Signal combiner

Abstract

Various embodiments provide a coaxial signal combiner. The signal combiner comprises a coaxial output branch having an output port for outputting a combined signal. The coaxial output branch comprises: a first inner conductor; a first outer conductor formed coaxially about the first inner conductor; and a first dielectric material separating the first inner conductor and the first outer conductor. The signal combiner comprises a coaxial radiofrequency branch having a radiofrequency input port for receiving a radiofrequency signal. The coaxial radiofrequency branch is electrically connected to the coaxial output branch to convey the radiofrequency signal thereto. The coaxial radiofrequency branch comprises: a second inner conductor connected to the first inner conductor; a second outer conductor connected to the first outer conductor and formed coaxially about the second inner conductor; and a second dielectric material separating the second inner conductor and the second outer conductor. The signal combiner comprises a coaxial microwave branch having a microwave input port for receiving a microwave frequency signal, the coaxial microwave branch being electrically connected to the coaxial output branch to convey the microwave frequency signal thereto. The coaxial microwave branch comprises: a third inner conductor; a third outer conductor formed coaxially about the third inner conductor; and a third dielectric material separating the third inner conductor and the third outer conductor. The coaxial microwave branch is connected to the coaxial output branch via first and second capacitive structures, the first capacitive structure formed by the first inner conductor and the third inner conductor, and the second capacitive structure formed by the first outer conductor and the third outer conductor.

Description

[0001] Signal Combiner

[0002] Field of the Invention

[0003] The present invention relates to signal combiners for combining signals onto a common signal line and particularly, although not exclusively, to signal combiners for combining signals of different frequencies onto a common signal line.

[0004] Background

[0005] Surgical resection is a means of removing sections of organs from within the human or animal body. Such organs may be highly vascular. When tissue is cut (divided or transected) small blood vessels called arterioles are damaged or ruptured. Initial bleeding is followed by a coagulation cascade where the blood is turned into a clot in an attempt to plug the bleeding point. During an operation, it is desirable for a patient to lose as little blood as possible, so various devices have been developed in an attempt to provide blood free cutting. For endoscopic procedures, it is also undesirable for a bleed to occur and not to be dealt with as soon as quickly as possible, or in an expedient manner, since the blood flow may obscure the operator's vision, which may lead to the procedure needing to be terminated and another method used instead, e.g. open surgery.

[0006] Electrosurgical generators are pervasive throughout hospital operating theatres, for use in open and laparoscopic procedures, and are also increasingly present in endoscopy suites. In endoscopic procedures the electrosurgical accessory is typically inserted through a lumen inside an endoscope. Considered against the equivalent access channel for laparoscopic surgery, such a lumen is comparatively narrow in bore and greater in length. In the case of a bariatric patient the surgical accessory may have a length of 300 mm from handle to RF tip, whereas the equivalent distance in a laparoscopic case can be in excess of 2500 mm.

[0007] Instead of a sharp blade, it is known to use radiofrequency (RF) energy to cut biological tissue. The method of cutting using RF energy operates using the principle that as an electric current passes through a tissue matrix (aided by the ionic contents of the cells and the intercellular electrolytes), the impedance to the flow of electrons across the tissue generates heat. When an RF voltage is applied to the tissue matrix, enough heat is generated within the cells to vaporise the water content of the tissue. As a result of this increasing desiccation, particularly adjacent to the RF emitting region of the instrument (referred to herein as an RF blade) which has the highest current density of the entire current path through tissue, the tissue adjacent to the cut pole of the RF blade loses direct contact with the blade. The applied voltage then appears almost entirely across this void which ionises as a result, forming a plasma, which has a very high volume resistivity compared to tissue. This differentiation is important as it focuses the applied energy to the plasma that completed the electrical circuit between the cut pole of the RF blade and the tissue. Any volatile material entering the plasma slowly enough is vaporised and the perception is therefore of a tissue dissecting plasma. It is also known to use microwave electromagnetic (EM) radiation for treating biological tissue, for example for coagulating blood or for ablating the tissue itself. Tissue ablation using microwave EM energy is based on the fact that biological tissue is largely composed of water. Human soft organ tissue is typically between 70% and 80% water content. Water molecules have a permanent electric dipole moment, meaning that a charge imbalance exists across the molecule. This charge imbalance causes the molecules to move in response to the forces generated by application of a time varying electric field as the molecules rotate to align their electric dipole moment with the polarity of the applied field. At microwave frequencies, rapid molecular oscillations result in frictional heating and consequential dissipation of the field energy in the form of heat. This is known as dielectric heating.

[0008] This principle is harnessed in microwave ablation therapies, where water molecules in target tissue are rapidly heated by application of a localised electromagnetic field at microwave frequencies, resulting in tissue coagulation and cell death. It is known to use microwave emitting probes to treat various conditions in the lungs and other organs. For example, in the lungs, microwave radiation can be used to treat asthma and ablate tumours or lesions.

[0009] Used in combination, RF energy may be used to cut biological tissue and microwave EM radiation may be used to coagulate blood at the site of the cut. As the space is limited in endoscopic applications, it is often not possible to provide two separate signal lines for delivering the RF energy and the microwave EM radiation through the probe to the treatment site; however, providing both signals on a common signal line runs the risk of the RF energy damaging the microwave signal generator and the microwave energy damaging the RF signal generator.

[0010] The present invention has been devised in light of the above considerations.

[0011] Summary of the Invention

[0012] At its most general, the present invention provides a signal combiner for supplying radiofrequency (RF) electromagnetic (EM) energy and microwave EM energy obtained from separate sources to a common signal pathway. The invention combines into a single unit all the necessary components to isolate the microwave and RF channels from one another whilst providing a high withstanding voltage (e.g. greater than 10 kV).

[0013] The invention improves on existing isolator-combiner structures by combining all the necessary components to isolate the microwave and RF channels from one another in a single unit with a significantly reduced footprint, meaning that an electrosurgical apparatus including the coaxial signal combiner described herein may be made more compact without adversely affecting the performance of the apparatus. In particular, the invention provides a signal combiner having a coaxial structure, for example the invention may be made substantially or entirely from one or more coaxial structures. It is to be understood that a coaxial structure comprises an inner conductor, an outer conductor formed coaxially about the inner conductor, and a dielectric material separating the inner conductor and the outer conductor. This coaxial structure can provide a transmission pathway for RF EM energy and / or microwave frequency EM energy.

[0014] According to an aspect of the invention, there is provided a coaxial signal combiner comprising: a coaxial output branch having an output port for outputting a combined signal, the coaxial output branch comprising: a first inner conductor; a first outer conductor formed coaxially about the first inner conductor; and a first dielectric material separating the first inner conductor and the first outer conductor; a coaxial radiofrequency branch having a radiofrequency input port for receiving a radiofrequency signal, the coaxial radiofrequency branch being electrically connected to the coaxial output branch to convey the radiofrequency signal thereto, the coaxial radiofrequency branch comprising: a second inner conductor connected to the first inner conductor; a second outer conductor connected to the first outer conductor and formed coaxially about the second inner conductor; and a second dielectric material separating the second inner conductor and the second outer conductor; and a coaxial microwave branch having a microwave input port for receiving a microwave frequency signal, the coaxial microwave branch being electrically connected to the coaxial output branch to convey the microwave frequency signal thereto, the coaxial microwave branch comprising: a third inner conductor; a third outer conductor formed coaxially about the third inner conductor; and a third dielectric material separating the third inner conductor and the third outer conductor, and wherein the coaxial microwave branch is connected to the coaxial output branch via first and second capacitive structures, the first capacitive structure formed by the first inner conductor and the third inner conductor, and the second capacitive structure formed by the first outer conductor and the third outer conductor.

[0015] In other words, there is provided a signal combiner for combining an RF EM signal and a microwave EM signal onto a single common output line. The signal combiner includes two coaxial input lines, or branches, (one for RF EM signals and the other for microwave EM signals) and one coaxial output line, or branch. The use of a coaxial structure throughout the entire signal combiner provides a reduced form factor as well as providing a means to isolate the two input branches from each other, thereby protecting the microwave generator from RF EM signals and thereby protecting the RF signal generator from the microwave EM signals.

[0016] Put another way, there is provided a compact signal combiner for combining RF and microwave EM signals onto a combined signal pathway whilst isolating the input signal pathways from each other.

[0017] The RF EM signals will propagate along the coaxial radiofrequency branch in the transverse electric magnetic mode, meaning that the electric field and magnetic field associated with the RF signals will be perpendicular to the direction of propagation, i.e., orthogonal to the second inner conductor. Put another way, the RF signals will propagate along the coaxial radiofrequency branch along the second inner conductor and the associated electric field and magnetic field will be contained within the coaxial radiofrequency branch by the second outer conductor.

[0018] Above a given frequency, an EM signal will propagate along a coaxial conductor as though it is a waveguide formed between the inner and outer conductors. Accordingly, the microwave EM signals will propagate along the coaxial microwave branch as though it is a waveguide formed between the third inner conductor and the third outer conductor.

[0019] The combined signal output at the coaxial output branch may be a combination of a RF EM signal and a microwave EM signal. Alternatively, the combined signal may include only a RF EM signal or only a microwave frequency EM signal. Put another way, the coaxial signal combiner may output any combination of radiofrequency and / or microwave EM signals depending on the signals being received at the radiofrequency input port and / or the microwave input port. The combined signal will propagate along the coaxial output branch, in the manner(s) outlined above according to the component parts of the combined signal.

[0020] The coaxial signal combiner includes capacitive structures provided along the coaxial microwave branch and / or the coaxial output branch. One capacitive structure is formed between the inner conductors of the coaxial output branch and the coaxial microwave branch and another capacitive structure is formed between the outer conductors of the coaxial output branch and the coaxial microwave branch.

[0021] The behaviour of a capacitor in an AC circuit, which may be referred to as the capacitive reactance, varies with frequency according to the formula: c = — 2nfC , where: Xcis the capacitive reactance in Ohms (Q); f is frequency in Hertz (Hz); and C is capacitance in Farads (F).

[0022] At higher frequencies, such as frequencies in the microwave frequency range, a capacitor, having an appropriate capacitance, will have a low capacitive reactance value and will approach a short circuit condition. Accordingly, the microwave frequency signals received at the microwave input port may pass from the coaxial microwave branch to the coaxial output branch with little, to no, loss across the first and second capacitive structures.

[0023] At lower frequencies, such as frequencies in the radiofrequency range (when compared to microwave frequency signals), a capacitor, having an appropriate capacitance, will have a high capacitive reactance value and will approach an open circuit condition. Accordingly, the radiofrequency signals received at the radiofrequency input port may be prevented from passing from the coaxial radiofrequency branch to the coaxial microwave branch by the first and second capacitive structures.

[0024] Accordingly, the first and second capacitive structures protect, or isolate, the microwave frequency branch, and any microwave signal generation circuitry connected to the microwave input port, from radiofrequency signals propagating from the coaxial radiofrequency branch to the coaxial output branch.

[0025] The first capacitive structure may be formed by a first overlap between the first inner conductor and the third inner conductor. A capacitive structure may be formed from two conductors positioned in close proximity to each other and separated by a non-conducting region. Therefore, the first capacitive structure may be formed by overlapping a portion of the first inner conductor and the third inner conductor, without requiring any additional components such as a discrete capacitor. In an embodiment the first inner conductor and the third inner conductor are separated by an air gap at the first overlap. In an example, the first inner conductor comprises a solid elongate body connected to a hollow elongate end portion, wherein the third inner conductor is located within a bore of the hollow elongate end portion of the first inner conductor such that the hollow elongate end portion of the first inner conductor surrounds the third inner conductor at the first overlap.

[0026] Put another way, the third inner conductor may be received within the first inner conductor in the overlapping region forming the first capacitive structure. In an alternative arrangement, the first inner conductor may be received within the third inner conductor to form the first overlap and so form the first capacitive structure.

[0027] The length of the first overlap may be between 2mm and 4mm, for example 3.2mm.

[0028] By making the length of the first overlap in the region of a quarter wavelength of a microwave signal, the first overlap between the first and third inner conductors behaves as a capacitor despite essentially being open-circuited lines with no direct electrical connection to each other. In particular, the use of a quarter wavelength overlap means that the first overlap becomes a lossless, or low loss, transmission line for the microwave EM signals received at the microwave input port.

[0029] The first capacitive structure further comprises an inner dielectric sheath which separates the first inner conductor from the third inner conductor at the first overlap. In this way, the capacitive behaviour of the first overlap may be improved by the presence of a dielectric provided between the overlapping conductors. For example, the material of the dielectric sheath may be chosen such that it has a higher dielectric constant than air so as to improve the capacitive behaviour of the first overlap over that provided by only an air gap. The inner dielectric sheath may be a thin film insulation layer, for example of polyamide or any other suitable dielectric.

[0030] The second capacitive structure may be defined by a second overlap between the first outer conductor and the third outer conductor. A capacitive structure may be formed from two conductors positioned in close proximity to each other and separated by a non-conducting region. Therefore, the second capacitive structure may be formed by overlapping a portion of the first outer conductor and the third outer conductor, without requiring any additional components such as a discrete capacitor. In an embodiment the first outer conductor and the third outer conductor are separated by an air gap at the second overlap.

[0031] The first outer conductor may comprise a hollow elongate body connected to a hollow elongate end portion, the hollow elongate end portion having a reduced thickness compared to the hollow elongate body, wherein the third outer conductor comprises a hollow elongate body connected to a hollow elongate end portion, the hollow elongate end portion having a reduced thickness compared to the hollow elongate body, wherein the hollow elongate end portion of the first outer conductor surrounds the hollow elongate body of the third outer conductor at the second overlap.

[0032] Put another way, the third outer conductor may be received within the first outer conductor in the overlapping region forming the second capacitive structure. In an alternative arrangement, the first outer conductor may be received within the third outer conductor to form the second overlap and so form the second capacitive structure. The length of the second overlap may be between 2mm and 4mm, for example 3.2mm.

[0033] By making the length of the second overlap in the region of a quarter wavelength of a microwave signal, the second overlap between the first and third outer conductors behaves as a capacitor despite essentially being open-circuited lines with no direct electrical connection to each other. In particular, the use of a quarter wavelength overlap means that the second overlap becomes a lossless, or low loss, transmission line for the microwave EM signals received at the microwave input port.

[0034] The second capacitive structure may further comprise an outer dielectric sheath which separates the first outer conductor from the third outer conductor at the second overlap. In this way, the capacitive behaviour of the second overlap may be improved by the presence of a dielectric provided between the overlapping conductors. For example, the material of the dielectric sheath may be chosen such that it has a higher dielectric constant than air so as to improve the capacitive behaviour of the second overlap over that provided by only an air gap. The outer dielectric sheath may be a thin film insulation layer, for example of polyamide or any other suitable dielectric.

[0035] The coaxial radiofrequency branch may comprise a low pass filter.

[0036] The low pass filter of the coaxial radiofrequency branch may be selected to allow radiofrequency EM signals received at the radiofrequency input port to pass along the coaxial radiofrequency branch to the coaxial output branch, but prevent microwave EM signals from passing along the coaxial radiofrequency branch to the radiofrequency input port. In this way, any circuitry connected to the radiofrequency input port may be protected from potentially damaging microwave EM signals. The low pass filter may be formed, at least in part, by the second inner conductor.

[0037] The low pass filter may be a stepped impedance low pass filter.

[0038] A stepped impedance low pass filter operates on the principle that impedance mismatches along a signal pathway will result in a signal, and in particular a high frequency signal, being reflected at the impedance mismatch. Accordingly, by intentionally providing impedance mismatches along the coaxial radiofrequency branch, microwave EM signals will be reflected and prevented from propagating along the coaxial radiofrequency branch, whilst radiofrequency EM signals will be able to pass through the low pass filter substantially unimpeded.

[0039] The stepped impedance low pass filter may comprise a plurality of capacitive segments and a plurality of inductive segments, wherein the plurality of capacitive segments and the plurality of inductive segments are arranged alternately between the radiofrequency input port and the coaxial output branch. Inductors and capacitors have opposite impedance relationships with respect to frequency according to the following formulae: where: ZLis the impedance of an inductor; j is the imaginary unit; co is the angular velocity of the signal, which is directly proportional to its frequency; L is the inductance of the inductor, which is the ratio of magnetic flux linkage against current for the given inductor; Zcis the impedance of the capacitor; and C is the capacitance of the capacitor.

[0040] This means that the greater the frequency, the greater the impedance mismatch will be experienced by the signal at an interface between a capacitor and an inductor. In this way, a stepped impedance low pass filter may include a series of frequency dependent impedance changes, which will therefore have a greater impact on the microwave signals than the radiofrequency signals.

[0041] One or more parameters of each capacitive segment and each inductive segment may be modified to tune the performance of the low pass filter at one or more predetermined operating frequencies of the microwave signal. For example, the one or more parameters may include one or more of the following: a length of the capacitive segment; a length of the inductive segment; a width or thickness of the second inner conductor in the capacitive segment; a width or thickness of the second inner conductor in the inductive segment; a separation between the second inner conductor and the second outer conductor in the capacitive segment; a separation between the second inner conductor and the second outer conductor in the inductive segment; the length of a conductive pathway between the second capacitive segment of the coaxial RF branch and the coaxial output branch (e.g. the first inner conductor or the output port); and, the separation between the first and second capacitive segments (e.g. a length of the first inductive segment).

[0042] Each of the plurality of capacitive segments may be formed by segments of the second inner conductor having an increased diameter compared to the plurality of inductive segments, thereby providing a reduced separation between the second inner conductor and the second outer conductor in the plurality of capacitive segments compared to the plurality of inductive segments. In this way, the stepped impedance low pass filter may be implemented in the coaxial radiofrequency branch by changing the diameter of the second inner conductor along its length without requiring any additional discrete electronic components. In an embodiment, these increased diameter segments are formed by sliding over the second inner conductor, or otherwise positioning on the second inner conductor, a ring-shaped conductive element (or a cylindrical conductive element having a central bore for receiving the second inner conductor). The ring-shaped conductive element may be formed from substantially the same material as the second inner conductor. The ring-shaped conductive element may replace the second dielectric material in the capacitive segments. In an embodiment, an air gap is maintained between the ring-shaped conductive element and the second outer conductor so that it operates as a capacitive structure.

[0043] In an embodiment, each of the plurality of inductive segments is formed by the absence of the ringshaped conductive element between the second inner conductor and the second outer conductor. That is, in the inductive segments, the second dielectric material is present between the second inner conductor and the second outer conductor.

[0044] The coaxial radiofrequency branch may further comprise an additional dielectric sheath provided between the second inner conductor and the second outer conductor. In this way, the capacitive effect of the capacitive sections may be increased, thereby improving the efficiency of the stepped impedance low pass filter. Additionally the dielectric sheath may be selected for its high breakdown characteristics, which enable a high voltage signal excitation at the radiofrequency input port. For example, the material of the additional dielectric sheath may be chosen such that it has a higher dielectric constant than air so as to improve the capacitive effect over that provided by only an air gap. The additional dielectric sheath may be a thin film insulation layer, for example of polyamide or any other suitable dielectric. Where a ringshaped conductive element is present to form the capacitive segment, the ring-shaped conductive element may abut the additional dielectric sheath provided between the second inner conductor and the second outer conductor. Alternatively, there may be an air gap between the ring-shaped conductive element and the second outer conductor. The shaping and spacing of the ring-shaped conductive element may be made with consideration to allow an adequate operating margin without breakdown for the excitation signal. Typically air breakdown occurs at 3kV / mm.

[0045] In an example, the stepped impedance low pass filter may comprise: a first capacitive segment provided between the radiofrequency input port and the coaxial output branch; a first inductive segment provided between the first capacitive segment and the coaxial output branch; a second capacitive segment provided between the first inductive segment and to the coaxial output branch; a second inductive segment provided between the second capacitive segment and the coaxial output branch. The filter topology implemented may be a 3rdorder Chebyshev type filter with the first element being a capacitive segment followed by an inductive segment. High order filters may provide improved isolation, but will increase the length and size of the coaxial radiofrequency branch, and the coaxial signal combiner as whole.

[0046] The coaxial output branch, the coaxial radiofrequency branch, and the coaxial microwave branch may have a circular cross section. In this way, the machining and manufacture of the coaxial signal combined may be simplified. Alternatively, the branches may have any suitable cross-sectional shape and each branch may have the same cross section or a different cross section. The cylindrical coaxial structure supports transverse electric (TE) and transverse magnetic (TM) modes over a wider range of operating frequencies. The circular cross section may be improve the area for incorporating thin film dielectric sheaths between the outer and inner overlapping conductors such that when these are fitted together they behave as a continuous low loss transmission path for the microwave signals.

[0047] The coaxial radiofrequency branch may further comprise a spacer element provided about the second inner conductor and receivable within the second outer conductor, wherein the spacer element is adapted to align the second inner conductor within the second outer conductor. The spacer element may be formed from an insulator, such as polytetrafluoroethylene (PTFE). The spacer element may be positioned adjacent to the radiofrequency input port.

[0048] According to a further aspect of the invention, there is provided an electrosurgical apparatus for treating biological tissue, the apparatus comprising: a radiofrequency (RF) signal generator for generating RF electromagnetic (EM) radiation having a first frequency; a microwave signal generator for generating microwave EM radiation having a second frequency that is higher than the first frequency; a probe arranged to deliver the RF EM radiation and the microwave EM radiation separately or simultaneously from a distal end thereof; and a feed structure for conveying the RF EM radiation and the microwave EM radiation to the probe, the feed structure comprising a coaxial signal combiner as described above, wherein the radiofrequency input port is connected to receive the RF EM radiation from the RF signal generator, the microwave input port is connected to receive the microwave EM radiation from the microwave signal generator, and the output port is connected to deliver the combined signal to the probe.

[0049] Treating biological tissue may include a number of different operations involving the delivery of RF and / or microwave EM radiation to the biological tissue, such as: ablation; resection; coagulation; desiccation; sealing; and the like.

[0050] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0051] Summary of the Figures

[0052] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0053] Figure 1 shows a schematic representation of an electrosurgical apparatus for treating biological tissue.

[0054] Figure 2 shows a cross section of a coaxial signal combiner according to an aspect of the invention.

[0055] Figure 3 shows a cross section of a portion of the coaxial signal combiner shown in Figure 2.

[0056] Figures 4A to 4C show graphs depicting the signal that is reflected and transmitted from the microwave input port across the coaxial signal combiner shown in Figure 2.

[0057] Figures 5A to 5C show graphs depicting the signal that is reflected and transmitted from the coaxial output port across the coaxial signal combiner shown in Figure 2.

[0058] Figures 6A to 6C show graphs depicting the signal that is reflected and transmitted from the radiofrequency input port across the coaxial signal combiner shown in Figure 2.

[0059] Figure 7 shows a graph of signal power that is stimulated and the power leaving the structure against frequency for the coaxial signal combiner shown in Figure 2. Detailed Description of the Invention

[0060] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0061] Fig. 1 shows a schematic diagram of coaxial signal combiner 100 for an electrosurgical apparatus that is an embodiment of the invention. The coaxial signal combiner 100 forms part of a feed structure for conveying RF EM radiation from an RF signal generator 118 and microwave radiation from a microwave signal generator 120 to a probe. In this embodiment, the probe (not shown) is connectable to an output port 128 provided in a housing 126. The feed structure comprises an RF Bipolar channel having a RF active and return pathway 112, 114 for conveying the RF EM radiation from the RF signal generator 118 to the radiofrequency input port 119. The feed structure further comprises a microwave channel having a microwave signal pathway 110 for conveying the microwave EM radiation from the microwave signal generator 120 to the microwave input port 111.

[0062] The signal pathways for the RF EM radiation and microwave radiation are physically separate from each other until the coaxial signal combiner 100. The RF signal generator is connected to the RF active and return pathway 112, 114 via a voltage transformer 116. The secondary coil of the transformer 116 (i.e. on the probe side of the arrangement) is floating, so there is not direct current path between the patient and the RF signal generator 118. This means that both the active conductor 112 and return conductor 114 of the RF active and return pathway 112, 114 are floating.

[0063] The coaxial signal combiner 100 has a coaxial radio frequency branch 121 for connecting to the RF active and return pathway 112, 114, by way of the radiofrequency input port 119, and a coaxial microwave branch 122 for connecting to the microwave signal pathway 110, by way of the microwave input port 111. The coaxial signal combiner joins the pathways to an isolated coaxial output branch 123, which is connected to a common signal pathway 108 that is floating. The common signal pathway 108, which may include a flexible cable (e.g. coaxial cable of the like) conveys the RF EM radiation and microwave EM radiation to the probe.

[0064] An insulating sleeve 129 is provided at the output port 128 of the housing to provide clearance and creepage distance for isolation and to mitigate any likelihood of a DC current path for connecting the grounded casing of the housing with the floating components connected to the output port 128. The output port 128 may comprise a Type N screw thread or a quick release connector, e.g. to allow different probes to be attached to the housing.

[0065] Fig. 2 shows a cross section of a coaxial signal combiner 100 according to an aspect of the invention.

[0066] The coaxial signal combiner 100 comprises a coaxial output branch 123 having an output port 130 for outputting a combined signal. The coaxial output branch 123 comprises a first inner conductor 131 and a first outer conductor 132 formed coaxially about the first inner conductor 131 . The first inner conductor 131 and the first outer conductor 132 are separated from each other by a first dielectric material 133. The coaxial signal combiner 100 further comprises a coaxial radiofrequency branch 121 having a radiofrequency input port 119 for receiving a radiofrequency signal. The coaxial radiofrequency branch 121 is electrically connected to the coaxial output branch 123 to convey the radiofrequency signal thereto. The coaxial radiofrequency branch 121 includes a second inner conductor 141 connected to the first inner conductor 131 and a second outer conductor 142 connected to the first outer conductor 132 and formed coaxially about the second inner conductor 141 . The second inner conductor 141 and second outer conductor 142 are separated from each other by a second dielectric material 143.

[0067] The coaxial signal combiner 100 further comprises a coaxial microwave branch 122 having a microwave input port 111 for receiving a microwave frequency signal. The coaxial microwave branch 122 is capacitively coupled and forms a high frequency electrical connection to the coaxial output branch 123 to convey the microwave frequency signal thereto. The coaxial microwave branch 122 comprises a third inner conductor 151 and a third outer conductor 152 formed coaxially about the third inner conductor 151 . The third inner conductor 151 and the third outer conductor 152 are separated from each other by a third dielectric material 153.

[0068] In the example shown in Figure 2, the first, second and third dielectric materials 133, 143, 153 are formed by a single continuous air gap present throughout the coaxial signal combiner.

[0069] As shown in Figure 2, the coaxial microwave branch 122 is connected to the coaxial output branch 123 by a first capacitive structure 161 and a second capacitive structure 162. The first capacitive structure 161 is formed by the first inner conductor 131 and the third inner conductor 151 and the second capacitive structure 162 formed by the first outer conductor 132 and the third outer conductor 152. The implementation of the first and second capacitive structures 161 , 162 is discussed in further detail below with respect to Figure 3.

[0070] In the example shown in Figure 2, the coaxial radiofrequency branch 121 , and in particular the second inner conductor 141 , comprises a low pass filter in the form of a stepped impedance low pass filter. The stepped impedance low pass filter comprises a first capacitive segment 171 , a first inductive segment 172, a second capacitive segment 173 and a second inductive segment 174 provided in an alternating order of capacitive and inductive segments from the radiofrequency input port 119 to the coaxial output branch 123, and in particular the first inner conductor 131 .

[0071] The first and second capacitive segments 171 , 173 are formed by segments of the second inner conductor 141 having an increased diameter compared to the first and second inductive segments 172, 174. Accordingly, the separation between the second inner conductor 141 and the second outer conductor 142 is smaller in the first and second capacitive segments 171 , 173 compared to the separation between the second inner conductor 141 and the second outer conductor 142 in the first and second inductive segments 172, 174. The reduced separation between the second inner conductor 141 and the second outer conductor 142 in the first and second capacitive segments 171 , 173 produces a capacitive effect, which is enhanced by the provision of an additional dielectric sheath 175 provided between the second inner conductor 141 and the second outer conductor 142. The additional dielectric sheath 175 is a thin film insulation layer of polyamide. As outlined above, the impedance of an inductor is directly proportional to the frequency of the signal being applied; whereas the impedance of a capacitor is indirectly proportional to the frequency of the signal being applied. Accordingly, the higher the frequency of the signal, the greater the impedance mismatch between the first capacitive segment 171 and the first inductive segment 172, between the first inductive segment 172 and the second capacitive segment 173 and between the second capacitive segment 173 and the second inductive segment 174. Each of these impedance mismatch interfaces between the capacitive segments and the inductive segments will cause any microwave signals incident on the coaxial radiofrequency branch 121 from the coaxial microwave branch 122 and / or any reflected microwave signals incident on the radiofrequency branch 121 from the coaxial output branch 123 to be reflected. In this way, microwave signals are prevented from propagating to the radiofrequency input port 119.

[0072] In the example shown in Figure 2, the coaxial radiofrequency branch 121 further comprises a spacer element 180 positioned adjacent to the radiofrequency input port 119, provided about the second inner conductor 141 and receivable within the second outer conductor 142. The spacer element is adapted to align the second inner conductor 141 within the second outer conductor 142, and in particular to maintain the separation between the second inner conductor 141 and the second outer conductor 142 at the first and second capacitive segments 171 , 173.

[0073] Figure 3 shows a cross section of a portion 190 of the coaxial signal combiner 100 shown in Figure 2. In particular, Figure 3 shows the first capacitive structure 161 and the second capacitive structure 162 between the coaxial microwave branch 122 and the coaxial output branch 123 in greater detail.

[0074] As shown in Figure 3, the first capacitive structure 161 is formed by a first overlap 163 between the first inner conductor 131 and the third inner conductor 151 . In particular, the third inner conductor 151 is located within a bore of a hollow elongate end portion 135 of the first inner conductor 131 such that the hollow elongate end portion 135 of the first inner conductor 131 surrounds the third inner conductor 151 at the first overlap 163. The length of the first overlap 163 is 3.2mm.

[0075] The first capacitive structure 161 further comprises an inner dielectric sheath 191 which separates the first inner conductor 131 from the third inner conductor 151 at the first overlap 163. The inner dielectric sheath 191 improves the capacitive performance of the first capacitive structure 161 . In particular, the inner dielectric sheath 191 separates the inner surface of the bore of the hollow elongate end portion 135 of the first inner conductor 131 from the third inner conductor 151 received therein. The inner dielectric sheath 191 is a thin film insulation layer of polyamide.

[0076] The third inner conductor 151 may be partially received in the bore of the hollow elongate end portion 135 of the first inner conductor 131 , such that an air gap is present between a distal end face of the third inner conductor and an end face of the bore of the hollow elongate end portion 135 of the first inner conductor 131 . The air gap may be sized to mitigate the risk of electrical breakdown or shorting between the third inner conductor and the first inner conductor.

[0077] As shown in Figure 3, the second capacitive structure 162 is formed by a second overlap 164 between the first outer conductor and the third outer conductor 152. In particular, a hollow elongate end portion the second overlap 164. The hollow elongate end portion 134 of the first outer conductor has a reduced thickness compared to the hollow elongate body that defines the rest of the first outer conductor. The hollow elongate end portion 154 of the third outer conductor 152 also has a reduced thickness compared to the hollow elongate body that defines the rest of the third outer conductor 152. The reduced thicknesses of the hollow elongate end portion 134 of the first outer conductor and the hollow elongate end portion 154 of the third outer conductor allows the conductors to overlap with a dielectric material provided therebetween whilst maintaining the ratio between the inside diameter of the outer conductors with the outside diameter of the inner conductors that forms the characteristic impedance of the coaxial structure for the microwave signal. The length of the second overlap 164 is 3.2mm.

[0078] The second capacitive structure 162 further comprises an outer dielectric sheath 192, which separates the first outer conductor from the third outer conductor 152 at the second overlap 164. The outer dielectric sheath 192 improves the capacitive performance of the second capacitive structure 162. In particular, the inner surface of the hollow elongate end portion 134 of the first outer conductor is separated from the hollow elongate end portion 154 of the third outer conductor received therein by the outer dielectric sheath 192. The outer dielectric sheath 192 is a thin film insulation layer of polyamide.

[0079] As outlined above, for frequencies in the microwave frequency range, an appropriate capacitor as defined by the first and second capacitive structures 161 , 162 will have a low capacitive reactance value and will approach a short circuit condition. Accordingly, the microwave frequency signals received at the microwave input port will pass from the coaxial microwave branch to the coaxial output branch with little to no loss across the first and second capacitive structures 161 , 162.

[0080] For frequencies in the radiofrequency range, an appropriate capacitor as defined by the first and second capacitive structures 161 , 162, will have a high capacitive reactance value and will approach an open circuit condition. Accordingly, the radiofrequency signals received at the radiofrequency input port will be prevented from passing from the coaxial radiofrequency branch to the coaxial microwave branch by the first and second capacitive structures 161 , 162.

[0081] The combination of the first and second capacitive structures 161 , 162 on the coaxial microwave branch shown in Figure 3 and the stepped impedance low pass filter shown in Figure 2 provides a coaxial signal combiner capable of combining both radiofrequency and microwave frequency signals onto a single common signal pathway, whilst also isolating the radiofrequency signal pathway and the microwave signal pathway from each other.

[0082] Figures 4A to 4C show graphs depicting the signal loss across the coaxial signal combiner 100 shown in Figures 2 and 3.

[0083] In particular, Figure 4A shows a graph 200 of the log ratio of forward power compared to reflected power as a measure of signal amplitude (dB) against signal frequency (GHz) as measured at the microwave input port 111 with markers at two microwave frequencies, 2.45GHZ (marker 210) and 5.8GHZ (marker 220). As shown in Figure 4A, at marker 210 the signal loss is approximately -10dB and at marker 220 the signal loss is approximately -17dB. Accordingly, the signal loss at the microwave input port 111 is relatively low at these frequencies.

[0084] Figure 4B shows a graph 300 of transmission loss signal amplitude (dB) against signal frequency (GHz) as measured between the microwave input port 111 and the output port 130 with markers at two microwave frequencies, 2.45GHZ (marker 310) and 5.8GHZ (marker 320).

[0085] As shown in Figure 4B, at both markers 310 and 320 the signal loss is negligible at greater than -1dB. Accordingly, the microwave EM signals pass from the coaxial microwave input branch, to the coaxial output branch in an essentially lossless fashion. Furthermore as shown in Figure 4B, the radiofrequency EM signals (< 1 MHz) are essentially blocked from flowing between the microwave output port 130 and the microwave input port 111.

[0086] Figure 4C shows a graph 400 of transmission loss signal amplitude (dB) against signal frequency (GHz) as measured between the microwave input port 111 and the radiofrequency input port 119 with markers at two microwave frequencies, 2.45GHZ (marker 410) and 5.8GHZ (marker 420).

[0087] As shown in Figure 4C, at marker 410 the signal loss is approximately -29dB and at marker 220 the signal loss is approximately -40dB. Accordingly, the microwave EM signals are essentially blocked from flowing from the microwave input port 11 1 to the radiofrequency input port 119. Figure 4C further demonstrates that radiofrequency EM signals (< 1 MHz) are essentially blocked from flowing between the radiofrequency input port 119 and the microwave input port 111.

[0088] Figures 4A to 4C show the signal loss characteristics between the microwave input port and the other signal ports of the coaxial signal combiner. However, similar behaviours are also seen between the radiofrequency input port and other ports, and between the output port and the other ports.

[0089] Figures 5A to 5C show graphs depicting the signal that is reflected and transmitted across the coaxial signal combiner 100 shown in Figures 2 and 3.

[0090] In particular, Figure 5A shows a graph 500 of the log ratio of forward power compared to reflected power as a measure of signal amplitude (dB) against signal frequency (GHz) as measured at the microwave output port 130 with markers at two microwave frequencies, 2.45GHZ (marker 510) and 5.8GHZ (marker 520).

[0091] As shown in Figure 5A, at marker 510 the signal loss is approximately -11 dB and at marker 520 the signal loss is approximately -17dB. Accordingly, the signal loss at the microwave output port 130 is relatively low at these frequencies.

[0092] Figure 5B shows a graph 600 of transmission loss signal amplitude (dB) against signal frequency (GHz) as measured between the microwave output port 130 and the input port 1 11 with markers at two microwave frequencies, 2.45GHZ (marker 610) and 5.8GHZ (marker 620).

[0093] As shown in Figure 5B, at both markers 610 and 620 the signal loss is negligible at greater than -1dB. Accordingly, the microwave EM signals pass from the coaxial microwave output branch, to the coaxial input branch in an essentially lossless fashion. Furthermore as shown in Figure 5B, the radiofrequency EM signals (< 1 MHz) are essentially blocked from entering the microwave input port 111.

[0094] Figure 5C shows a graph 700 of transmission loss signal amplitude (dB) against signal frequency (GHz) as measured between the output port 130 and the radiofrequency input port 119 with markers at two microwave frequencies, 2.45GHZ (marker 710) and 5.8GHZ (marker 720).

[0095] As shown in Figure 5C, at marker 710 the signal loss is approximately -30dB and at marker 720 the signal loss is approximately -40dB. Accordingly, reflected microwave EM signals are essentially blocked from flowing from the output port 130 to the radiofrequency input port 119. Figure 5C further demonstrates that radiofrequency EM signals (< 1 MHz) pass through in a lossless fashion between the microwave output port 130 and radiofrequency input port 119.

[0096] Figures 6A to 6C show graphs depicting the signal that is reflected and transmitted across the coaxial signal combiner 100 shown in Figures 2 and 3.

[0097] In particular, Figure 6A shows a graph 800 of the log ratio of forward power compared to reflected power as a measure of port match signal amplitude (dB) against signal frequency (GHz) as measured at the radiofrequency input port 119 with markers at two microwave frequencies, 2.45GHZ (marker 810) and 5.8GHZ (marker 820).

[0098] As shown in Figure 6A, at both markers 810 and 820 the signal loss is negligible at greater than -1dB indicative that microwave frequencies 2.45GHZ and 5.8GHZ are more likely to reflect at this port

[0099] Figure 6B shows a graph 900 of transmission loss signal amplitude (dB) against signal frequency (GHz) as measured between the radiofrequency input port 119 and the output port 130 with markers at two microwave frequencies, 2.45GHZ (marker 910) and 5.8GHZ (marker 920).

[0100] As shown in Figure 6B, at marker 910 the signal loss is approximately -30dB and at marker 920 the signal loss is approximately -40dB. Accordingly, reflected microwave EM signals are essentially blocked from flowing from the output port 130 to the radiofrequency input port 119, whereas lower frequency, i.e., radiofrequency, signals may pass through.

[0101] Figure 6C shows a graph 1000 of through loss signal amplitude (dB) against signal frequency (GHz) as measured between the radiofrequency input port 1 19 and the microwave input port 111 with markers at two microwave frequencies, 2.45GHZ (marker 1010) and 5.8GHZ (marker 1020).

[0102] As shown in Figure 6C, at marker 1010 the signal loss is approximately -35dB and at marker 1020 the signal loss is approximately -45dB. Accordingly, the microwave EM signals are essentially blocked from flowing from the microwave input port 11 1 to the radiofrequency input port 119. Further, the graph shows a sharp increase in signal loss for lower frequency, i.e., radiofrequency, signals showing the blocking of radiofrequency signals from flowing from the radiofrequency input port 119 to the microwave input port 111.

[0103] Figure 7 shows a graph 1 100 of signal power (W) against signal frequency (GHz) with markers at two microwave frequencies, 2.45GHZ (marker 11 10) and 5.8GHZ (marker 1120). The graph 1100 includes a first plot 1 130 representing the power accepted by the coaxial signal combiner and a second plot 1140 representing the power radiated by the coaxial signal combiner, for example from the first and second overlaps 163, 164 as shown in Figure 3.

[0104] According to the results illustrated in Figure 7, the total efficiency of the coaxial signal combiner at 2.45GHz is approximately -73dB. This indicates the radiated signal is approximately -73dB below the stimulated power level at the microwave input port. The approximate radiated power based on the stimulated level is P(W) = 10<-734 / 1°) / 1000 = 45 x 10'12W.

[0105] According to the results illustrated in Figure 7, the total efficiency of the coaxial signal combiner at 5.8GHz is approximately -65dB. This indicates the radiated signal is approximately -63dB below the stimulated power level at the microwave input port. The approximate radiated power based on the stimulated level is P(W)= 10<-634 / 10> / 1000 = 0.46 x 109W

[0106] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0107] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0108] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0109] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0110] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0111] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

Claims:1 . A coaxial signal combiner comprising: a coaxial output branch having an output port for outputting a combined signal, the coaxial output branch comprising: a first inner conductor; a first outer conductor formed coaxially about the first inner conductor; and a first dielectric material separating the first inner conductor and the first outer conductor; a coaxial radiofrequency branch having a radiofrequency input port for receiving a radiofrequency signal, the coaxial radiofrequency branch being electrically connected to the coaxial output branch to convey the radiofrequency signal thereto, the coaxial radiofrequency branch comprising: a second inner conductor connected to the first inner conductor; a second outer conductor connected to the first outer conductor and formed coaxially about the second inner conductor; and a second dielectric material separating the second inner conductor and the second outer conductor; and a coaxial microwave branch having a microwave input port for receiving a microwave frequency signal, the coaxial microwave branch being electrically connected to the coaxial output branch to convey the microwave frequency signal thereto, the coaxial microwave branch comprising: a third inner conductor; a third outer conductor formed coaxially about the third inner conductor; and a third dielectric material separating the third inner conductor and the third outer conductor, and wherein the coaxial microwave branch is connected to the coaxial output branch via first and second capacitive structures, the first capacitive structure formed by the first inner conductor and the third inner conductor, and the second capacitive structure formed by the first outer conductor and the third outer conductor.

2. The coaxial signal combiner claimed in claim 1 , wherein the first capacitive structure is formed by a first overlap between the first inner conductor and the third inner conductor.

3. The coaxial signal combiner claimed in claim 2, wherein the first inner conductor comprises a solid elongate body connected to a hollow elongate end portion, wherein the third inner conductor is located within a bore of the hollow elongate end portion of the first inner conductor such that the hollow elongate end portion of the first inner conductor surrounds the third inner conductor at the first overlap.

4. The coaxial signal combiner claimed in any of claims 2 to 3, wherein a length of the first overlap is between 2mm and 4mm, for example 3.2mm.

5. The coaxial signal combiner claimed in any of claims 2 to 4, wherein the first capacitive structure further comprises an inner dielectric sheath which separates the first inner conductor from the thirdinner conductor at the first overlap.

6. The coaxial signal combiner claimed in claim 5, wherein the inner dielectric sheath is a thin film insulation layer.

7. The coaxial signal combiner claimed in any of claims 1 to 6, wherein the second capacitive structure is defined by a second overlap between the first outer conductor and the third outer conductor.

8. The coaxial signal combiner claimed in claim 7, wherein the first outer conductor comprises a hollow elongate body connected to a hollow elongate end portion, the hollow elongate end portion having a reduced thickness compared to the hollow elongate body, wherein the third outer conductor comprises a hollow elongate body connected to a hollow elongate end portion, the hollow elongate end portion having a reduced thickness compared to the hollow elongate body, wherein the hollow elongate end portion of the first outer conductor surrounds the hollow elongate body of the third outer conductor at the second overlap.

9. The coaxial signal combiner claimed in any of claims 7 to 8, wherein a length of the second overlap is between 2mm and 4mm, for example 3.2mm.

10. The coaxial signal combiner claimed in any of claims 7 to 9, wherein the second capacitive structure further comprises an outer dielectric sheath which separates the first outer conductor from the third outer conductor at the second overlap.11 . The coaxial signal combiner claimed in claim 10, wherein the outer dielectric sheath is a thin film insulation layer.

12. The coaxial signal combiner claimed in any of claims 1 to 11 , wherein the coaxial radiofrequency branch comprises a low pass filter.

13. The coaxial signal combiner claimed in claim 12, wherein the low pass filter is a stepped impedance low pass filter.

14. The coaxial signal combiner claimed in claim 13, wherein the stepped impedance low pass filter comprises: a plurality of capacitive segments; and a plurality of inductive segments, wherein the plurality of capacitive segments and the plurality of inductive segments are arranged alternately between the radiofrequency input port and the coaxial output branch.

15. The coaxial signal combiner claimed in claim 14, wherein each of the plurality of capacitive segments are formed by segments of the second inner conductor having an increased diameter compared to the plurality of inductive segments, thereby providing a reduced separation between the second innerconductor and the second outer conductor in the plurality of capacitive segments compared to the plurality of inductive segments.

16. The coaxial signal combiner claimed in any of claims 14 to 15, wherein the coaxial radiofrequency branch further comprises an additional dielectric sheath provided between the second inner conductor and the second outer conductor.

17. The coaxial signal combiner claimed in claim 16, wherein the additional dielectric sheath is a thin film insulation layer.

18. The coaxial signal combiner claimed in any of claims 13 to 17, wherein the stepped impedance low pass filter comprises: a first capacitive segment provided between the radiofrequency input port and the coaxial output branch; a first inductive segment provided between the first capacitive segment and the coaxial output branch; a second capacitive segment provided between the first inductive segment and to the coaxial output branch; a second inductive segment provided between the second capacitive segment and the coaxial output branch.

19. The coaxial signal combiner claimed in any of claims 1 to 18, wherein the coaxial output branch, the coaxial radiofrequency branch, and the coaxial microwave branch have a circular cross section.

20. The coaxial signal combiner claimed in any of claims 1 to 19, wherein the coaxial radiofrequency branch further comprises a spacer element provided about the second inner conductor and receivable within the second outer conductor, wherein the spacer element is adapted to align the second inner conductor within the second outer conductor.21 . The coaxial signal combiner claimed in claim 20, wherein the spacer element is positioned adjacent to the radiofrequency input port.

22. An electrosurgical apparatus for treating biological tissue, the apparatus comprising: a radiofrequency (RF) signal generator for generating RF electromagnetic (EM) radiation having a first frequency; a microwave signal generator for generating microwave EM radiation having a second frequency that is higher than the first frequency; a probe arranged to deliver the RF EM radiation and the microwave EM radiation separately or simultaneously from a distal end thereof; and a feed structure for conveying the RF EM radiation and the microwave EM radiation to the probe, the feed structure comprising a coaxial signal combiner as claimed in any of claims 1 to 21 , wherein the radiofrequency input port is connected to receive the RF EM radiation from the RF signalgenerator, the microwave input port is connected to receive the microwave EM radiation from the microwave signal generator, and the output port is connected to deliver the combined signal to the probe.

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