GB2274374A - Receiver - Google Patents

Abstract

The receiver Rx in a communication system includes a magnetostatic wave device 14 which is configured so as to modify signals having an amplitude above a predetermined level with respect to signals having an amplitude below the predetermined level. A transmitter Tx is operative to randomly frequency hop a carrier signal 3 over the bandwidth of a broadband noise signal 6 and, as long as the amplitude of the received signal is above the predetermined level and the average broadband noise level is below it, the former is attenuated less and the signal can be retrieved without any synchronisation between the transmitter and receiver. In another embodiment, a receiver includes a parallel arrangement of two such devices and the transmitter is operative to randomly hop two information carrying signals over a frequency band. The information can then be retrieved by a respective one of the devices. <IMAGE>

Description

RECEIVER This invention relates to a receiver for use in a communication system, in particular to a receiver which can receive a frequency hopped signal without any synchronisation being necessary between the transmitter and receiver.

This invention provides a receiver, for use in a communication system, including a magnetostatic wave device operative to modify signals having an amplitude above a predetermined level with respect to signals having an amplitude below the predetermined level, whereby a modulated signal having an amplitude above the level can be retrieved from accompanying signals each having an amplitude below the level.

Magnetostatic wave devices have the property that, over their bandwidth of operation, signals having an amplitude above a predetermined level can be either enhanced or attenuated with respect to signals having an amplitude below that level. The applicant has appreciated that this property allows them to be used within a receiver as a means of retrieving a hopped signal from accompanying signals, such as broadband noise, even when the total power level in the accompanying signals exceeds that in the desired signal, as long as that signal going into the device has an amplitude above the predetermined or threshold level. The receiver has a great advantage in that no synchronisation is required between the transmitter and receiver - i.e. the receiver does not need to have any knowledge of the manner in which the signal has been hopped.

In another aspect the invention provides a communication system comprising a transmitter operative to hop an information carrying signal over a frequency band to which a broad band noise signal has been added; and a receiver as defined, whereby the information carrying signal can be retrieved from the noise signal.

In yet another aspect, the invention provides a communication system comprising; a transmitter operative to frequency hop two amplitude modulated signals over a frequency band, the two signals being modulated between two respective sets of levels with the sets being disposed on opposing sides of a further level; and a receiver including at least one magnetostatic wave device operative to attenuate received signals having an amplitude above the further level by a lesser or a greater amount than signals having an amplitude below the further level, whereby one of the two modulated signals within the frequency band can be retrieved.

In order that the invention may be well understood, an embodiment thereof will now be described by way of example with reference to the drawings, in which: Figure 1 shows schematically a communication system comprising a receiver according to the invention; Figure 2(a) shows the attenuation transfer characteristics of a magnetostatic wave device configured as a signal to noise enhancer for use in the system of Figure 1; Figure 2(b) shows a typical spectrum of a signal generated by the transmitter shown in Figure 1 entering the magnetostatic wave device; Figure 2(c) shows the spectrum of Figure 2(b) after having been processed by the magnetostatic wave device; Figure 3 shows schematically a communication system according to another embodiment of the invention; Figure 4(a) shows a typical spectrum of a signal being emitted from the transmitter of the system of Figure 3;; Figure 4(b) shows the spectrum after having been processed by one of the magnetostatic wave devices; and Figure 4(c) shows the spectrum after having been processed by the other magnetostatic wave device.

The communication system in Figure 1 comprises a transmitter Tx and a receiver Rx. The transmitter Tx comprises a broadband noise source 1, the output from which is filtered by a base band filter 2. The pass band width of the filter 2 is selected according to the operating bandwidth of the magnetostatic wave device 14 in the receiver Rx which is described later on. A voltage controlled oscillator 3 is driven by the output from the filter 2 so that the oscillator can generate a carrier signal whose frequency hops in a random way over a frequency range corresponding to the pass band of the same magnetostatic wave device 14. An interface 4 supplies data to an amplitude modulator 5 for varying the output power from the oscillator 3 in accordance with the message data.The output from a broadband noise source 6 is filtered by a band pass filter 7 to select a frequency band which corresponds to the operating bandwidth of the magnetostatic wave device and is then passed to an amplifier 8 before being mixed with the frequency hopped signal by a combiner 9. In this way, the transmitter can hop an information carrying signal in a genuinely random way over a spectrum of noise. The combined signal is then fed through a power amplifier 10 before being radiated by an antenna 11.

The receiver Rx includes a receiving antenna 12 connected through an amplifier 13 to a magnetostatic wave device 14 for retrieving the randomly hopped transmitted signal. An amplifier 15 and a broadband diode detector 16 is provided for retrieval of the applied data which is then passed onto further signal processing circuitry 17.

The magnetostatic wave device 14 may comprise a Gadolinium Gallium Garnet (GGG) substrate upon which an epitaxial Yttrium Iron Garnet (YIG) film, or other magnetic material has been deposited. A transducer electrode is provided on top of the film and the device is placed, in use, within a magnetic field. R.F.Coupling occurs between the electrode and the magnetic film over a finite bandwidth (determined by the magnetic field strength) such that, according to the direction of the applied magnetic field, the device can have attenuation characteristics similar to those shown in Figure 2(a), where input signals within the finite bandwidth having an amplitude above 1P1 are selectively attenuated to a lesser extent than signals having an amplitude below that value provided that the smaller signals are separated in frequency from the desired signal by an amount which depends on the magnetic material ferromagnetic linewidth (typically a few MHz). A magnetostatic wave device configured to have the characteristics shown in Figure 2(a) is known as a "signal to noise enhancer".The system is arranged such that the amplifier 13 boosts the signal entering the magneto static wave device 14 to the level shown in Figure 2(a) where the amplitude of the data carrying signal F1 is above the predetermined or threshold level IP1 and the broadband noise signal is at or below this level.

Accordingly, after passing through the magnetostatic wave device 14, the ratio between the amplitudes of signal F1 and the noise signal is greatly enhanced, as is shown schematically in Figure 2(c), and this signal can then be easily retrieved using the simple diode detector 16. In effect, a self centering narrow band filtering action is provided to track and recover the hopped wanted signal, with the filtering bandwidth defined by the ferromagnetic linewidth of the material. This bandwidth is potentially much less than the magnetostatic wave device band of operation and corresponding noise signal bandwidth. The receiver may be used as a means of retrieving the signal from any type of frequency hopping or other transmitter.

Figure 3 shows another embodiment of the invention in which two separate randomly hopped frequency band signals can each be retrieved. As with the Figure 1 embodiment, each channel includes a noise source 11, 12; the outputs from these are filtered by a respective base band filters 21t 22.

Voltage controlled oscillators 31' 32 are driven by the outputs from the respective filters 21, 22. Because separate noise generators are used, the VCO signal frequencies will rarely be coincident within the wide operating range available. Respective interfaces 41 42 supply data to respective amplitude modulators 51 52. Channel 1, as shown, includes an extra amplifier 20 for boosting both a.m. levels of the signal transmitted by that channel above the signal levels transmitted by channel 2. As shown in Figure 4(d), the signals from Channel 1 and Channel 2 are amplitude modulated between levels A and B, and C and D respectively, with the sets of levels being disposed on opposing sides of a further level, V1.

The receiver Rx comprises an antenna 12, amplifier 13 and a parallel arrangement of magnetostatic wave devices 21, 22. The device 21 has the same characteristics as that used in the Figure 1 embodiment and, as shown in Figure 4(b), attenuates signals having an amplitude below IP2 more than signals having an amplitude above that level. The amplifier B is of an automatic gain type and boosts the incoming signals such that signal level V1 aligns with the predetermined or threshold level IP2. Thus the modulation depth of signal 1 can be enhanced through device 21 and the information carried can then be retrieved using a diode detector 161. The other device 22 is configured as a frequency selective limiter having opposite characteristics to the device 21 such that any signals having an amplitude above IP2 are relatively more attenuated or limited to a constant output level.These characteristics can be provided by adjustment of the direction and strength of the magnetic field applied to the device, as is well known in the art.

Alternatively a different device architecture can be used in the form of a magnetostatic wave delay line. The signal from channel 1 is thereby modified to such an extent that, as shown in Figure 4(c), the only time varying amplitude component of substantial size is that due to the signal from channel 2 and this can then be retrieved using the diode detector 162.

In an alternative embodiment, where only one of the channels carries useful information, one or other of the devices 161, 162 can be dispensed with so that the receiver is generally similar to that shown in Figure 1.

It is to be appreciated that this technique could be applied to a system in which more than two signals were transmitted simultaneously and separately recovered. Means other than the diode arrangements could be provided for retrieving the data.

Claims (8)

1. A receiver, for use in a communication system, including a magnetostatic wave device operative to modify signals having an amplitude above a predetermined level with respect to signals having an amplitude below the predetermined level, whereby a modulated signal having an amplitude above the level can be retrieved from accompanying signals each having an amplitude below the level.

2. A receiver as claimed in claim 1 in which the magnetostatic wave device is configured as a signal to noise enhancer operative to attenuate signals having an amplitude below the level more than signals having an amplitude above the level.

3. A communication system comprising a transmitter operative to frequency hop an information carrying signal over a frequency band to which broadband noise has been added; and a receiver as claimed in claim 1 or 2, whereby the information carrying signal can be retrieved from the noise signal.

4. A communication system according to claim 3 in which the information carrying signal is randomly hopped over the frequency band using a source generating a broad band spectrum of noise.

5. A communication system comprising; a transmitter operative to frequency hop two amplitude modulated signals over a frequency band, the two signals being modulated between two respective sets of levels with the sets being disposed on opposing sides of a further level; and a receiver including at least one magnetostatic wave device operative to attenuate received signals having an amplitude above the further level by a lesser or a greater amount than signals having an amplitude below the further level, whereby one of the two modulated signals within the frequency band can be retrieved.

6. A communication system as claimed in claim 5, in which the receiver includes two magnetostatic wave devices disposed in parallel and which are operative to attenuate received signals having an amplitude above the further level by a lesser and a greater amount respectively than signals having an amplitude below the further level, whereby both of the modulated signals within the frequency band can be retrieved.

7. A receiver for use in a communication system substantially as described with reference to the drawings.

8. A communication system comprising a transmitter and receiver substantially as described with reference to the drawings.