This invention relates to a heterodyning arrangement whereby a heterodyne (frequency mixed) output signal can be obtained from a plurality of input signals. In particular, it relates to an arrangement in which such a heterodyne signal can be formed through the use of the thermoacoustic effect.

Heterodyning is a signal processing technique in which two or more input signals are mixed or combined in such a manner as to produce an output signal including additional frequencies over and above those present in the input signals. Typically, if input signals at the frequencies f _A and f _B are combined using a heterodyning technique, the output signal will include signal components at the frequencies f _A +f _B and ί _Α -ίβ- Depending upon the application in which the combined signal is being used, one or other of these heterodyne components is often filtered out. A number of devices are known for producing heterodyne outputs from input signals, these devices typically comprising diodes or other non-linear components.

Heterodyning is used extensively in telecommunications and signal processing applications. In addition, it is used in acoustic metrology, gas sensing and in chemical spectroscopy.

The present invention relates to a relatively simple arrangement whereby such heterodyning may be accomplished. According to the present invention there is provided a heterodyning arrangement comprising a conductive material element, at least one terminal whereby first and second electrical signals can be applied to the element, the application of the electrical signals to the element result in Joule heating thereof which, in turn, results in the generation of a pressure wave via the thermoacoustic effect, and a sensor operable to produce an electrical output signal related to the pressure wave, the electrical output signal comprising a heterodyne of the first and second electrical signals.

Such an arrangement is advantageous in that, unlike typical heterodyning devices, it is linear in nature. The conductive material element conveniently comprises a graphene material element.

It has been found that the output pressure wave, and hence the output from the sensor, contains a heterodyne mix of the input electrical signals, and hence that the arrangement can be used in the generation of heterodyne mixes of, for example, audio signals represented by the electrical signals.

The sensor may take the form of a microphone, for example a condensing microphone. It will be appreciated, however, that other forms of sensor may be used.

The graphene material element is preferably mounted upon a support substrate. By way of example, it may be mounted upon a substrate of Si0 ₂ or quartz.

The first input terminal may be of comb-like form.

The conductive material element is further provided with a ground terminal. The ground terminal, like the first input terminal, is conveniently of comb-like form. Where both terminals are of comb like form, the teeth of the combs are conveniently arranged in an interlaced or interdigitated fashion.

The invention also relates to a method of producing a heterodyne output from a first input electrical signal and a second input electrical signal comprising the steps of applying the first and second electrical signals to a conductive material element, the application of the electrical signals to the element resulting in Joule heating thereof and thus in the generation of a pressure wave via the thermoacoustic effect, the pressure wave including heterodyne components of the first and second input electrical signals.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 and 1 a are diagrammatic representations illustrating a heterodyning arrangement in accordance with an embodiment of the invention; and

Figure 2 is a graph illustrating an output achieved using the arrangement of Figures 1 and 1 a. Referring firstly to Figure 1 , a heterodyning arrangement in accordance with an embodiment of the invention is illustrated. The arrangement comprises a graphene material element 10 upon which are provided input and ground terminals 12, 14. Each of the terminals 12, 14 is of comb-like form, each including a series of generally parallel teeth 12a, 14a, the teeth 12a of the input terminal 12 being interconnected by a common region 12b, and the teeth 14a being interconnected by a common region 14b. The teeth 12a are interleaved or interdigitated with the teeth 14a. Such an arrangement is advantageous in that the resistance of the arrangement may be relatively low.

The common region 12b of the input terminal 12 is connected, via a suitable electrical lead (not shown) to the outputs of a pair of signal generators arranged to output respective electrical signals. The two electrical signals are intended to be received simultaneously, in use. The common region 14b of the ground terminal 14 is connected to ground.

It is well known that the application of electrical currents to conductive materials results in Joule heating thereof. In many applications, steps are taken to minimise the level of heating that occurs, and to dissipate heat generated in this manner. However, in the arrangement of the invention the Joule heating effect that arises from the application of an electrical current to the graphene element 10 through the application of the electrical signal to the input terminal 12 (and the connection of the ground terminal 14 to the element 10) is used in the formation of a heterodyne output. Graphene, having a very high thermal conductivity and a low heat capacity, is particularly suitable for use in applications in which thermoacoustic effects are to be utilized. The Joule heating of the element 10 results in the formation of pressure waves in the air surrounding the element 10, the pressure waves containing heterodyne components of the two input audio signals.

The sound pressure in air resulting from Joule heating can be represented by the equation where P is the power, f is the frequency of the input signal, r is the distance from the element 10, and ε is related to the thermal effusivity of the air relative to the total effusivity of the system. It will be appreciated, therefore, that the pressure is related to the signal power and to the frequency of the input signal.

Where, as in the arrangement of the invention, an element 10 is driven using a signal made up of two (or more) component signals that may be of different frequencies, f _A and f _B , the power dissipated through the Joule heating effect can be approximated as

P = Po + P2A + PiB + PA-B + PA+B where P, represents the power at frequency f,. It will be appreciated that the second and third components of this expression are related to the second harmonics of the input signals and that the final two terms of the above expression are related to the heterodyne components of the mixed input signals. Accordingly, the pressure waves resulting from the application of the input signals to the element 10 and the accompanying Joule heating will include components at these frequencies.

As shown in Figure 1 , a sensor 16 sensitive to the air pressure in the vicinity of the element 10 is provided. The sensor 16 conveniently takes the form of a microphone.

Figure 2 illustrates, diagrammatically, the output signal 18 from the sensor 16 where the first and second input signals are at frequencies f _A and f _B . It is clear from Figure 2 that peaks occur at the frequencies 2f _A and 2f _B , these peaks being indicative of the second harmonics, as mentioned above. Furthermore, a peak arises at the frequency f _A+ B, indicative of one of the heterodyne components. Although not shown, a peak representative of the other of the heterodyne frequencies would also exist.

Clearly, therefore, the arrangement shown in Figure 1 serves as a heterodyning arrangement, producing an output in the form of a heterodyne mix of the input signals. Whilst described as producing a heterodyne mix of two input signals, the arrangement may be used, if desired, in producing a mix of a greater number of inputs. Furthermore, it is clear that the methodology described hereinbefore may be used in the production of a heterodyne mix. Whilst the graphene element 10, terminals 12, 14 and sensor 16 may take a wide range of forms, in one example arrangement, the graphene element 10 is of monolayer form of generally square shape, having sides of length 6mm. The graphene element 10 is located upon a Si0 ₂ or quartz substrate. The sensor 16 takes the form of a condenser microphone. The graphene element 10 and terminals 12, 14 may be of back gated or top gated form. The fabrication of one example of a back gated arrangement involves transferring a CVD grown monolayer of graphene to a degenerately doped silicon substrate coated with 300nm silicon dioxide. 50nm thick gold terminals 12, 14 are formed in the desired shape by thermal evaporation directly onto the graphene. A thin layer of Cr may be used to anchor extensions of the terminals 12, 14 to the substrate surface. The spacings between the interleaved or interdigitated parts of the terminals 12, 14 are preferably in the region of 100-200μιη.

The heterodyning arrangement of the invention may be employed in a wide range of applications in which it is desired to produce a heterodyne mix of two or more input signals. By way of example, it may be employed in a number of telecommunications and signal processing applications. Other applications in which the arrangement may be used include as a switch suitable for use in supercomputing or the like technologies, or in acoustic metrology, gas sensing and in chemical spectroscopy technologies.

The arrangement of the invention is advantageous compared to known heterodyning arrangements in that the arrangement is linear. The arrangement further allows the user a great deal of control over the output mixed signal produced in that the amplitude of the output mixed signal scales linearly with the amplitudes of the input signals, and these can readily be controlled independently of one another.

If one of the input signals takes the form of a non-time varying, DC signal, then the mixed output will have the same frequency as the other of the input signals, plus the heterodyne outputs, but has an amplitude that is linearly related to the amplitude of the input DC signal. By controlling the amplitude of the input DC signal, it will be appreciated that the amplitude of the output mixed signal can be readily controlled.

In the arrangement described hereinbefore, the element takes the form of a monolayer graphene element. It will be appreciated, however, that whilst graphene is a material particularly suitable for use in the invention, a number of other conductive materials may be used without departing from the scope of the invention.

Whilst one specific embodiment of the invention is described herein, it will be appreciated that the invention is not restricted to the specific arrangement described and illustrated, but rather that a wide range of modifications and alterations may be made without departing from the scope of the invention as defined by the appended claims.