GB2105939A - A spectrum analyser and other apparatus for detecting at least one particular frequency component in an electrical signal - Google Patents

Abstract

A spectrum analyser for analysing an electrical signal into its different frequency components, first converts the signal into surface acoustic waves. These surface acoustic waves are separated spatially into different frequency components by, for example, using an interference effect (Fig. 1 not shown) or by using a refraction technique Fig. 4. Referring to Fig. 4 the signal to be analysed is applied to a transducer 16A, 16B and 17, which launches surface acoustic waves along a silicon member 15. Different frequency components of the surface acoustic waves are focussed by a dispersive lens 22 onto respective different focal spots 18 and generate voltages in a piezoelectric layer 20. These voltages are rectified by diodes formed integrally in the member 15 to produce rectified D.C. outputs at terminals f1, f2, f3, and f4. <IMAGE>

Description

SPECIFICATION A spectrum analyser and other apparatus for detecting at least one particular frequency component in an electrical signal.

This invention relates to apparatus for detecting a particular frequency component in an electrical signal. More particularly it relates to a spectrum analyser which of course detects a whole range of different particular frequency components in the electrical signal.

Conventional spectrum analysers mix the signal to be analysed with an internally generated signal whose frequency is swept during the analysing process. The lower side band of the mixed signal is passed through a narrow band filter and the output of this, at different times during the sweep, represents respective different frequency components of the signal to be analysed. This technique is not satisfactory where, the signal to be analysed is presented as a pulse or a series of pulses whose duration is shorterthan the duration ofthe sweep. The invention arose when considering how this problem could be overcome though it is emphasised that the invention is also applicable to non-pulsed signals.

The invention provides apparatus for detecting a particular frequency component in an electrical signal, the apparatus comprising means for converting the electrical signal into mechanical waves and for separating different frequency components of the said waves into respective different spacial positions, and detecting means for detecting the waves at one of said positions.

In a simple form of the invention the detecting means comprises just one detector which detects and measures the intensity of just one frequency component of the mechanical waves and therefore of the original electrical signals. In a more complex form of the invention the detecting means comprises a plurality of detectors arranged at the said respective different spacial positions to detect respective different frequency components. In this latter form the invention can be considered to constitute a spectrum analyser.

Separation of the different frequency components of the mechanical waves, which are preferably surface acoustic waves, can be achieved in many different ways. One possibility is to use an interference effect. Another possibility is to use some form of frequency discriminating deflector. The deflector could be a single frequency discriminating reflector or could be a series of frequency discriminating reflectors. Another possibility is to use a refractive member arranged to focus different frequency components onto the different positions.

The detecting means can take the form of one or more piezoelectric transducers located at one or more than one of said spacial positions. In another form of the invention a relatively high stress concentration at the or each said position causes a change in the refractive index of the medium at that position which therefore acts as a light scattering centre. A read-out may thus be obtained using an optic sensor preferably in conjunction with a light source.

Examples of how the invention may be performed will now be described by way of example with reference to the accompanying drawings in which: Figure 7 is a plan view of a spectrum analyser constructed in accordance with the invention; Figure 2 is a cross-section through the line ll-ll on Figure 1; Figure 3 is a cross-section through the line Ill-Ill of Figure 1; Figure 4 is a view, partly perspective and partly a cross-section of a second embodiment of the invention; and Figure 5 is a cross-section through part of a third embodiment of the invention.

A member 1 (Fig. 1) of material exhibiting piezoelectric and elasto-optic properties carries an input terminal 2 to which a pulsed RF signal, derived for example from a receiving antenna, and having a plurality of different frequency components, is applied. The signal arrives via terminals 2 & 6, alternately in and out of phase at first electrodes 3 of an array of surface acoustic wave transmitting transducers 4D, 4B, 4A, 4C and 4E. In practice many more than the five transmitting transducers illustrated would be employed and it should be appreciated that the illustrated apparatus is shown onlyschemat- ically.

Surface acoustic waves are launched from the transducers along the surface of the piezoelectric member 1 towards a linear receiving area ofthe member 1 located directly beneath a longitudinal light source 7. Considering a point 8 in this receiving area it can be seen that this receives surface acoustic waves in the directions denoted by the dotted lines from transducers 4A, 4B, 4C and 4D. The path differences from point 8 to the transducers 4A, 4B, 4C and 4D are each equal to an amount A1/2 multiplied by an integar (e.g. 0, 1, 2, 3, etc.). Thus, any frequency component of the signal applied to 2 and 6 resulting in surface acoustic waves of wavelength At will cause constructive interference of the surface acoustic waves at the point 8.The rest ining mechanical disturbance at point 8 causes the material of the member 1 at that point to scatter light incident on it in a manner which will be described later.

Referring now to point 9 of the receiving area of the member 1, the path lengths between this point and each of the transducers 4A, 4B, 4C and 4D differ by a value X2/2 or an integral multiple thereof. Thus, constructive interference of components of wavelength A2. occurs at point 9 which then acts as a light scattering point. Similar comments apply to point 10 which is located at a position where constructive interference of the surface acoustic wave occurs for a wavelength of A3.

Located between the light source 7 and the aforementioned longitudinal receiving area containing points 8,9 and 10 is a longitudinal convex lens 11 which focusses light on the receiving area. In the The drawing(s) originally filed were informal and the print here reproduced is taken from a later filed formal copy.

absence of any incoming signal at terminals 2 and 6 this i ght, after passing through the transparent mat eria of member 1, is totally inte1-ceptad by a stop 12 in the form of an opaque line prin'recl on a lower surface of the member 1 as shown in F.ure 2; and is thus prevented from reaching a second linear convex lens 13 and from being focussed onto a linear array of photodetectors shown at 14Ato 141. However, when a received signal causes a point such as point 9 to scatter the light focussed on it, such light is scattered past the edges of the stop 12so as to reach the lens 13 and one of the detectors, e.g. detector 14F associated with point 9.A signal is thus presented at the output of detector 14E indicating the presence of a particular frequency component fE in the input signal. Since the extent to which the point 9 scatters the light incident on it is dependent on the strength of the fE component in the input signal, the magnitude ofthe output of detector 14E is also so dependent.

For simplicity of description, only nine detectors are shown in Figure 2 but, in a practical embodiment of the invention, a very large number of such detectors could be included.

It will be appreciated that the illustrated spectrum analyser will operate irrespective of whether or not the input signal is pulsed. The previously mentioned disadvantage of existing systems is thus eliminated.

The invention can be embodied in many forms other than that illustrated. For example, the whole of the substrate 1 need not be piezoelectric this only being necessary in the regions of the transducers 4A-4D where a non-piezoelectric substrate can be coated with ZnO or some other piezoelectric material, preferably applied over the top of the transducers 4Ato 4D. In this example the substrate still needs to be elasto-optic, unless an elasto-optic overlay (e.g.

AS2S3) is used over region 11.

In another modification instead of having separate transmitting transducers such as shown at 4A-4D a single transducer is placed behind a suitable screen having apertures (in positions like those occupied by alternate transducers of Fig. 1) which behave like discrete coherent sources of the surface acoustic wavec In another modification, instead of using an interference effect to separate the different frequency components ofthe surface acoustic waves, this is done by using a refraction technique as follows.

The transducers 4A-4D are replaced by a single transducer and in insert is placed between the transmitting and receiving transducers, the insert being chosen from material in which surface acoustic waves travel at a different velocity to their velocity in the remainder of the support 1 and which the wave velocity is dispersive (i.e. is a function of wavelength). This insert thus acts as a focussing member or "lens" which focusses the surface acoustic waves onto an area adjacent an array of receiving transducers each of which may be like that shown at 14 in Figure 2. The focal point is different for different frequency components of the surface acoustic waves, in a way analogous to chromatic aberration of an optical lens, so that the different frequency components are sensed by respective different transducers, spaced at different positions along an optical axis.

At the point where surface acoustic waves are brought to a focus or otherwise concentrated on a piezoelectric surface, a varying surface potential arises: and this may be sensed as an alternative to the technique of using an optical system to sense an elasto-optic effect.

Figure 4 illustrates an embodiment of the invention in which the varying surface potential is sensed in the manner just described. A p-type silicon member 15 carries, at one end, interdigitated sets 1 6A and 16B of conductive fingers over which a layer 17 of ZnO, which is piezoelectric, is deposited. The opposite end of the silicon member 15 has discrete n-type spots 18 and each of these has a plurality of p-type islands 19. A layer 20 of ZnO is deposited over the spots 18 and has an earthed conductive layer 20A on its upper surface. Conductors 21, shown schematically, connect the spots 18 to respective output terminals T" T2, T3, and T4.

Between the ends of the member 15 is a lens 22 which is set into the silicon member 15 and is of a material which conducts surface acoustic waves at a different velocity to their velocity on the rest of the member 15 and which is dispersive. In operation, a signal to be analysed and containing different frequency components f1, f2, f3, and f4 is applied across the sets 1 6A and 16B of interdigitated fingers and, because of the piezoelectric properties of the ZnO layer 17, surface acoustic waves are launched along the surface of the member 15, these surface acoustic waves having a spectrum containing the same frequency components f., f2, f3 and f4 and the input signal.

The lens 22 focusses the different frequency components onto respective different focal spots coincident with the spots 18 of n-type silicon. This produces an alternating potential on the lower side of the piezoelectric layer 20 with respect to its earthed upper side. The alternating potential is rectified by the p-n interfaces between the islands 19 and their associated n-type spots 18 so that rectified voltages appear at appropriate terminals T, toT4 depending on the spectrum to be analysed. The magnitude of each voltage depends on the power of the associated frequency component of the input signal.

For a given frequency of the input signal, sayf1, the phase of the voltage generated across the layer 20 is different for adjacent different areas of the associated spot 18. It is for this reason that a number of p-type islands 19 are provided in each spot 18. The linear dimension of each ofthe islands in a direction of propagation of the surface acoustic waves is less than a half of the wavelength and is preferably less than a third of the wavelength. The voltage across the layer 20 is approximately in phase over each of the small islands 19 so that a resultant rectified cur rent passes through each diode interface defined between the p-type islands and the n-type spots.

Another embodiment of the invention, partially shown in Figure 5, is similar to that of Figure 4 but utilises a piezoelectric member 1 5A and therefore does not need a ZnO layer like that shown at 17 on Figure 4. Since the member 15A is not made of sili con, the diodes cannot be integrated into the struc ture of the member 15A. Instead, a separate p-type silicon member 23 is spaced above spots (e.g. spot 24) where the surface acoustic waves of different wavelength are brought to a focus. Above each such focal spot is a spot (e.g. as shown at 18A) of n-type silicon in the otherwise p-type member 23. Islands 1 9A of n-type silicon are arranged in each of these spots, the dimensions of elements 18A and 19A being chosen as for the elements 18 and 19 of Figure 3.

Operation ofthe embodiment of Figure 5 is similar to that of Figure 4 except that the varying potential producing a focal spot such as indicated at 24 induces corresponding potentials across the p-n junctions between islands 19A and spots 18A. Out puts are taken, as before, from terminals e.g. as shown at Ts, T, and T7 connected to respective islands.

In yet another embodiment of the invention a sensing arrangement as shown in Fig. 4 or 5 can be used on a silicon substrate in combination with an array of transducers like that of Fig. 1 overlaid if necessary with a layer of ZnO or other piezoelectric substance.

Claims (13)

1. Apparatus for detecting a particular frequency component in an electrical signal, the apparatus comprising means for converting the electrical signal into mechanical waves and for separating different frequency components of the said waves into respective different spacial positions, and detecting means for detecting the waves at one of said positions.

2. Apparatus according to claim 1 for detecting different frequency components of the electrical signal in which the detecting means is adapted to detect the waves at said respective different positions, separately.

3. Apparatus according to claim 1 or 2 in which the mechanical waves are surface acoustic waves.

4. Apparatus according to claim 1,2 or 3 including different frequency discriminating deflection means for separating the different frequency components.

5. Apparatus according to claim 4 in which the deflection means is constructed so as to refract different frequency components through different angles.

6. Apparatus according to claim 5 in which the deflection means is adapted to focus different frequency components of the waves onto different pos itions.

7. Apparatus according to claim 1,2 or 3 including means for directing said mechanical waves towards said positions from a plurality of points such that different frequency components of said waves interfere constructively at the respectively different positions.

8. Apparatus according to any preceding claim in which said different positions are in a material, a light scattering or transmission property of which is altered in response to said mechanical waves, and in which the detecting-means includes an optical detector designed to detect such an alteration of light scattering or transmission property.

9. Apparatus according to any one of ciaims 1-7 in which said detecting means is constructed to detect a piezoelectric voltage generated by the mechanical waves.

10. Apparatus according to claim 9 in which said detecting means comprises a plurality of diodes arranged to rectify piezoelectric voltages generated by the mechanical waves.

11. Apparatus according to claim 10 and comprising a silicon member along which the mechanical waves travel, the silicon member having a piezoelectric coating in a region of said different spatial positions and the silicon being suitably doped to define said plurality of diodes.

12. Apparatus for receiving pulsed electromagnetic signals comprising a spectrum analyser constructed in accordance with any preceding claim and arranged to analyse received pulses into different frequency components.

13. A spectrum analyser substantially as described with reference to the accompanying drawing and substantially as illustrated therein.