ADAPTIVE RADIO FREQUENCY FILTER WITH IMPROVED LINEARITY

Description

[0001] The invention relates to the field of signal processing, particularly for radio communications and radar systems.

[0002] The invention relates more particularly to adaptive radio frequency filters, also known as agile filters or reconfigurable filters. Adaptive filters are capable of being reconfigured to change certain properties in real time, such as bandwidth and cutoff frequency, according to requirements.

[0003] Generally, adaptive filters are constructed either from passive filters or from active filters.

[0004] Passive filters contain passive inductors, the value of which is fixed. The inductors themselves cannot be changed, but the filter's performance and properties can be modified in real time by adjusting variable capacitors associated with the inductors.

[0005] However, this solution has some drawbacks.

[0006] On the one hand, passive filters are quite bulky due to the size of the passive inductors. Indeed, passive filters are generally only efficient over a relatively narrow frequency band. To create a passive filter capable of operating over a wide frequency band, it is often necessary during the design phase to duplicate entire sections of the filter, each optimized for a specific frequency band. The user then switches between these sections to select the one that best suits their needs. It is clear that this duplication of entire filter sections increases both the size and the complexity of the filter's manufacturing.

[0007] On the other hand, for practical reasons, passive filters are generally manufactured using silicon integration technologies. However, passive inductors produced using these technologies often exhibit a low quality factor, which contributes to significant insertion losses.

[0008] Active filters, on the other hand, include variable active inductances, that is to say, active electronic circuits simulating the behavior of inductances, these circuits being able to include elements such as gyrators and / or amplifiers.

[0009] US patent 6184747 B1 describes an example of an active filter incorporating a gyrator.

[0010] The CN 103 905 012 document describes a conductance combining a variable active inductance and a fixed passive inductance.

[0011] The document "Active tunable inductor using non-Foster element" by E. Vorobev et al describes a variable active inductance L neg connected to a fixed passive inductance L pos in order to make a circuit with a negative resistance inductance.

[0012] However, the linearity performance of active filters, for example quantified by the third-order intercept point (IIP3), is generally insufficient for many applications. Furthermore, the power consumption of variable active inductors is typically high, which can be a prohibitive factor in certain applications.

[0013] It is these drawbacks that the invention intends to remedy in particular by proposing an improved adaptive radio frequency filter.

[0014] To this end, one aspect of the invention relates to an adaptive radio frequency filter according to claim 1.

[0015] According to advantageous but not mandatory aspects, such a radio frequency filter is according to any one of claims 2 to 10.

[0016] The invention will be better understood and other advantages thereof will become more apparent in the light of the following description of an embodiment of an adaptive radio frequency filter, given solely by way of example and with reference to the accompanying drawings, in which: there figure 1 represents an example of a radio frequency filter; the figure 2 schematically represents a variable composite inductance that is part of the adaptive radio frequency filter of the figure 1 and comprising a fixed inductance and a variable inductance; the figure 3 represents an adaptive radio frequency filter according to a first embodiment of the invention; the figure 4 represents an adaptive radio frequency filter according to a second embodiment of the invention; the figure 5 illustrates the performance of an adaptive radio frequency filter according to the invention.

[0017] There figure 1 This represents an example of an adaptive radio frequency filter 2 with an input (IN) for receiving a radio frequency signal and an output (OUT). For example, the input (IN) has one or more input terminals. The output (OUT) has one or more output terminals.

[0018] The adaptive radio frequency filter 2 is an electronic circuit comprising a plurality of electronic components, including inductors.

[0019] One aspect of the invention aims in particular to provide an adaptive radio frequency filter comprising one or more fixed passive inductances combined with one or more variable active inductances.

[0020] For this, we advantageously use at least one variable composite inductance comprising a fixed inductance and a variable inductance.

[0021] Combining the two types of inductances allows for better linearity in the same filter than using only active inductances, while reducing the size and insertion losses associated with passive inductances.

[0022] This also allows for finer adjustment of the equivalent inductance and thus provides greater agility, for example, greater ease in reconfiguring the filter.

[0023] For example, the figure 2 represents a variable composite inductance 4 (or hybrid inductance) comprising a fixed passive inductance 6 and a variable active inductance 8, both connected in parallel.

[0024] The fixed passive inductance 6 is, for example, a passive component such as a coil.

[0025] Assembly 4 is, for example, a portion of an adaptive radio frequency filter 2.

[0026] Alternatively, other ways of coupling a fixed passive inductance 6 with a variable active inductance 8 are possible. For example, a variable composite inductance can be constructed in which the fixed passive inductance 6 is connected in series with the variable active inductance 8.

[0027] In practice, to form an adaptive radio frequency filter 2, several variable composite inductances 4 can be formed, each comprising one or more fixed passive inductances 6 coupled (in series, or in parallel, for example) with one or more variable active inductances 8, these variable composite inductances 4 then being connected together in the manner of modules.

[0028] Optionally, as illustrated on the figure 2 The variable composite inductance 4 can be associated with a negative resistance 9, which will be described in more detail below. For example, the negative resistance is connected in parallel with the variable active inductance 8; other ways of associating the negative resistance 9 are possible as alternatives.

[0029] There figure 3 represents an adaptive radio frequency filter 10 according to a first embodiment of the invention.

[0030] In this embodiment, the filter 10 comprises several sets of components, called interstages, including a first interstage 12, a second interstage 14, a third interstage 15, a fourth interstage 16 and a fifth interstage 18. These interstages are, for example, connected in series between the input and output terminals of the filter 10. At least part of these sets of components includes a variable composite inductance 4.

[0031] The interstage 12 includes a variable capacitor 20, here connected in parallel with the input of the filter 10, and a fixed passive inductance 22 (analogous to the inductance 6), such as a coil, here connected in parallel with the variable capacitor 20.

[0032] In other embodiments, the fixed passive inductance 22 could be connected, in series or in parallel, to at least one variable active inductance to form a variable composite inductance.

[0033] The interstage 18 includes a variable capacitor 24, here connected in parallel with the output of the filter, and a fixed passive inductance 26 (analogous to inductance 6), here connected in parallel with the variable capacitor 24.

[0034] In other embodiments, the fixed passive inductance 26 could be connected, in series or in parallel, to at least one variable active inductance to form a variable composite inductance.

[0035] For example, the interstage 18 can be constructed similarly or symmetrically with respect to the interstage 12, although other embodiments are possible as alternatives. However, the capacitance and inductance values ​​of the components of the interstage 18 are not necessarily identical to those of the interstage 12.

[0036] The interstage 14 includes at least one variable active inductance, and preferably two variable active inductances 8, here included in the module 28, the variable active inductances of the module 28 being here connected in series with the interstage 12.

[0037] For example, in differential configuration, block 28 is equivalent to two variable inductances 8 of the same value: one connected between the positive input and the positive output and the other between the negative input and the negative output of block 28.

[0038] The interstage 14 also includes a first variable capacitor 30, a first fixed passive inductance 32, a second fixed passive inductance 34 and a second variable capacitor 36.

[0039] For example, each of these elements 30, 32, 34 and 36 is connected in parallel with variable active inductances 28.

[0040] In this example, a first variable composite inductance of the interstage 14 is formed by the association of the first fixed passive inductance 32 and one of the variable active inductances of the module 28. A second variable composite inductance of the interstage 14 is formed by the association of the second fixed passive inductance 34 and the other of the variable active inductances of the module 28. The two variable composite inductances are here of the same value.

[0041] Combining a variable composite inductance with a variable capacitor allows for the formation of a variable LC resonant circuit.

[0042] Interstage 14 is connected in series with interstage 16, here via interstage 15, the latter comprising a variable capacitor 40 and a fixed passive inductance, connected in parallel with each other.

[0043] The interstage 16 includes at least one variable active inductance 8 and preferably two variable active inductances 8, here included in a module 48 (analogous to module 28), the variable active inductances of module 48 being here connected in series between the interstage 15 and the interstage 18.

[0044] The interstage 16 also includes a first variable capacitor 50, a first fixed passive inductance 52, a second fixed passive inductance 54 and a second variable capacitor 56.

[0045] For example, each of these elements 50, 52, 54 and 58 is connected in parallel with the variable active inductance 28. For example, the interstage 16, which has a differential configuration, is constructed in an analogous or symmetrical or identical manner to the interstage 14, although other embodiments are possible as alternatives.

[0046] In this example, a first variable composite inductance of the interstage 16 is formed by the association of the first fixed passive inductance 52 and one of the variable active inductances of the module 48. A second variable composite inductance of the interstage 16 is formed by the association of the second fixed passive inductance 54 and the other of the variable active inductances of the module 48.

[0047] Although this is not illustrated in detail on the figure 2 , the filter 10 may also include one or more negative resistances 9, this or these negative resistances 9 being preferably associated with at least one of the variable active inductances, or even with each of the variable active inductances.

[0048] Filter 10 also includes a power supply system, which is not shown on the figure 3 , and which is configured, for example, to electrically power active components of filter 10, such as variable capacitors and active inductors.

[0049] The filter 10 may also include an electronic control system configured to modify at least one filter operating parameter, such as the cutoff frequency, bandwidth, or gain. This modification is achieved, for example, by controlling one or more of the variable elements of the filter 10, such as variable capacitors and / or variable active inductors.

[0050] It will be understood that this example of filter 10 is given for illustrative purposes and that, alternatively, many other filters can be constructed in a similar way, for example by combining sets of components (interstages) in different ways according to the desired properties, at least part of these sets of components preferably including a variable composite inductance 4.

[0051] There figure 4 represents an adaptive radio frequency filter 10' according to a second embodiment of the invention.

[0052] The radio frequency filter 10' is largely identical to the radio frequency filter 10, except that the radio frequency filter 10' has two additional variable active inductances 70 and 72 placed respectively in the input stage 12 and in the output stage 18, for example by being connected in parallel with the fixed passive inductance 22 or 26.

[0053] In practice, the additional variable active inductances 70 and 72 do not necessarily have the same values ​​and can be controlled independently, although they may be identical in some examples.

[0054] For example, each of the additional variable active inductances 70 and 72 includes an adjustable variable inductance 8 as previously described.

[0055] In this example, a variable composite inductance of the interstage 12 is formed by combining the fixed passive inductance 22 and an additional variable active inductance (one of the inductances of module 70). Similarly, a variable composite inductance of the interstage 18 is formed by combining the fixed passive inductance 26 and an additional variable active inductance (for example, one of the inductances of module 72).

[0056] Apart from these differences, everything described with reference to filter 10 of the figure 3 also applies to the 10' radio frequency filter in this second embodiment.

[0057] Many other variations of the 10 or 10' filter can be constructed by combining fixed passive inductors, variable capacitors, and variable active inductors. In these variations, the number and arrangement of the various filter components may differ from those in the examples shown above.

[0058] In many embodiments, the variable capacitors 30, 36, 50, and 56 are implemented using variable capacitance diodes (e.g., varactor diodes). Alternatively, capacitor banks can be used, comprising several capacitors that can be selectively connected or disconnected by means of controllable switches, such as transistors used as switches.

[0059] The equivalent capacitance values ​​of these variable capacitors are selected according to the target frequency band for filter operation. This selection can be made in software, for example by an electronic control system for the filter 10, in order to control the commands by means of a control signal.

[0060] For example, in the case of a capacitor bank, the switches can be controlled digitally with a control voltage applied to a control electrode of the switches (for example, on the gate of a field-effect transistor).

[0061] In many embodiments, each adjustable active inductance 8, 28, 48, 70, 72 includes an active electronic circuit simulating the behavior of an inductance.

[0062] This electronic circuit may, for example, include one or more gyrators, and / or amplifiers, or any other suitable element.

[0063] Preferably, each adjustable active inductance 8, 28, 48, 70, 72 comprises several gyrators connected in series and / or in parallel, with a variable capacitor associated with at least some of the gyrators, preferably with each gyrator.

[0064] For example, gyrators can be constructed from transconductance amplifiers, also known as transconductance cells.

[0065] Each transconductance cell is associated with a variable capacitor C out placed at the output, such that the value of the variable inductance (Lvar) is a function of the transconductance (gm) and the capacitance (C out) of the corresponding capacitor.

[0066] For example, each transconductance cell is a four-terminal element that has two input terminals (e.g., a positive terminal and a negative terminal) and two output terminals (e.g., a positive terminal and a negative terminal).

[0067] In a first example of construction, we can form a variable active inductance 8 by constructing a gyrator by associating two transconductance cells connected in parallel between input terminals of the active inductance circuit and output terminals of the active inductance circuit, each of these gyrators.

[0068] For example, in each of the gyrators, the positive terminal of the output of the second transconductance cell is connected to the negative terminal of the input of the first transconductance cell, and the negative terminal of the output of the second transconductance cell is connected to the positive terminal of the input of the first transconductance cell. The negative terminal of the output of the first transconductance cell is connected to the negative terminal of the input of the second transconductance cell, and the positive terminal of the output of the first transconductance cell is connected to the positive terminal of the input of the second transconductance cell.

[0069] A variable capacitor is connected between the first and second output terminals of each transconductance cell. In some embodiments, these capacitors can be directly integrated into each transconductance cell, or they can be connected to the output of each transconductance cell.

[0070] In order to make the active inductance variable, each of these capacitors C out is configured so as to be variable and adjustable (i.e. the capacitance value can be changed by a user), for example by being electrically controlled by an electronic control system of the filter 10. To this end, each variable capacitor can be made as previously described, for example by having voltage-controllable variable capacitance diodes, or by any suitable means.

[0071] In a second construction example, a variable active inductance 8 can be formed by connecting two gyrators in series, between input terminals of the active inductance circuit and output terminals of the active inductance circuit, each of these gyrators comprising the association of two transconductance cells connected in parallel with each other, as explained previously.

[0072] Here again, a variable capacitor is connected between the first and second output terminals of each transconductance cell. In some embodiments, these capacitors can be directly integrated into the transconductance cell, or alternatively, connected to the output of each transconductance cell.

[0073] Many other implementation methods are possible.

[0074] Preferably, each transconductance cell includes an adjustable negative resistance, or negative impedance converter, which serves to compensate for filter losses.

[0075] For example, the equivalent value of a negative resistance, such as negative resistance 9, can be controlled by placing an adjustable positive resistance in parallel with the negative resistance. In such an example, the adjustable positive resistance can be constructed from a field-effect transistor (such as a MOSFET) operating as a triode. Connected in this way, the transistor functions as a resistor, and its equivalent resistance value can be adjusted by varying the gate voltage applied to the transistor. Other implementations are also possible.

[0076] Negative resistances are known and are not described in further detail. An example of a negative resistance is described in the article by C. Andriesei et al., "Negative resistance based tuning of an RF bandpass filter," Proceedings of the 4th European Conference on Circuits and Systems for Communications, ECCSC, July 2008, Bucharest, Romania, DOI: 10.1109 / ECCSC.2008.4611651. This example is not exhaustive, and other implementations are possible.

[0077] Thanks to the invention, by combining one or more fixed passive inductances with one or more variable active inductances with one or more negative resistances 9, it is possible to construct an adaptive radio frequency filter exhibiting better linearity than by using only active inductances, while reducing the size and insertion losses associated with the use of only passive inductances.

[0078] Variable components in the radio frequency filter, such as active variable inductors and variable capacitors, allow the filter to operate from one frequency band to another without requiring the duplication of entire sections of the circuit to achieve satisfactory performance over wide frequency ranges. As a result, the radio frequency filter is less bulky and less complex to design than conventional passive radio frequency filters.

[0079] Unlike known passive inductance solutions, insertion losses can be advantageously reduced and controlled by means of negative resistances integrated into variable active inductances.

[0080] There figure 5 highlights the performance of filter 10 of the figure 3 , in particular to illustrate the improvement in linearity performance of filter 10, by means of intermodulation distortion measurements carried out on filter 10.

[0081] On graph 80, the x-axis corresponds to the input power of test signals supplied to filter 10, while the y-axis corresponds to the output power of the measured signals.

[0082] The first curve 82 corresponds to the fundamental frequency of the test signals, while the second curve 84 corresponds to the third-order intermodulation product. The frequencies of the test signals are, in this example, equal to 9.75 GHz ("1st< tone") and 9.74 GHz ("2nd< tone"), respectively.

[0083] In these examples, the filter supply voltage is equal to 1.2V, the amplitude of the filter supply current is equal to 80mA, and the third-order intercept point (IIP3) at the input, extrapolated from curves 82 and 84, is equal to +6dBm.

[0084] Other tests, not illustrated, carried out for other supply voltages and for other frequencies, have shown that linearity performance can be further improved, for example to obtain a third-order intercept point (IIP3) of up to more than +20dBm.

[0085] These linearity and power consumption performances are better than those of conventional adaptive filters, such as filters built solely from active inductors.

[0086] Similar tests carried out on adaptive filters having a similar function but constructed solely from gyrator-based active inductors have shown that these active filters exhibit, by comparison, a third-order intercept point of -10dBm and a power consumption of 160mA for the whole filter for a supply voltage of 1.2V, which is double the consumption of filter 10 in the example illustrated above.

[0087] Thus, judiciously combining active and passive inductances to form an adaptive filter provides better performance, particularly in terms of linearity and power consumption, while reducing the complexity of the filter design.

Claims

1. An adaptive radio frequency filter (10; 10') comprising an input (IN), an output (OUT), at least one variable composite inductor (4) including a fixed passive inductor (6, 22) and at least one variable active inductor (8), the variable composite inductor (4) being connected between the input and the output of the radio frequency filter; and said at least one variable active inductor (8, 28, 48, 70) includes a gyrator and a variable capacitor (Cout) connected to the output of the gyrator.

2. The radio frequency filter (10; 10') according to claim 1, wherein the filter further includes a variable capacitor associated with said at least one variable active inductor (8).

3. The radio frequency filter (10; 10') according to claim 1 or 2, wherein the filter further includes a negative resistor (9) associated with said at least one variable active inductor.

4. The radio frequency filter (10; 10') according to any of the preceding claims, wherein the filter further includes a first inter-stage (12) connected to the filter input, the first inter-stage (12) including the fixed passive inductor (22) connected in series or in parallel with the variable active inductor so as to form the variable composite inductor.

5. The radio frequency filter (10; 10') according to claim 4, wherein the filter further includes another inter-stage (18) connected to the filter output, said inter-stage (18) including another fixed passive inductor (26) connected in series or in parallel with another variable active inductor so as to form another variable composite inductor.

6. The radio frequency filter (10') according to claim 4 or 5, wherein a variable active inductor (70, 72) is connected in parallel with the fixed passive inductor (22, 26) so as to form a variable composite inductor.

7. The radio frequency filter (10; 10') according to any of the preceding claims, wherein the filter further includes a variable active inductor (28, 48) connected between the input and the output of the filter, as well as a first variable capacitor (30, 50), a first fixed passive inductor (32, 52), a second fixed passive inductor (34, 54) and a second variable capacitor (36, 56) respectively connected in parallel with the variable active inductor (28, 48).

8. The radio frequency filter (10; 10') according to any of the preceding claims, wherein the gyrator is built from transconductance amplifiers (gm).

9. The radio frequency filter (10; 10') according to claim 8, wherein each transconductance amplifier (gm) has an adjustable negative resistance for compensating the losses of the radio frequency filter (10; 10').

10. The radio frequency filter (10; 10') according to any of the preceding claims, wherein the filter further includes an electronic control system configured for changing at least one operating parameter of the radio frequency filter, such as the cut-off frequency, or the bandwidth, or the gain.