FR3036231A1 - RESONATOR AND FREQUENCY TUNABLE FILTER COMPRISING AT LEAST ONE RESONATOR. - Google Patents

Abstract

Resonator (7) with two access ports (2, 3) and a loop comprising: - a first half-loop comprising two quarter-wave lines (4, 5) separated by a controlled element (6) to variable capacity; and a second half-loop comprising a half-wave line (9) forming an overcoupling circuit (8).

Description

Resonator and frequency tunable filter comprising at least one resonator The invention relates to a resonator and a frequency tunable filter comprising at least one resonator. The present invention therefore applies to any microwave equipment incorporating one or more planar filters, such as a receiver or a frequency converter. There is no effective tunable filter solution to date, so it is necessary to use different versions of the same equipment to cover all frequency plans. This makes complex the industrial management of such products because it is necessary to have multiple manufacturing records. In any frequency converter, for example from 6 GHz to 4 GHz or from 14 GHz to 12 GHz, a conversion output filtering function is necessary in order to eliminate the spurious responses produced by the conversion element. In the majority of cases, the output frequency bandwidth is subband cut for a better efficiency of the filtering function, each subband then imposing a different physical filter. In a passive and electrically tunable bandpass filter, the maintenance of useful bandwidth remains a problem for the following reasons: the useful band decreases by sub-coupling when the frequency decreases, the useful band decreases increases by over-coupling when the frequency increases; the poor quality factor of the tuning active elements affects the useful band of the filter when the frequency decreases, by sub-coupling. These two combined reasons make up to date such an unusable device and therefore inapplicable to most needs of frequency flexibility of the converters. FIG. 1 represents a resonator 1 of the state of the art, comprising two access ports 2 and 3, and two line sections 4 and 5, connecting the two access ports 2 and 3, separated by a controlled element 6 with variable capacity. A line section is a section of resonant line.

In the microwave domain, it is known to use the properties of the resonant lines to perform a filtering function. The basic resonator, as schematically represented in FIG. 1, illustrates a filter of order 1 (1 resonator).

Figure 2 illustrates the principle of an order 1 (1 resonator) filter initially set at 16.5 GHz and moved up to 13.5 GHz by varying the capacitance. This filter frequency shift is illustrated by the three curves A1, A2 and A3. This frequency shift of the filter is accompanied by a decrease in the bandwidth of the resonator, mainly related to the variation of the impedance as a function of the frequency inducing a sub-coupling effect towards the low frequencies. Undercoupling is the inability or inefficiency to transfer electrical or magnetic energy from one segment of a circuit to another. The same problem arises for a higher order filter realized by putting in series a plurality of resonators according to FIG. 1. By generalizing to a higher order filtering function, the same problem is strictly observed, namely a strong one. decrease of the bandwidth of the filter towards the low frequencies. To maintain a constant relative bandwidth, the coupling coefficient between adjacent resonators must be inversely proportional to the operating frequency of the filter. FIG. 3 illustrates the response obtained for a second-order filtering function by varying the capacitances of the variable-capacity controlled elements of the circuit comprising two resonators of FIG. 1. The useful bandwidth of the filter decreases sharply. when the filtering frequency 5 decreases, as illustrated by the three curves B1, B2 and B3. An object of the invention is to overcome the problems mentioned above. According to one aspect of the invention, there is provided a resonator provided with two access ports and a loop comprising: a first half-loop comprising two quarter-wave lines separated by a variable capacity controlled element; and a second half-loop comprising a half-wave line forming an overcoupling circuit. Such a resonator makes it possible to maintain a useful band or constant filter bandwidth when the filtering frequency varies. By overcoupling is meant the property to excessively transfer electrical or magnetic energy from one segment of a circuit to another. In one embodiment, the variable capacity controlled element comprises two variable capacity diodes mounted upside down.

Such an embodiment of the variable capacity controlled element allows an excitation of the device by injecting the DC voltage control at its midpoint without affecting the properties of the main circuit.

It is also proposed, in another aspect of the invention, a frequency tunable filter comprising a single resonator as previously described.

Such a frequency-tunable filter comprising a single resonator remains an effective and simple device to implement when the useful bandwidth to be treated is low or reduced to a single frequency.

It is also proposed, according to another aspect of the invention, a frequency tuneable filter comprising at least two resonators as described above, connected in series, so that two consecutive resonators are connected by two associated access ports respectively. and a coupling element.

Such a tunable frequency filter comprising at least two resonators allows for better attenuated band filtering efficiency while maintaining the necessary bandwidth characteristics. For example, the frequency tunable filter comprises five resonators. Such an embodiment makes it possible to widen the useful bandwidth while preserving the attenuated band filter characteristics.

In one embodiment, the coupling element is capacitive or inductive. According to one embodiment, the capacitive or inductive coupling element comprises a section of line of electrical length close to the quarter wave considered as an inverter or an impedance transformer. Thus, in a structure with several resonators, the action on the electrical length of each section allows alternating coupling types while keeping the anodes of the diodes at the same electrical potential. The invention will be better understood from the study of some embodiments described by way of non-limiting examples and illustrated by the appended drawings, in which: FIGS. 1 to 3 show schematically a resonator and variations of gains of such a resonator, as well as gain variations of a filter comprising two such resonators, according to the state of the art; FIGS. 4 and 4b schematically illustrate a resonator according to one aspect of the invention; FIG. 5 schematically represents gain variations of a filter comprising two resonators of FIG. 4b, according to one aspect of the invention; FIG. 6 shows a filter comprising 5 resonators of FIG. 4b, according to one aspect of the invention. In the various figures, the elements having identical references are identical.

Figure 4a schematically shows a resonator according to one aspect of the invention. FIG. 4a shows a resonator 7 according to one aspect of the invention, provided with a first half-loop comprising two access ports 2 and 3, and two line sections 4 and 5, connecting the two access ports. 2 and 3, separated by a controlled element 6 with variable capacity. The resonator 7 also comprises a second half-loop comprising an overcoupling circuit 8, produced by a half-wave line 9. The overcoupling circuit 8 then makes it possible to compensate for the sub-coupling effect observed when the frequency central filter decreases. Figure 4b schematically shows a particularly advantageous embodiment of the resonator of Figure 4a. The variable capacity controlled element 6 is thus produced by means of two variable capacity diodes 10 and 11 mounted upside down. The two diodes 10 and 11 are controlled by a variable resistor 12 controlled by a control port 13.

Figure 5 shows the response of an ideal filter to two resonators 7 of Figure 4b. It is clear from FIG. 5 that the invention makes it possible to keep the useful band or bandwidth of the filter constant when the filtering frequency is modified by control, as illustrated by the three curves Ci, O2 and C3. FIG. 6 shows a particularly relevant embodiment of a filter 15 comprising five resonators 7 of FIG. 4b connected together by coupling elements 14, in this case inductive, of the quarter-wave line section type. Thus, in a structure with several resonators 7, the action on the electrical length of each section allows alternating coupling types while maintaining at the same electrical potential the anodes of the diodes 10, 11.

Claims (7)

REVENDICATIONS1. Resonator (7) with two access ports (2, 3) and a loop comprising: - a first half-loop comprising two quarter-wave lines (4, 5) separated by a controlled element (6) to variable capacity; and a second half-loop comprising a half-wave line (9) forming an over-coupling circuit (8). 2. Resonator (7) according to claim 1, wherein the variable capacity controlled element (6) comprises two variable capacity diodes (10, 11) mounted upside down. 3. Frequency tunable filter comprising a single resonator (7) according to claim 1 or 2. 20 4. Frequency tunable filter (15) comprising at least two resonators (7) according to claim 1 or 2, connected in series, so that two consecutive resonators (7) are connected by two access ports (2, 3) respectively associated and a coupling element (14). 25 The frequency tunable filter (15) of claim 4, wherein the coupling element (14) is capacitive or inductive. 30 The frequency tunable filter (15) of claim 5, wherein the capacitive or inductive coupling element (14) comprises a quarter-wave length line section. 35 7. frequency tunable filter (15) according to one of claims 4 to 6, comprising five resonators.