating negative capacitors) or phase shifter circuits. B) The active filters resulting from a transposition to radiofrequencies and microwaves of concepts developed for the low frequencies namely:

- recursive and transversal filters, - continuous analog active filters (RC filters, Gm-C ... etc),

- active inductance filters and active resonator filters,

filters based on the paralleling (or cascade) of passive and active cells: "channeled" filters, filters (or resonators) actively coupled.

Current research on active filtering focuses on several areas of research, including:

- the development of synthesis techniques and new principles of filtering;

the optimization of existing topologies to, for example, increase their dynamics while making them less noisy and more linear,

- The improvement of their frequency tuning by association of control devices, to control the central frequency and selectivity to overcome technological dispersions and self-heating which are important limitations.

Some techniques for producing active filters rely on the use of resonant cells separated from each other by "active blocks", that is to say amplifier blocks comprising active elements. That is a known filter function, characterized by a known set of transmission poles. The objective is to perform this function by a series of p active stages or cells Ch, CIj-- .. _I, .. CIj .. Cl _p cascade. In a simplifying assumption, the active element of the stage is a unilateral transistor T and the passive elements constitute a resonator Re for each stage. The one-sided term indicates that a signal applied to the input of transistor T propagates towards its output (with a large or low amplification) and that another signal called "return signal" propagates in the opposite direction with attenuation. important (or isolation). FIG. 1 represents a diagram showing the principle of the active filter synthesis method. For this purpose we exploit the unilaterality of the chosen model of the transistor T used in the cells in order to decompose the filtering structure into "cascadable" cells according to the diagram of FIG. 1. A signal applied to a cell propagates without attenuation (or with amplification) in the direction indicated by the arrow fa in Figure 1 and with attenuation in the opposite direction indicated by the crossed-out arrow fr.

The cell CIj is shown in detail in FIG. 1 in the dashed central frame. Transistors T are, for example, field effect transistors

(TECs).

The cell CIj comprises the resonator Rβj _, an input circuit Ce in the dashed frame on the left of the resonator Re in the form of the equivalent diagram of the drain circuit of a TEC Tj of the cell CIj. The equivalent circuit diagram of input transistor Tj comprises a current generator G1 supplying a current Gm. Vj _-1 , Vj _-1 being the output voltage of the cell CIj- _I , Gm the current gain of the TEC, a gate capacitance C _ds and a gate impedance G _ds -

An output circuit Cs of the cell CIj represented in the dashed line on the right of the resonator Rei in the form of the equivalent circuit diagram of the transistor of the transistor T _{i + 1} of the following cell CIi ₊ - _I, comprising a gate capacitance C _gs and a gate impedance G ₉ . Vj is the output voltage at the output of the cell CIj.

The resonator Rei is the filtering part of the cell C1 of order i of central pulsation Co ₁ .

The choice of this filter cut-out in cells C ₁ , C ₁ ,... CI 1, .. C _p leads us to express the transfer functions in the form of voltage gains. With the unilateral nature of transistors, a fundamental property of this structure is that the cell C _{i +} i does not interfere with the previous cell Cl _1.

Each cell realizes, according to its characteristics, pairs of real or complex conjugated poles. A theoretical development shows that this filtering structure has very interesting characteristics (but also limitations) which differentiates it fundamentally from the synthesis of passive filters namely: each cell can produce several pairs of poles that can be of different types (real poles and complex poles). A cell CIj is characterized by its order n (an order n corresponds to n pairs of poles). An odd-order cell makes it possible to integrate in its construction the parasitic capacitances that form part of the equivalent circuit diagram of a transistor or any other active block. In practice, this synthesis method therefore imposes the use of odd order cells. On the other hand, the theory shows that a cell whose order is odd and greater than 2 must have a pair of real poles (we can combine 2 cells of order 1 to obtain 2 pairs of complex conjugated poles). Strictly speaking, it is therefore possible to include only one cell of order greater than or equal to 3 because a filtering function comprises only one pair of real poles.

whatever the order and the type of poles made by each cell of the filter, they will all have different central frequencies f = 2 different.

the unilateral character of each cascadable cell (derived from that of the transistors) makes it possible to treat the cells independently of one another. In practice, this will result in a very important possibility of adjusting each of them separately.

However, when it is desired to achieve high order filters, this promising technique has a serious drawback related to the large number of cells needed to produce these types of filters.

It is known that the higher the order of a filter, the more "flanks are steep". This means that its attenuation versus frequency response curve will have larger slopes at the ends of its bandwidth, or that the filter will be able to more efficiently reject frequencies outside the useful band.

According to the theoretical characteristics described above, this filter can consist of an odd-order cell greater than or equal to 3 and cells of order 1 grouped by two to make the remaining complex conjugate poles. In addition, the multi-order cell can not be of high order because the problems of passive filtering in planar technology (low voltage overvoltage coefficients) would then rest. In practice, a high order n filter may consist of a 3 order cell followed by (n-3) order 1 cells. Each cell being surrounded by an active block, this results in a total of p = (n-1) active blocks. The gain of the whole, its consumption and the difficulties to maintain the stability and linearity of the structure increase in proportion to this number.

To avoid the disadvantages of active filters of the state of the art, the invention provides a p microwave active filter cells Ch, Cl _2, ..CIM, CIj .., .. _ICps filter cascade a microwave signal traveling through the filter propagating in the direction of the first cell Ch to the last cell C1 _p , p being an integer greater than or equal to 1, each filter cell comprising a resonator Re-i _, Re ₂ ,. ..Re _M! ..Re _i .. Re _P or _respectively _{2 n,} _rii-i, _ni, n _p poles each, the number of poles

of a resonator Re of index i being an integer greater than or equal to 1, a resonator Re ₁ of a cell CIj being isolated from a resonator Re _M of a preceding cell CIj- ₁ , at least in the direction inverse propagation the microwave signal passing through the filter, through an isolation circuit attenuating a microwave signal back from said cell C ₁ to the previous _I CIj-- cell, characterized in that the isolation circuit at at least one of the cells of the filter, but not all the cells, is an AT value attenuator attenuating the return signal of said AT attenuation value, the respective isolation circuits of the filter cells other than those having an attenuator as a circuit isolation means being active isolation circuits comprising at least one amplifier transistor.

In one embodiment of the filter, the active isolation circuit or circuits consist of a circuit comprising one or more transistors used to control the amplification gain of the active isolation circuit and to obtain a large attenuation of the microwave return signal. .

In another embodiment, the transistors are bipolar transistors mounted either as a follower emitter or as a common collector, to form an active isolation circuit with low amplification and large isolation. In another embodiment, the transistors are field effect transistors mounted either as a follower source or as a common drain to form an active isolation circuit with low amplification and large isolation.

In another embodiment, the filter comprises an even-numbered cell CIj when at least said even-numbered cell CIj and the following cell C1 _{i +} i each have an attenuation attenuator (Att-i, Att ₂ ) of its respective resonant circuit (Rθj, .. Re _{i +} -ι).

The invention will be better understood with the aid of examples of embodiments according to the invention, with reference to the indexed drawings in which:

FIG. 1 represents a diagram showing the principle of the active filter synthesis method;

FIG. 2 represents a diagram of an ideal passive filter of order 3 consisting of ideal localized components;

FIG. 3 shows the attenuation as a function of the frequency of the filter of FIG. 2;

FIG. 4 represents the diagram of a passive filter having an attenuation as a function of the shape frequency identical to that of the filter of FIG. 2;

FIG. 5 shows the attenuation as a function of the frequency of the filter of FIG. 4;

FIG. 6 shows the block diagram of a filter of the state of the art having a filtering curve of identical shape to that of the filter of FIG. 2;

FIG. 7 shows an exemplary embodiment of a filter according to the invention having an attenuation response curve as a function of the frequency very close to that of the filter of FIG. 6;

FIG. 8 shows respectively the frequency response curve of the filter of the state of the art of FIG. 6 and that of the filter according to the invention of FIG. 7 and;

FIG. 9 shows an exemplary embodiment of a filter according to the invention of order 7. A main object of the invention is to substantially reduce the total number of cells necessary to achieve a given order filter.

Another goal is to effectively control the total gain of a filter having a given number of cells.

This solution essentially consists in replacing, in certain places, a filtering circuit comprising several cascade filtering cells, at least one of the cells, an amplifier block by an attenuator. The principle on which this invention is based indeed relies on the possibility of considering the resonators as isolated from each other. In the case of an active amplifier block, for example a common source transistor, a "cascode" assembly or any other configuration, this property was ensured by the unilaterality (at least ideally) of such an active block .

In the case of an attenuator, this isolation is ensured by the attenuation itself, with the advantage that the isolation is done in this case in the 2 directions of propagation of a signal in the filter and with a counterpart , which is to lower the transmission gain of the filter.

To illustrate this solution, Figures 2, 3, 4 and 5 describe two versions of a filter function derived from a so-called Chebyshev filter with three poles. Figure 2 shows a diagram of an ideal passive order 3 filter consisting of ideal localized components. The filter of FIG. 2 comprises two parallel LC circuits L1 / C1 and L3 ^ C3 separated by a third series LC circuit, or L2, C2 in FIG. 2. The filter is charged at both ends by a characteristic impedance ZO. FIG. 3 shows the attenuation ATf as a function of the frequency F of the filter of FIG. 2.

FIG. 4 represents the diagram of another passive filter having an attenuation as a function of the shape frequency identical to that of the filter of FIG. 2. FIG. 5 shows the attenuation ATf as a function of the frequency F of the filter of FIG. 4.

The filter represented in FIG. 4 is synthesized by 3 first-order cells separated by two attenuators Att1, Att2 of 15 dB each. The synthesis is carried out according to the method proposed by Laurence Darcel in the document "Method of synthesis of active MMIC filters", 14th National Microwave Days, 1 1 -12-13 May 2005 Nantes. Its frequency response curve representing the attenuation ATf as a function of the frequency F is shown in FIG. 5. The comparison between the frequency response curves of the filters of FIGS. 2 and 4 shows a quasi-similarity of shape (same coefficient quality) and a difference in gain of 51.8dB (gains at m1 points respectively on the curves of Figures 3 and 5). This difference in gain comes on the one hand from the decomposition of a filter of order 3 into 3 cells of order 1 and on the other hand from the attenuation of 15 dB of each of the attenuators att1 and Att2.

The similarity of shapes of the curves of FIGS. 2 and 4 is also reflected in a constant gain difference between the two filters, within 0.1 dB, over the entire band (one octave) shown in FIGS. 3 and 5. The value of 15dB for each attenuator Att1, Att2 is given as an indication. A lower attenuation value of the attenuators Att1 and Att2 makes it possible to reduce the difference in gain with the theoretical attenuation curve as a function of the frequency of FIG. 2, but degrades the similarity of shape. This remarkable result shows that it is indeed possible to reproduce almost perfectly an ideal transfer function, retaining the relative rejections, by synthesizing a filter by independent cells, centered on different frequencies and separated by attenuators. The filter may also include amplifier stages for separating the resonator circuits Re and obtaining a gain of the filtered signal.

FIG. 6 shows the block diagram of a filter of the state of the art having a filtering curve of identical shape to that of the filter of FIG. 2. The filter of FIG. 6 is synthesized by 3 cells Ch, Cl ₂ , Cl _3, of order 1 separated by active amplifier blocks A _1, A _2, A _3, A _4.

These amplifier blocks may for example be assemblies of transistors known as "cascodes", each consisting of a field effect transistor (FET) in a so-called "common source" configuration followed by a field effect transistor in a "common grid" configuration. ".

The filtering circuit of FIG. 6 is made according to the method proposed by Laurence Darcel in the aforementioned document and in another document "Synthesis and Realization of Integrated Microwave Active Filters in MMIC Technology- Integration of the Method in a Tool of

CAD ", Ph.D. dissertation of Paris VI University, December 2005.

In the following, a filter according to the invention is described. FIG. 7 shows an exemplary embodiment of a filter according to the invention having an attenuation response curve as a function of the frequency very close to that of the filter of FIG. 6.

The filter of FIG. 7 comprises three filtering cells in cascade, a first cell Ch comprising a first amplifier A ₁ followed by a first resonator Re ₁ , a second cell Cl ₂ comprising, according to a main characteristic of the invention, a attenuator AH ₁ followed by a second resonator Re ₂ , then a third cell Cl ₃ having a third amplifier A ₃ followed by a third resonator Re ₃

The amplifier A ₂ of the second cell Cl ₂ of the filter of FIG. 6 has been replaced by an attenuator AH ₁ . The attenuator AH ₁ is, in this embodiment of Figure 7, a resistance T-attenuator.

The filters respectively comprise input impedance matching circuits ADe 10 and ADs output 20.

FIG. 8 shows respectively the frequency response curve F1 of the state of the art filter of FIG. 6 and that of the filter F2 according to the invention of FIG. 7.

On the one hand, it can be seen that the state-of-the-art filter represented by the curve F1 in FIG. 6 has a frequency response very close to the ideal curve of FIG. 3, with comparable relative rejections. The maximum gain of the filter of Figure 6 is 26.8dB at the center of bandwidth (point m8 on the curve), which is a very high value. We can understand that a filter of significantly higher order would present a gain too high if it was made entirely with active blocks.

On the other hand, it can be seen that the filter according to the invention of FIG. 7, whose frequency response is represented by curve F2 of FIG. 8, has a shape frequency response curve close to the previous F1, with rejections preserved but a gain limited to 5.5dB. We can then consider higher order filters made according to this principle.

The associated response curves F1, F2 of FIG. 8 have been calculated for a circuit formed of realistic components belonging to the commercial reference die PH25 (pHEMT GaAs with a gate length of 0.25 μm) of the UMS trading company.

FIG. 9 shows an exemplary embodiment of a filter according to the invention of order 7.

The filter of FIG. 9 shows the advantage of filters comprising attenuators between certain resonators Re instead of amplifiers.

The filter of FIG. 9 comprises four resonant cells: a first cell Ch comprising an amplifier A ₁ followed by a resonator Re _1,

a second cell Cl ₂ comprising an attenuator Atti followed by a resonator Re _2,

a third cell Cl ₃ comprising an attenuator Att ₂ followed by a resonator Re _3,

a fourth cell Cl ₄ comprising an amplifier A ₂ followed by an resonator R ₄

The filter further comprises input matching circuit ADe 10 connected between the input Ef of the filter and the first cell Ch, Cl ₄ the fourth cell being connected, by its output, to the output Sf from the filter by intermediate of an amplifier A ₃ and an output adaptation circuit ADs

20.

The filter of FIG. 9 comprises three amplifiers A _1, A _2, A ₃ and two attenuators AH _1, Att ₂ for a total number of four resonant cells. The gain of such a filter can be well controlled as well as its characteristics of linearity and stability. In addition, the filter according to the invention makes it possible to use second order resonators (Re ₂ ) between two attenuators.

(Att-i _, Att ₂ ) to win a filter cell compared to a filter of the state of the art to use two cells of order 1.

The advantage of the filters according to the invention with attenuator between one or more resonator circuits is twofold:

on the one hand, the replacement of an amplifier unit by an attenuator makes it possible to significantly reduce the overall gain of the filter and thus to control its linearity and stability constraints,

- On the other hand, an attenuator does not have parasitic capacitance as a transistor. It is then possible to use even-order resonant cells, in particular cells of order 2. These cells only provide conjugated complex poles, it is possible to use several of them. For example, it is theoretically possible to produce a filter of order n (high and odd) by using a cell of order 3 and ((n-3) / 2) cells of order 2, to be compared to a cell of order 3 followed by (n-3) first-order cells with only active blocks. In fact, the advantage of this solution is to combine stages comprising active blocks with stages comprising attenuators. It would be very penalizing to use only attenuators because the overall gain would be much too low.

In other embodiments of the filter according to the invention, the amplifiers separating certain resonator blocks may be amplifiers with low amplification, such as, for example, transistor amplifiers in a "source follower" configuration in the case of the TECs or in a "follower transmitter" In the case of the use of bipolar transistors, ie a so-called "common drain" stage for the TECs or "common collector" for the bipolar transistors.

Claims

1. Active microwave filter with p cells Ch, Cl _2, ..CI _M , .. CI _J .. CI _P cascading filtering of a microwave signal traveling through the filter propagating in the direction of the first cell Ch towards the last cell CIp, p being an integer greater than or equal to 1, each filtering cell comprising a resonator Re-i _, Re _2, ... Re _M! ..Re _i .. Re _P or _respectively _{2 n,} _rii-i, _ni, n _p poles each, the number of poles

of a resonator Re of index i being an integer greater than or equal to 1, a resonator Re ₁ of a cell CIj being isolated from a resonator Re _M of a preceding cell CIj- ₁ , at least in the direction reverse propagation of the microwave signal passing through the filter, an isolation circuit _(Ai, a _2, a ₃ a ₄₎ reducing a microwave return signal from said cell C ₁ to the previous cell CIj-- _I, characterized in that the circuit for isolating at least one of the cells (Cl ₂ , Cl ₃ ) of the filter, but not all the cells, is an AT attenuator (Att-i, Att ₂ ) attenuating the return signal said attenuation value AT, the respective isolation circuits of the filter cells other than those (Cl ₂ , Cl ₃ ) having an attenuator as the isolation circuit being active isolation circuits comprising at least one amplifier transistor.

2. Active filter according to claim 1, characterized in that the active isolation circuit or circuits are formed of a mounting comprising one or more transistors used to control the amplification gain of the active isolation circuit and to obtain a large attenuation of the microwave return signal.

3. Active filter according to claim 2, characterized in that the transistors are bipolar transistors mounted, either emitter follower or common collector, to form an active isolation circuit low amplification and large isolation.

4. Active filter according to claim 2, characterized in that the transistors are field effect transistors mounted either as a source follower or common drain, to form an active isolation circuit low amplification and large isolation.

5. Active filter according to one of claims 1 to 4, characterized in that it comprises an even-order cell CIj when at least said even-numbered cell CIj and the following cell C1 _{i +} i each have an attenuator d Isolation (Att-i, Att ₂ ) of its respective resonant circuit (Rei, .. Re _{i +} i).