Adaptive radio frequency filter with improved linearity

Abstract

Adaptive radio frequency filter exhibiting improved linearity. An adaptive radio frequency filter (10) comprises an input, an output, at least one fixed passive inductor (22, 26, 32, 34, 50, 54) and at least one variable active inductor (28, 48) connected between the input and the output. Figure for the abbreviation: Figure 3

Description

Title of the invention: Adaptive radiofrequency filter with improved linearity

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

[0002] The invention relates more particularly to adaptive radiofrequency filters, also called agile filters, or reconfigurable filters. The adaptive filters are capable of being reconfigured to change in real time certain of their properties, such as the bandwidth and the cut-off frequency, according to needs.

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

[0004] Passive filters have passive inductors, the value of which is fixed. The inductors cannot therefore be changed, but the performance and properties of the filter can be changed in real time by adjusting variable capacitors that are associated with the inductors.

[0005] This solution nevertheless has drawbacks.

[0006] On the one hand, passive filters are very bulky, due to the size of the passive inductances. In fact, passive filters are generally only efficient over a relatively narrow frequency band. To have a passive filter capable of being used over a wide frequency band, it is often necessary during design to duplicate entire parts of the filter, each of these filter parts being optimized over a given frequency band, the filter then having to be switched by the user to select the filter part corresponding to the needs. It is understood that this duplication of entire parts of the filter increases the size and the manufacturing complexity of the filter.

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

[0008] Active filters comprise variable active inductances, i.e. active electronic circuits simulating the behavior of inductances, these circuits being able to comprise elements such as gyrators and / or amplifiers.

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

[0010] However, the linearity performance of active filters, for example quantified by the third-order intercept point (IIP3), are generally not sufficient for many applications. In addition, the power consumption of variable active inductors is generally high, which can be prohibitive in certain applications.

[0011] It is these drawbacks that the invention more particularly intends to remedy by proposing an improved adaptive radiofrequency filter.

[0012] To this end, one aspect of the invention relates to an adaptive radiofrequency filter comprising an input, an output, at least one variable composite inductance comprising at least one fixed passive inductance and at least one variable active inductance, the variable composite inductance being connected between the input and the output of the radiofrequency filter.

[0013] According to advantageous but not obligatory aspects, such a radiofrequency filter may incorporate one or more of the following characteristics, taken in isolation or in any technically admissible combination:

[0014] - the radiofrequency filter further comprises a variable capacitor associated with said at least one variable active inductance.

[0015] - the radiofrequency filter may further comprise a negative resistance associated with said at least one variable active inductance.

[0016] - the radiofrequency filter comprises a first inter-stage connected to the input of the filter, the first inter-stage comprising a fixed passive inductor connected in series or in parallel with a variable active inductor to form a variable composite inductor.

[0017] - the radiofrequency filter has another inter-stage connected to the output of the filter, said interstage comprising a fixed passive inductance connected in series or in parallel with a variable active inductance to form a variable composite inductance.

[0018] - a variable active inductance is connected in parallel with the passive inductance fixed to form a variable composite inductance.

[0019] - the radio frequency filter comprises a variable active inductance connected between the input and output of the filter, as well as a first variable capacitor, a first fixed passive inductor, a second fixed passive inductor and a second variable capacitor respectively connected in parallel with the variable active inductor.

[0020] - said at least one variable active inductance comprises a gyrator and a variable capacitor connected to the output of the gyrator.

[0021] - the gyrator is constructed from transconductance amplifiers.

[0022] - each transconductance amplifier may include a negative resistance adjustable to compensate for radio frequency filter losses.

[0023] - the filter comprises an electronic control system configured to modify at least one operating parameter of the radio frequency filter, such as the cutoff frequency, or the bandwidth, or the gain.

[0024] The invention will be better understood and other advantages thereof will appear more clearly in the light of the following description of an embodiment of an adaptive radiofrequency filter, given solely by way of example and with reference to the appended drawings, in which:

[0025] [Fig-1] [Fig.l] represents an example of a radio frequency filter;

[0026] [Fig.2] [Fig.2] schematically represents a variable composite inductance forming part of the adaptive radio frequency filter of [Fig.l] and comprising a fixed inductance and a variable inductance;

[0027] [Fig.3] [Fig.3] represents an adaptive radiofrequency filter according to a first mode of carrying out the invention;

[0028] [Fig.4] [Fig.4] represents an adaptive radiofrequency filter according to a second mode of carrying out the invention;

[0029] [Fig.5] [Fig.5] illustrates the performance of an adaptive radio frequency filter in accordance with the invention.

[0030] [Fig.l] represents an example of an adaptive radiofrequency filter 2 comprising an input IN for receiving a radiofrequency signal and an output OUT. For example, the input IN comprises one or more input terminals. The output OUT comprises one or more output terminals.

[0031] The adaptive radiofrequency filter 2 is an electronic circuit comprising a plurality of electronic components, in particular inductances.

[0032] One aspect of the invention aims in particular to propose an adaptive radiofrequency filter comprising one or more fixed passive inductances combined with one or more variable active inductances.

[0033] For this, at least one variable composite inductance comprising a fixed inductance and a variable inductance is advantageously used.

[0034] The combination of the two types of inductances makes it possible to benefit, in the same filter, from better linearity than by using only active inductances, while reducing the size and insertion losses linked to passive inductances.

[0035] This also makes it possible to adjust the equivalent inductance more finely and thus to benefit from greater agility, for example greater ease in reconfiguring the filter.

[0036] By way of illustration, [Fig.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.

[0037] The fixed passive inductance 6 is, for example, a passive component such as a coil.

[0038] The assembly 4 is, for example, a portion of an adaptive radiofrequency filter 2.

[0039] Alternatively, other ways of coupling a fixed passive inductor 6 with a variable active inductor 8 are possible. For example, a variable composite inductor can be constructed in which the fixed passive inductor 6 is connected in series with the variable active inductor 8.

[0040] In practice, to form an adaptive radiofrequency 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 to each other in the manner of modules.

[0041] Optionally, as illustrated in [Fig.2], the variable composite inductor 4 may be associated with a negative resistor 9, which will be described in more detail in the following. For example, the negative resistor is connected in parallel with the variable active inductor 8, other ways of associating the negative resistor 9 being possible alternatively.

[0042] [Fig. 3] represents an adaptive radiofrequency filter 10 according to a first embodiment of the invention.

[0043] In this embodiment, the filter 10 comprises several sets of components, called inter-stages, including a first inter-stage 12, a second inter-stage 14, a third inter-stage 15, a fourth inter-stage 16 and a fifth inter-stage 18. These inter-stages are for example connected in series between the input and output terminals of the filter 10. At least part of these sets of components comprises a variable composite inductance 4.

[0044] The inter-stage 12 comprises 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.

[0045] In other embodiments, the fixed passive inductor 22 could be connected, in series or in parallel, to at least one variable active inductor to form a variable composite inductor.

[0046] The inter-stage 18 comprises a variable capacitor 24, here connected in parallel with the output of the filter, and a fixed passive inductance 26 (analogous to the inductance 6), here connected in parallel with the variable capacitor 24.

[0047] In other embodiments, the fixed passive inductor 26 could be connected, in series or in parallel, to at least one variable active inductor to form a variable composite inductor.

[0048] For example, the inter-stage 18 may be constructed similarly or symmetrically with respect to the inter-stage 12, although other embodiments are nevertheless possible as variants. However, the capacitance and inductance values ​​of the components of the inter-stage 18 are not necessarily identical to those of the inter-stage 12.

[0049] The inter-stage 14 comprises 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 inter-stage 12.

[0050] For example, in differential configuration, block 28 is equivalent to two variable inductors 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.

[0051] The inter-stage 14 also comprises a first variable capacitor 30, a first fixed passive inductance 32, a second fixed passive inductance 34 and a second variable capacitor 36.

[0052] For example, each of these elements 30, 32, 34 and 36 is connected in parallel with the variable active inductances 28.

[0053] In this example, a first variable composite inductance of the inter-stage 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 inter-stage 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.

[0054] The association of a variable composite inductor with a variable capacitor makes it possible to form a variable LC resonant circuit.

[0055] The inter-stage 14 is connected in series with the inter-stage 16, here via the inter-stage 15, the latter comprising a variable capacitor 40 and a fixed passive inductance, connected in parallel with each other.

[0056] The inter-stage 16 comprises at least one variable active inductance 8 and preferably two variable active inductances 8, here included in a module 48 (analogous to the module 28), the variable active inductances of the module 48 being here connected in series between the inter-stage 15 and the inter-stage 18.

[0057] The inter-stage 16 also comprises a first variable capacitor 50, a first fixed passive inductance 52, a second fixed passive inductance 54 and a second variable capacitor 56.

[0058] For example, each of these elements 50, 52, 54 and 58 is connected in parallel with the variable active inductance 28. For example, the inter-stage 16, which has a differential configuration, is constructed in a manner analogous or symmetrical or identical to the inter-stage 14, other embodiments being however possible as a variant.

[0059] In this example, a first variable composite inductance of the inter-stage 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 inter-stage 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.

[0060] Although this is not illustrated in detail in [Fig.2], the filter 10 may also comprise one or more negative resistors 9, this or these negative resistors 9 preferably being associated with at least one of the variable active inductances, or even with each of the variable active inductances.

[0061] The filter 10 also comprises an electrical power supply system, which is not shown in [Fig. 3], and which is configured, for example, to electrically supply active components of the filter 10, such as the variable capacitors and the active inductors.

[0062] The filter 10 may also comprise an electronic control system configured to modify at least one operating parameter of the filter, such as the cutoff frequency, or the bandwidth, or the gain. This modification is for example carried out by controlling one or more of the variable elements of the filter 10, such as the variable capacitors and / or the variable active inductors.

[0063] It will be understood that this example filter 10 is given for illustrative purposes and that alternatively, many other filters can be constructed in a similar manner, for example by combining sets of components (inter-stage) differently according to the desired properties, at least a part of these sets of components preferably comprising a variable composite inductance 4.

[0064] [Fig.4] represents an adaptive radiofrequency filter 10' according to a second embodiment of the invention.

[0065] The radio frequency filter 10' is largely identical to the radio frequency filter 10, except that the radio frequency filter 10' comprises 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.

[0066] 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 certain examples.

[0067] For example, each of the additional variable active inductors 70 and 72 comprises an adjustable variable inductor 8 as previously described.

[0068] In this example, a variable composite inductance of the inter-stage 12 is formed by the association of the fixed passive inductance 22 and an additional variable active inductance (one of the inductances of the module 70). Similarly, a variable composite inductance of the inter-stage 18 is formed by the association of the fixed passive inductance 26 and an additional variable active inductance (for example one of the inductances of the module 72).

[0069] Apart from these differences, everything that has been described with reference to the filter 10 of [Fig. 3] also applies to the radiofrequency filter 10' in this second embodiment. lization.

[0070] Many other variants of the 10 or 10' filter can be constructed by combining fixed passive inductors, variable capacitors and variable active inductors. In these variants, the number and arrangement of the various constituent elements of the filter may be different from those of the examples presented above.

[0071] In many embodiments, the variable capacitors 30, 36, 50, and 56 are formed using variable-capacitance diodes (e.g., varactor diodes). Capacitor banks may also be used having multiple capacitors capable of being selectively connected or disconnected by means of controllable switches, such as transistors used as switches.

[0072] The equivalent capacitance values ​​of these variable capacitors are selected according to the frequency band targeted for the operation of the filter. This selection can be done in software, for example by an electronic control system of the filter 10, in order to drive the controls by means of a control signal

[0073] 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).

[0074] In many embodiments, each adjustable active inductor 8, 28, 48, 70, 72 comprises an active electronic circuit simulating the behavior of an inductor.

[0075] This electronic circuit may, for example, comprise one or more gyrators, and / or amplifiers, or any other suitable element.

[0076] Preferably, each adjustable active inductance 8, 28, 48, 70, 72 comprises several gyrators connected in series and / or in parallel, a variable capacitor being associated with at least part of the gyrators, preferably with each gyrator.

[0077] For example, gyrators can be constructed from transconductance amplifiers, also called transconductance cells.

[0078] Each transconductance cell is associated with a variable capacitor Cout placed at the output, such that the value of the variable inductance (Lvar) is a function of the transconductance (gm) and the capacitance (Cout) of the corresponding capacitor.

[0079] For example, each transconductance cell is a quadrupole 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).

[0080] In a first construction example, a variable active inductance 8 can be formed by constructing a gyrator by associating two transconductance cells connected in parallel with each other between input terminals of the active inductance circuit and output terminals of the active inductance circuit, each of these gyrators.

[0081] 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.

[0082] A variable capacitor is connected between the first and second output terminals of each of the transconductance cells. According to embodiments, these capacitors may be directly integrated into each transconductance cell, or they may be connected to the output of each transconductance cell.

[0083] In order to make the active inductance variable, each of these capacitors Cout is configured so as to be variable and adjustable (i.e. the capacitance value can be modified 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 produced as previously described, for example by comprising voltage-controllable variable capacitance diodes, or by any appropriate means.

[0084] 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.

[0085] Again, a variable capacitor is connected between the first and second output terminals of each of the transconductance cells. According to embodiments, these capacitors may be directly integrated into the transconductance cell, or alternatively may be connected to the output of each transconductance cell.

[0086] Many other embodiments are possible.

[0087] Preferably, each transconductance cell includes an adjustable negative resistor, or negative impedance converter, which serves to compensate for filter losses.

[0088] For example, the equivalent value of the negative resistor, such as negative resistor 9, is controllable by placing an adjustable positive resistor in parallel with the negative resistor. In such an example, the adjustable positive resistor may be constructed from a field effect transistor (such as a MOSFET transistor) operating as a triode. Connected in this way, this 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 nevertheless possible as a variant.

[0089] Negative resistances are known and are not described in further detail. An example of 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 construction example is not limiting and other implementations are possible as variants.

[0090] Thanks to the invention, by combining one or more fixed passive inductances with one or more variable active inductances with one or more negative resistors 9, it is possible to construct an adaptive radiofrequency filter having better linearity than using only active inductances, while reducing the size and insertion losses linked to the use of only passive inductances.

[0091] The variable elements of the radio frequency filter, such as active variable inductors and variable capacitors, allow the radio frequency filter to switch from one frequency band to another, without the need to duplicate entire portions of the circuit to achieve satisfactory operation over wide frequency bands. Thus, the radio frequency filter is less bulky and less complex to design than known passive radio frequency filters.

[0092] Unlike known passive inductance solutions, insertion losses can advantageously be reduced and controlled thanks to the negative resistances integrated into the variable active inductances.

[0093] [Fig.5] highlights the performance of the filter 10 of [Fig.3], in particular to illustrate the improvement in the linearity performance of the filter 10, by means of intermodulation distortion measurements carried out on the filter 10.

[0094] On graph 80, the abscissa axis corresponds to the input power of test signals supplied to filter 10, while the ordinate axis corresponds to the output power of the measured signals.

[0095] 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.

[0096] 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 interception point (IIP3) at the input, extrapolated from curves 82 and 84, is equal to +6dBm.

[0097] Other tests, not illustrated, carried out for other supply voltages and for other frequencies, have shown that the linearity performances can be improved even further, for example to obtain a third-order intercept point (IIP3) of up to more than +20dBm.

[0098] These performances in linearity and in electrical consumption are better than those of classic adaptive filters, such as filters constructed solely from active inductances.

[0099] Similar tests carried out on adaptive filters having a similar function but constructed solely from active inductances based on gyrators have shown that these active filters have, by comparison, a third order intercept point equal to -10dBm and a power consumption of 160mA for the entire filter for a supply voltage of 1.2V, which is double the consumption of filter 10 in the example illustrated above.

[0100] Thus, judiciously combining active inductances 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.

[0101] Many other embodiments are possible.

[0102] Any feature of one of the embodiments or variants described above may be implemented in the other embodiments and variants described.

Claims

Claims

1. Adaptive radiofrequency filter (10; 10') comprising an input (IN), an output (OUT), at least one variable composite inductance (4) comprising a fixed passive inductance (6) and at least one variable active inductance (8), the variable composite inductance (4) being connected between the input and the output of the radiofrequency filter; and said at least one variable active inductance (8, 28, 48, 70) comprising a gyrator and a variable capacitor (Cout) connected to the output of the gyrator.

2. Radiofrequency filter (10; 10') according to claim 1, which further comprises a variable capacitor associated with said at least one variable active inductance (8).

3. Radio frequency filter (10; 10') according to claim 1 or claim 2, which further comprises a negative resistance (9) associated with said at least one variable active inductance.

4. A radio frequency filter (10; 10') according to any preceding claim, which comprises a first interstage (12) connected to the input of the filter, the first interstage (12) comprising a fixed passive inductance (22) connected in series or in parallel with a variable active inductance to form a variable composite inductance.

5. Radio frequency filter (10; 10') according to any one of the preceding claims, which comprises a further inter-stage (18) connected to the output of the filter, said inter-stage (18) comprising a fixed passive inductance (26) connected in series or in parallel with a variable active inductance to form a variable composite inductance.

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

7. Radio frequency filter (10; 10') according to any one of the preceding claims, which comprises a variable active inductance (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 inductance (32, 52), a second fixed passive inductance (34, 54) and a second variable capacitor (36, 56) respectively connected in parallel with the variable active inductance (28, 48).

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

9. Radio frequency filter (10; 10') according to claim 8, in which each transconductance amplifier (gm) comprises an adjustable negative resistance, to compensate for the losses of the radio frequency filter (10; 10').

10. Radio frequency filter (10; 10') according to any one of the preceding claims, which comprises an electronic control system configured to modify at least one operating parameter of the radio frequency filter, such as the cut-off frequency, or the bandwidth, or the gain.

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