Electronic Surveillance System

Abstract

An electronic surveillance system in which a passive label is attached to goods to be placed under surveillance and the label is interrogated by electromagnetic signals from a transmitter, the label acting to transmit a reply signal to a receiver which gives a characteristic response in the presence of a label, the transmitter being arranged to transmit signals, preferably two, at widely different frequencies and the label acts to mix these frequencies to produce a reply signal which is distinct from the original transmitted signal. The receiver is provided with a signal processing system which analyses and compares characteristics of the transmitted signal and the reply signal and produces different responses in the presence or absence of a label.

Description

In certain electronic surveillance systems, for example those devoted to the control of merchandise in shops and warehouses by the extraction of information from prepared passive labels by electromagnetic interrogation.

The basic principle of operation of any interrogating system for passive labels, is as follows: Energy is some form is transmitted to the label by a transmitter and transmitting antenna unit. This energy is then processed in some way by the label, and the resulting energy retransmitted by the label as a "reply" signal. This "reply" energy is then detected, suitably processed and information extracted therefrom by a sensitive receiver and receiving antenna unit. It is basic to all interrogation systems that the very small reply energy from the label be distinguished from the very much larger transmitter or "interrogation" energy, and provision must be made in the design of the system to enable it to distinguish unwanted responses from the desired reply signals. These undesired responses are of two main kinds:

A. Unwanted responses from labels located outside the scanning area which are accidentally interrogated by the system,

B. Spurious responses from naturally occurring objects, located inside the scanning area, which produce signals capable of being confused with the desired label response.

Each of these two problems may be dealt with by the techniques discussed in general terms in turn below, and described in particular form later in this document. The general form of the surveillance system to which the invention relates is shown in FIG. 1. The principal components are a set (perhaps only one) of transmitting units 1, a coded label 2 located inside the scanning area 3, and a set (perhaps only one) of receiver units 4, which detect and process the output signals from the label. The receiver units 4 also contain whatever signal processing systems are needed to distinguish between genuine reply signals and undesired signals. Certain direct interconnections (shown as a solid line in FIG. 1) between the transmitter and receiver units may be necessary to enable the receiver to perform this function.

The elimination of the first class of undesired responses described above involves the restriction of the area scanned by the transmitter - receiver system to the required region There are four basic principles on which this restriction may be based, any number of which can be used in combination.

1. Use of high frequency radiation for some of the information carrying signals (of which there may be one or several) so that directionally sensitive transmitting and receiving antennas of reasonable proportions became possible.

2. Employ time gating in the receiver adjusted in such a way that the time width and time position of the receiver gate determine, in conjunction with the propogation velocity of the signals being used, a spatial location an extension of the area being scanned.

3. Select at least one of the signal frequencies to be sufficiently low that the scanning area lies in the near field of the transmitter and receiver antennas. The discrimination against distant spurious responses is enhanced by the rapid decay with distance which the near fields possess as compared with propogating fields.

4. Select at least one of the signal frequencies so that the natural attenuation of the propogating medium (generally air) at that frequency contribuites to the discrimination against distant responses. If signals are propogated by electromagnetic means, one such suitable frequency occurs at the oxygen molecular resonance absorption band at about 60 GHz.

An important difference between the last two methods and between those methods which employ the natural decrease with distance of propogating fields lies in the mathematical form of the attenuation loss encountered. In the propagating case the fields undergo a l/r decrease with distance, in the near field case a l/r.sup. 3 or higher power law may be achieved depending upon complexity of the antenna system, and when attenuation in the propogating medium is made use of, the fields decay exponentially with distance. This last effect will always predominate at sufficiently large distances.

In extreme situations when a large degree of discrimination is required, the shortcomings of one system may be overcome by using several of these principles in conjunction as their characteristics are in a sense complementary. For example a highly directive microwave antenna pattern may be disturbed by reflections from objects or persons moving through the scanning area, but a low frequency signal for which the scanning area is in the near field region is not so disturbed.

For the discrimination against spurious responses produced by naturally occurring objects sited within the scanning area, the general approach is to use a combination of propogation means to the label, physical processes within the label, and subsequent signal analysis techniques which are unlikely to be duplicated in nature. A general discussion of the various means and combinations is included immediately below.

The sensing signals may be carried between the transmitter and the label, and from there back to the receiver by any combination of the following means.

a. Electromagnetic radiation at wave lengths comparable with or less than the dimensions of the scanning region, which therefore produce both electric and magnetic radio frequency fields in that region.

b. Electromagnetic radiation at much longer wave lengths designed to produce magnetic fields with negligible electric field in the scanning region.

c. Electromagnetic radiation at long wave lengths arranged to produce electric fields with negligible magnetic fields in the scanning region.

The sensing signals, once they have been received by the label, may be processed in several ways by making use of the following physical processes.

a. Frequency selective transmission of energy in its various forms by the use of resonant systems.

b. Harmonic and subharmonic frequency generation.

c. Generation of new frequencies by mixing techniques.

d. Creation of unusual and possibly anharmonic frequency or part time patterns in the response produced by the label.

The signals which reach the receiver can be analyzed in various ways. Two techniques which are important in this connection are:

a. Synchronous detection keyed, both in carrier phase and modulation envelope phase to the several kinds of signal transmitted.

b. Use of automatic gain control derived from signals in some of the receiver channels to control the gain of others, in such a way as to compensate for variations in the transmission losses between the labels and the transmitter and receiver antennas. Both these techniques are used in the particular embodiment of the present invention described below.

An electronic surveillance system having means to transmit electromagnetic signals, a passive label for attachment to an article to be placed under surveillance, the label having means to receive a signal from said transmitting means and transmit a reply signal and a receiver to receive and process said reply signal, the system being characterized in that the transmitting means is constructed and arranged to transmit signals simultaneously at widely spaced frequencies, the label has means to receive signals at said frequencies, means to mix the frequencies and means to transmit a reply signal produced by mixing those frequencies which is distinct from the original transmitted signal and the receiver having a signal processing system responsive both to the original transmitted signal and the reply signal and mean to analyze characteristics of the reply signal by reference to characteristics of the transmitted signal to produce different responses in the presence or absence of a label.

The invention further consists in a label for use in a system as defined above.

In order that the invention may be better understood and put into practice preferred forms thereof are hereinafter described, by way of example with reference to the accompanying drawings in which:

FIG. 1 is a block diagram showing the main elements constituting an electronic surveillance system of the kind with which the present invention is concerned,

FIG. 2 is a block diagram illustrating a first system according to the invention,

FIG. 3 illustrates the constructional features of a label for use in the system,

FIG. 4 is a block diagram illustrating a second system,

FIG. 5 is a block diagram illustrating a third system and

FIG. 6 illustrates the constructional features of a label for use in the third system.

The general form of a first surveillance system according to the present invention with a high degree of rejection of unwanted responses from prepared labels located outside the scanning area, and from spurious signals produced within the scanning area by means other than the labels, is shown in FIG. 2.

The principle components comprise:

1. Main transmitter and antenna system operating at a carrier frequency of 915MHz, a peak power output of 10 watts, a pulse length of 10/.mu. sec and a pulse repetition rate of 5,000 pulses per second.

2. An auxiliary transmitter and magnetic coil antenna system operating at a carrier frequency of 100 KHz, a pulse length of 1 m sec, and a pulse repetition rate of 93 per second.

3. A prepared label which contains the following elements

A. A strip line microwave antenna which is capable of operating at frequencies of 915 and 1,830 MHz.

B. A microwave semi-conductor diode capable of frequency doubling and frequency mixing.

C. A magnetic loop antenna operating at 100 KHz which receives energy from the auxiliary transmitter and couples this to the diode.

These elements and the details of their interconnections are shown in FIG. 3. The two sections of the antenna A are separated by a thin di-electric film in the region where they overlap. The resulting capacitance provides isolation at the 100 KHz frequency and an rf bypass at the 915 MHz frequency.

4. A receiver system tuned to 1,830 MHz with a 300/.mu. pass band, and a high level of rejection of both the 915 MHz and 100 KHz carrier frequencies.

5. An AGC system which controls the receiver gain. This system accepts signals from the receiver and from the two transmitters and acts only on the signals received while the main transmitter is on and the auxiliary transmitter is off. The function of the AGC system is to bring the 1,830 MHz received signal to a standard level, in order to compensate by changes of receiver gain for variations in the propogation path losses between the transmitter, the label, and the receiver.

6. A signal analyzer system. This unit processes the output of the receiver, and has the task of distinguishing genuine from spurious responses. The signal analyzer processes only those signals which are produced by the receiver while the auxiliary transmitter pulse is on. During these periods, the output from a label will contain the normal 1,830 MHz second harmonic signal, as well as the 1,830 .+-. 0.1. MHz sidebands of this signal in a substantial proportion. The signal analyzer examines the receiver output and passes as genuine those responses which contain the sidebands in sufficient proportion.

Because the system employs a 100 KHz signal in the near field region, it discriminates well against responses from coded labels located outside the scanning region. It is unlikely that naturally occurring objects in the scanning area, such as non-linear magnetic materials, will possess in sufficient degree all the characteristics needed to produce by accident an acceptable response. The essential characteristics are:

a. Coupling to electromagnetic fields at 915 and 1,830 MHz

b. Coupling to magnetic fields at 100 KHz.

c. Harmonic generation and frequency mixing at microwave frequencies.

The system may be varied by constructing the receiver to detect simply the 100 kc/s sidebands on reradiated 915 Mc/s carrier. Such a system would avoid the added losses associated with frequency doubling in the label to 1,830 Mc/s. On the other hand it would have the disadvantages of combining fewer processes unlikely to be duplicated in nature in the label also receiver design would have to reject the unmodulated 915 Mc/s carrier either direct or reradiated; such a system is described in more detail below.

Potential problems arising from variations in transmission path losses at the microwave frequencies are avoided by the use of an AGC system. There are no significant variations in the propogation path loss at 100 KHz which can be introduced by the interpolation of common body, clothing, or packaging materials.

The general arrangement of the second form of the invention is shown in block diagram form in FIG. 4. The principal components are:

1. A microwave transmitter and antenna system 15 operating at a frequency of 915 MHz, a peak power level of 10 watts, a pulse length of 10/.mu. sec and a pulse repetition frequency of 1 MHz.

2. An auxiliary transmitter and magnetic coil antenna system 16 operating at a frequency of 100 KHz with a power level of 10 watts delivered to the internal losses of the coil.

3. The coded label 17 which is the same label as used for the first form of the invention, and is shown in FIG. 3. 3 .times. 10.sup. 8

4. A receiver system 18 which is tuned to receive the 915.1 MHz sideband generated by frequency mixing between the two transmitted signals, and which employs high selectivity to reject the 915 MHz carrier. The receiver can employ the technique of synchronous detection to advantage to achieve a high sensitivity and a law noise bandwidth. The direct connections shown in FIG. 4 from the two transmitter units to the receiver carry the phase information which makes this possible.

The signal processing in the receiver consists in part of ensuring that sufficient side band energy exists in a suitably narrow band width centered on the sum of the two transmitter frequencies before a response is considered genuine Further signal processing which is designed to diseniminate against responses from labels located outside the scanning area is implemented in the receiver by incorporation of a time gate, adjusted in time position in relation to the microwave transmitter gate, so as to define in conjunction with the velocity of propogation of electromagnetic signals (3.times.10.sup.8 meter sec .sup.-.sup.1) a definite spatial volume from which the responses will be accepted by the receiver.

The general arrangement of the third form of the invention is shown in block diagram form in FIG. 5. The principle components comprise:

1. A microwave transmitter and antenna system 19 operating at a carrier frequency of 915 MHz, a peak power output of 10 watts, a pulse length of 250/.mu. sec and a pulse repetition frequency of 400 KHz.

2. An auxiliary transmitter and magnetic coil antenna system 20 operating at a CW frequency of 100KHz with a power of 10 watts, delivered to the internal losses in the coil.

3. A prepared label 21, shown in more detail in FIG. 6 containing the following elements:

a. A single turn magnetic dipole antenna 24 which receives the microwave pulses from the transmitter.

b. A surface accoustic wave delay line 25 which produces after a time of approximately 500/.mu. sec an electroaccoustic echo, of each transmitter pulse, which is reradiated by the microwave antenna.

c. A modulation diode 26 connected in parallel with the delay line, which can amplitude modulate at a frequency of 100 KHz the return signal from the delay line.

d. A multiturn magnetic antenna 27, designed to receive the 100 KHz signal but whose inductame is sufficiently large that it produces a negligible admittance across the delay line at the microwave frequencies.

4. A time gated receiver and signal processor system 22 which receives the modulated echo from the label 21 as well as reference signals directly from the two transmitters. The time position of the receiver gate is set in relation to the transmitted pulse envelope so that the receiver responds only to the electroaccoustic echo signal, and has a high degree of disenimination against the transmitted signal, part of which will unavoidably be present in the receiver antenna. A high degree of isolation in the receiver gate is required.

The significant signals received by the receiver system 2 consist of:

1. Pulses of 915 MHz energy retarded by the 500/.mu. sec time delay provided by the surface wave delay line.

2. Pulses of 915.1 MHz and 914.9 MHz energy, resulting by the modulation provided by diode 26, which however, are delayed by the same 500/.mu. sec as are the carrier echo pulses in (1) above.

The essential signal processing functions performed by the receiver consist of measuring the absolute and relative proportions of the return signals described above. Responses for which the relative magnitudes of all signals fall simultaneously within their respective preassigned acceptance levels are regarded as genuine.

All three forms described above of the invention involve the use of two widely separated frequencies in the transmitted signal, which combined with the use of signal analysis technique in the receiver, provides both improved definition of the scanning area (over the prior art), and also detailed identification of the characteristics of the non-linear elements in the coded label as a means of distinguishing between genuine and spurious responses from objects located within the scanning area.

Particulars of the actual circuits used in the various parts of these systems are not given in the interests of brevity and clarity as the design of the circuitry involved in conventional and obvious to those skilled in the art.

Claims

We claim:

1. An electronic surveillance system, comprising electronic interrogating means for transmitting interrogating signals, passive means attachable to an article under surveillance for sensing the interrogating signals and retransmitting reply signals, receiver means for responding to the reply signals, said interrogating means including zone defining means for establishing a surveillance zone outside of which said passive means and said receiver means are together substantially unresponsive to the interrogating signals, said zone defining means including first transmitter means for transmitting a first portion of the interrogating signals at a first frequency at which the first portion of the signals decays rapidly, said interrogating means including second transmitter means for transmitting a second portion of the interrogating signals at a second frequency substantially higher than the first frequency, said passive means including signal mixing means for forming a reply signal so that the reply signal includes beat frequency components, said passive means being sufficiently small to be attachable to an article under surveillance and to form a label on the article, said passive means including an antenna, said second frequency being sufficiently high to be coupled to said antenna and to allow coupling of said antenna to said receiver means, said receiver means including discriminator means for responding substantially only to a beat frequency component.

2. A system as in claim 1, wherein said receiver means is tuned to frequencies including a beat frequency and at least one of the fundamental and harmonics of the second frequency.

3. A system as in claim 1, wherein said second frequency exceeds said first frequency by at least two orders of magnitude.

4. A system as in claim 2, wherein said second frequency exceeds said first frequency by at least two orders of magnitude.

5. A system as in claim 1, wherein said receiver means is tuned over a passband including a beat frequency and one of the fundamentals and harmonics of said second frequency.

6. A system as in claim 1, wherein the second frequency exceeds the first frequency by at least three orders of magnitude.

7. A system as in claim 1, wherein the second frequency is of the order of 1,000 MHz and the first frequency is of the order of 0.1 MHz.

8. A system as in claim 1, wherein said passive means includes harmonic generating means for producing a second harmonic of the second frequency and a beat frequency of said second harmonic and said first frequency.

9. A system as in claim 8, wherein the second frequency exceeds the first frequency by at least three orders of magnitude.

10. A system as in claim 1, wherein said discrimination means produces a response only when a beat frequency has a predetermined strength in relation to the strength of said second frequency.

11. A system as in claim 10, wherein the higher frequency is of the order of 1,000 MHz and the lower frequency is of the order of 0.1 MHz.

12. A system as in claim 1, wherein one of said transmitter means transmits the signals in the form of pulses, and wherein said receiver means includes a synchronized detector, and connecting means connecting said one of transmitter means to carry phase information from said one of said transmitter means to said synchronous detector.

13. A system as in claim 11, wherein said passive means includes time delay means.

14. A system as in claim 1, wherein said passive means includes a first antenna responsive to signals of the first frequency, a second antenna responsive to signals of the second frequency, said second antenna having a first part and a second part, a capacitance coupling said first part to said second part and providing isolation of the first frequency and a bypass at the second frequency, and a non-linear circuit element connected between the two parts of said second antenna.

15. A system as in claim 1, wherein said passive means includes a first antenna responsive to signals at said first frequency, a second antenna responsive to signals at the second frequency, a surface accoustic delay line connected to said first antenna and arranged to produce an electroaccoustic echo of a signal received by said second antenna after a predetermined delay, and a non-linear circuit element connected in parallel with said delay line and connected to said first antenna.

16. A system as in claim 15 wherein said non-linear circuit element includes a semiconductor diode.