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Dielectric Heating Device for Aerosol Generation Using an Oscillator with Resonant Feedback Loop Abstract Provided is an aerosol-generating device 120 for dielectrically heating an aerosol-forming substrate 110. The device 120 comprises an oscillation circuit 150 comprising a switching unit configured for inverting operation and a feedback loop connected across the switching unit. The feedback loop comprises two electrical contacts 160, 165 configured to interconnect with an electrode arrangement that forms a load capacitor CL for dielectrically heating the aerosol-forming substrate 110. The feedback loop is configured to perform resonant oscillating operation. The feedback loop comprises a phase-shift component configured to provide a 180° phase shift between an output signal of the switching unit and an input switching signal of the switching unit above a resonant frequency of the oscillation circuit 150. Background The present publication relates to aerosol-generating devices, and specifically to aerosol-generating devices configured to heat an aerosol-forming substrate by dielectric heating. The publication also relates to dielectric heating circuits for use in a dielectric heating aerosol-generating device and an aerosol-generating system. Known electrically operated aerosol-generating systems typically heat an aerosol-forming substrate by one or more of: conduction of heat from a heating element to an aerosol-forming substrate, radiation of heat from a heating element to an aerosol-forming substrate or drawing heated air through an aerosol-forming substrate. Most commonly, heating is achieved by passing an electrical current through an electrically resistive heating element, giving rise to Joule heating of the heating element. Inductive heating systems have also been proposed, in which Joule heating occurs as a result of eddy currents induced in a susceptor heating element. A problem with these heating mechanisms is that they may give rise to non-uniform heating of the aerosol-forming substrate. The portion of the aerosol-forming substrate closest to the heating element is heated more quickly or to a higher temperature than portions of the aerosol-forming substrate more remote from the heating element. Systems that dielectrically heat an aerosol-forming substrate have been proposed, which advantageously provide uniform heating of the aerosol-forming substrate. However, known dielectric heating systems are less efficient than inductive heating systems and require complex electrical circuitry to achieve the necessary voltages and frequencies for dielectric heating of an aerosol-forming substrate, sometimes also referred to as microwave heating. It would be desirable to provide a system that dielectrically heats an aerosol-forming substrate with greater efficiency, while still being realisable in a compact or handheld system. Summary and Examples According to the present publication, an aerosol-generating device is provided including any one or more of the features described below. The aerosol-generating device may comprise an oscillation circuit. The oscillation circuit may comprise a switching unit and a feedback loop connected across the switching unit. The feedback loop may comprise two electrical contacts configured to interconnect with an electrode arrangement that forms a load capacitor for dielectrically heating the aerosol-forming substrate. The aerosol-generating device may further comprise any of the features described below alone or in combination with any other feature of the publication. As used herein, the term “aerosol-generating device” relates to a device that interacts with an article comprising an aerosol-forming substrate to generate an aerosol. As used herein, the term “aerosol-forming substrate” relates to a substrate capable of releasing volatile compounds that can form an aerosol. Such volatile compounds can be released by heating the aerosol-forming substrate. According to an example of the publication, provided is an aerosol-generating device for dielectrically heating an aerosol-forming substrate. The device comprises an oscillation circuit comprising a switching unit configured for inverting operation and a feedback loop connected across the switching unit. The feedback loop comprises two electrical contacts configured to interconnect with an electrode arrangement that forms a load capacitor for dielectrically heating the aerosol-forming substrate. The feedback loop is configured to perform resonant oscillating operation. The feedback loop is configured to provide a 180° phase shift between an output signal of the switching unit and an input switching signal of the switching unit. The oscillation circuits of the present publication enable more efficient dielectric heating of an aerosol-forming substrate by facilitating the coupling of a load capacitor electrode arrangement within the feedback loop of the oscillation circuit. The resonant oscillating operation of the feedback loop is able to generate high peak voltages across the load capacitor at high frequencies to deliver power to an aerosol-forming substrate within the electrode arrangement while maintaining the supply voltage across the switching unit, thereby keeping switching losses to a minimum. The oscillation circuits of the present publication may be configured to be self-oscillating; that is, the oscillation circuit itself controls the phase with which the external power acts on it. Where the switching unit is configured for inverting operation, the feedback unit is configured to provide a 180° phase shift between an output signal of the switching unit and an input switching signal of the switching unit. This allows for effective resonant oscillating operation to occur. In some examples, the feedback loop further comprises the electrode arrangement coupled between the two electrical contacts. In some examples, the electrode arrangement is coupled to, or configured to be coupled to a resonant cavity for housing an aerosol-forming substrate to be dielectrically heated via the electrode arrangement. According to an example of the publication, provided is an aerosol-generating device for dielectrically heating an aerosol-forming substrate. The device comprises an oscillation circuit comprising a switching unit and a feedback loop connected to the switching unit. The feedback loop comprises a first inductor having no more than five turns. The first inductor is connected in series with one of two electrical contacts configured to interconnect with an electrode arrangement that forms a load capacitor for dielectrically heating the aerosol-forming substrate. At the operation frequency of the oscillation circuit, the first inductor in the feedback loop is configured to deliver a 90° phase shift between an output signal of the switching unit and an input switching signal of the switching unit. A further 90° phase shift may be provided by a capacitive element, as described in further detail below. By providing an inductor with no more than five turns, the required phase shift for effective resonant oscillating operation may be achieved while also minimizing the presence of parasitic inductance and capacitance in the feedback loop which negatively impact efficient dielectric heating of the aerosol-forming substrate and cause additional power losses that do not contribute to the dielectric heating. In an example, the first inductor comprises no more than three turns. In preferred examples, the inductor has less than three turns, and preferably no more than one turn. In some examples, the first inductor may have less than one turn, for example, a half turn or omega-shaped inductor. In an example, a diameter of the turn(s) of the first inductor is less than 15 mm and more than 3 mm, preferably less than 12 mm and more than 5 mm.  According to an example of the publication, there is provided an aerosol-generating device for dielectrically heating an aerosol-forming substrate. The device comprises an oscillation circuit. The oscillation circuit comprises a switching unit and a feedback loop connected across the switching unit. The feedback loop comprises a series connection of a first inductor, two electrical contacts configured to interconnect with an electrode arrangement that forms a load capacitor for heating the aerosol-forming substrate, and a second inductor. Providing a first inductor coupled to a first side of the load capacitor and a second inductor coupled to a second side of the load capacitor enables more symmetrical voltages to be generated across the load capacitor, while also providing a 90° phase shift for effective resonant oscillating operation. In some examples, the second inductor may comprise the same number of turns as the first inductor to facilitate the symmetrical generation of voltages across the load capacitor. In some examples, the turn(s) in the second inductor have the same diameter as those in the first inductor. In some examples, the first inductor and the second inductor are inductively coupled to one another to form a mutual inductance. In order to achieve high peak voltages across the load capacitor, a high inductance is needed in the feedback loop. However, a high inductance also limits the maximum oscillation frequency attained in the feedback loop, and therefore, the power deliverable to the load capacitor. Inductively coupling the first inductor with the second inductor creates a slightly distributed inductor having an amplified effective inductance. Utilising an inductive coupling between the first and second inductors therefore enables the use of inductors having lower inductance values to mitigate the limitations on the achievable oscillation frequency in the feedback loop, while also providing a high effective inductance to amplify the peak voltages generated across the load capacitor.  In an example, the mutual inductive coupling between the first and second inductor may be between 40% and 70% (or has an inductive coupling coefficient from 0.4 to 0.7), and preferably greater than 50% (or has an inductive coupling coefficient greater than 0.5).  In an example, the mutual inductive coupling between the first and second inductors may be achieved by the close proximity between the first and second inductors. In alternative examples, a mutual inductive coupling between the first and second inductors is achieved with the use of a magnetic core extending through both the first and second inductors. The use of a magnetic core may facilitate a stronger inductive coupling between the first and second inductors than that achievable based on close proximity alone. In an example, a coil axis of the first inductor is arranged to be in parallel with and offset from a coil axis of the second inductor to minimize a capacitive coupling between the first and second inductors. In an example, the first and second inductors are formed as planar inductors. In an example, a coil axis of the first inductor is arranged to coaxially align with a coil axis of the second inductor.  In an example, a planar extension of the first inductor intersects the second inductor.  According to an example of the publication, provided is an aerosol-generating device for dielectrically heating an aerosol-forming substrate. The device comprises an oscillation circuit. The oscillation circuit comprises a switching unit and a feedback loop connected to the switching unit. The feedback loop comprises a series connection of a first inductor and two electrical contacts configured to interconnect with an electrode arrangement that forms a load capacitor for heating the aerosol-forming substrate. The oscillation circuit further comprises a delay element configured to impede the switching speed of the switching unit. Specifically, the delay element may delay a switching signal received by the switching unit. An oscillation circuit comprising a delay element is sometimes referred to as a delay-line oscillator. A delay-line oscillator is a form of electronic oscillator that uses a delay line, or delay element as its principal timing element. A delay-line oscillator may be set to oscillate by inverting the output of the delay line or delay element and feeding that signal back to the input of the delay line or delay element with appropriate amplification. The delay element may be realized with a physical delay line (such as an LC network or a transmission line). In some examples, capacitances and inductances may be distributed across the length of the delay element. In some examples, the delay element comprises a cascade of logic gates for creating a gate delay. The timing of an oscillation circuit using a physical delay element may be much more accurate. It is also easier to get such an oscillation circuit to oscillate in the desired mode. In an example, the oscillation circuit is configured to operate a frequency of between 100MHz-2.5GHz, and preferably between 300MHz-1.5GHz.  In operation, the oscillation frequency in the feedback loop will increase towards a certain resonance frequency based on th...