GB2289602A

GB2289602A - Improvements in detection systems

Info

Publication number: GB2289602A
Application number: GB9409450A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1994-05-12
Filing date: 1994-05-12
Publication date: 1995-11-22
Anticipated expiration: 2014-05-12
Also published as: GB9409450D0; GB2289602B

Abstract

A system for the detection of non-conductive articles comprises a tag comprising a semi-conductor diode operatively associated with an antenna which is attached to or embedded in an article to be detected a transceiver including means for transmitting an audio frequency modulated VHF/microwave carrier signal via a filter (18) for removing the harmonic (2 fo) of the carrier frequency (fo), and further including a direct conversion receiver having a harmonic mixer (25) to which received signals are fed via a filter (23) for removing the carrier frequency (fo) and to which a local oscillator signal is fed from the carrier frequency source (21) to provide an output signal at the audio frequency. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO DETECTION SYSTEMS This invention relates to detection systems and more especially, although not exclusively, it relates to detection systems suitable for the detection of buried plastics pipes.

There is a requirement for the detection of plastics pipes or other non-conductive articles such as optical cables for example.

which are buried in the ground or in walls. Detection of such nonconductive articles, even when carrying water or other liquid, in the case of pipes, is difficult if not impossible with presently known techniques especially when a pipe is deeply buried.

It is therefore an object of the present invention to provide a system for the detection of such articles which is simple to implement and relatively inexpensive to put into use.

According to the present invention a system for the detection of non-conductive articles comprises a tag which is attached to or embedded in an article to be detected, the tag comprising a semi-conductor diode operatively associated with an antenna, the system further comprising a transceiver which includes a transmitter having a modulator fed with an audio modulation frequency from a modulation frequency source and fed with a VHF/microwave carrier frequency from a carrier frequency source an audio frequency modulated carrier signal being fed from the modulator to a transmitter antenna via amplifier means and first filter means, which first filter means serves to discriminate against the second harmonic of the carrier frequency in favour of the fundamental of the carrier frequency and which transceiver further includes a direct conversion receiver having a harmonic mixer to which received signals are fed from a receiver antenna via second filter means, which second filter means discriminates against the fundamental of the carrier frequency in favour of the second harmonic thereof and to which harmonic mixer a local oscillator signal is fed from the carrier frequency source, thereby to provide in the presence of a signal at the receiving antenna cor r esponding to the second harmonic of the carrier frequency an output signal at the audio frequency which is fed via audio frequency filter and amplifier means to detection indicator means.

Thus, when the diode of a tag associated with an article to be detected is irradiated by a VHF/microwave modulated carrier signal from the transmitter of the transceiver, due to the nonlinear characteristics of the diode, signals at harmonics of the VHF/microwave signal (and especially the second harmonic) are re-radiated to be picked up by the direct conversion receiver of the transceiver thereby to provide the output signal which is used to operate the detection indicator means.

It should be understood that a characteristic of harmonic mixers, which is exploited by a system according to the present invention, and which is well documented, is that harmonic mixers tend to work in a mixer mode better at the second harmonic of an applied local oscillator frequency than at the fundamental of that frequency and thus direct conversion to audio frequency is facilitated in the present system without the necessity of providing a frequency multiplier or doubler for the carrier frequency which is applied as a local oscillator to the harmonic mixer.

The semi-conductor diode of the tag is preferably a germanium diode.

The tag may comprise a single diode and two elongate dipole elements which comprise the antenna and which are connected to the anode and cathode respectively of the diode.

The tag may be supported on a self adhesive flexible insulative planar substrate whereby it is easily affixed to an article to be detected.

The harmonic mixer may comprise a pair of diodes connected mutually back-to-back in parallel, a choke connected to the parallel diode combination, and a capacitor connected at a junction between the diodes and the choke, the local oscillator signal being applied to the said mixer via the capacitor, the diodes being fed at one end thereof with received signals from the second filter means and being arranged to feed at the other end thereof the audio frequency filter and amplifier via the choke.

Some embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: FIGURE 1 is a somewhat schematic sectional view showing a plastics pipe buried in the ground and tagged to facilitate detection; FIGURE 2 is a perspective view of one kind of tag which may be applied to the plastics pipe shown in Figure 1 to facilitate its detection; FIGURE 3 is a generally schematic block diagram of a transceiver suitable for the detection of tagged pipes; FIGURE 4 is a detailed circuit diagram of a part of the block diagram as shown in Figure 3 which is shown inset and, FIGURE 5 is a plan view of a spiral antenna for use as a transmitter or receiver antenna:: Referring now to Figure 1, as hereinbefore explained, with existing detection apparatus it is very difficult, if not impossible to detect a pipe, such as a plastics pipe 1 as shown in Figure 1, which is buried in the ground 2. In order to facilitate detection however, the plastics pipe is marked by means of special electrically detectable identifying tags 3. 4 and 5, which are carried on self adhesive tape by means of which they have been secured to the plastics pipe 1.

Referring now to Figure 2, wherein a tag is shown in greater detail, the tag comprises a self adhesive flexible plastics tape strip 6 which is used to carny a geranium diode 7. The germanium diode has dipole conductors 8 and 9 electrically connected thereto at its anode and cathode respectively. The dipole conductors 8 and 9 are arranged to have a length which corresponds approximately to a quarter wavelength of the frequency of the signal used to detect the tag and thus the overall length of the tag is approximately half of this wavelength.

In order to facilitate detection of tagged pipes as hereinbefore described, a transceiver may be used as shown in Figure 3.

The transceiver includes a transmitter having a transmitter antennal6 which is fed via an amplifier 17 and a low pass filter 18 with an amplitude modulated signal from a modulator 19. The modulator 19 is fed from a modulation source 20 with an audio frequency modulation signal of about 1 KHz and a carrier frequency from a driver oscillator 21 at a frequency of 870 MHz.

Thus in operation of the transmitter, a carrier frequency of 870 MHz is radiated by the aerial 16, amplitude modulated by an audio frequency of 1 KHz. The design of the filter 18 is arranged such that the second and higher order harmonics are substantially eliminated from the output signal radiated by the aerial 16.

The transceiver further includes a receiver having a receiver antenna 22 which is arranged to feed a bandpass filter 23. The characteristics of the bandpass filter 23 are such that the pass band is centred on the second harmonic of the carrier frequency, i.e. 1740 MHz, the fundamental of the carrier frequency itself, i.e. 870 MHz being rejected. The bandpass filter 23 is arranged to feed an amplifier 24 which in turn feeds a harmonic mixer 25. The harmonic mixer is also fed via a line 26 from the local oscillator with a signal of 870 MHz.

The construction of the harmonic mixer 25 is shown in more detail in Figure 4, which in inset in Figure 3 and wherein the parts of the mixer are enclosed by a broken line 27. The mixer comprises a pair of diodes 28 and 29 which are connected in parallel and back-to-back i.e. the cathode of one diode is connected to the anode of the other diode and vice-versa. The diodes 28 and 29 are connected to a choke 30 and a junction between the diodes and the choke 30 is connected to a capacitor 31.

As shown in Figure 4. the capacitor 31 is connected via the line 26 to the local oscillator 21 and the choke 30 is connected via a line 32 to an audio frequency bandpass filter 33 which is fed from the harmonic mixer 25 an input signal being fed via a line 34 to the harmonic mixer 25 5 from the amplifier 24. The audio frequency bandpass filter 33 is arranged to feed a serially connected amplifier/filter chain comprising an amplifier 35, an audio frequency bandpass filter 36, and a further audio frequency amplifier 37. The amplifier 37 is arranged to feed detection/indicator means comprising a loud speaker 38 which is fed via a still further amplifier 39 and a signal level meter 40 which is fed via a signal level detector 41.

Although any suitable antenna may be used for the transmit and receive antennas 16 and 22, a very efficient antenna comprising an Archimedian spiral is as shown in Figure 5.

As shown in Figure 5. an antenna suitable for transmission and reception purposes comprises a pair of conductors 11 and 12 which are wound together in spaced apart relationship to define a spiral which is supported on a flexible planar insulative substrate 13. At the centre of the spiral defined by the conductors 11 and 12, terminal pads 14 and 15 are formed to which connections are made.

In operation of the system, signals from the transmitter antenna 16 of the transceiver are received by the tags 3, 4, and 5 as shown in Figure 1, which are used to mark the plastics pipe 1.

Due to the non-linear character of the diodes in the tags, signals are radiated therefrom at the second harmonic of the radiated frequency, which second harmonic signals pass through the filter 23 to be mixed in the harmonic mixer 25 with signals from the local oscillator 21.

As hereinbefore explained, it is a characteristic of harmonic mixer design that signals corresponding to the second harmonic of the carrier frequency tend to participate most efficiently in the mixing function and accordingly output signals on the line 32 are produced at baseband which correspond to the frequency of the modulation source 20, i.e. 1 KHz. These audio frequency signals are further filtered by the filters 33 and 36 and amplified by the amplifiers 35 and 37 so as to produce output signals which are detected by the signal level detector 41 to provide an indication on the signal level meter 40 . These are also amplified by the amplifier 39 to produce an output from the loud speaker 38.

It is important to appreciate that by using the harmonic mixer 25 in the receiver and by using direct conversion techniques, wherein the local oscillator 21 is used both for the transmitter and for the receiver functions, a particularly efficient and inexpensive system is afforded. Additionally, it has been found that by using germanium diodes in preference to any other kind of diode, performance is quite surprisingly considerably enhanced whereby detection of quite deeply buried pipes is facilitated, especially if transceiver antennas as shown in Figure 5 are also used.

Claims

1. A system for the detection of non-conductive articles comprising a tag which is attached to or embedded in an article to be detected, the tag comprising a semi-conductor diode operatively associated with an antenna, the system further comprising a transceiver includes a transmitter having a modulator fed with an audio modulation frequency from a modulation frequency source and fed with a VHF/microwave carrier frequency from a carrier frequency source, an audio frequency modulated carrier signal being fed from the modulator to a transmitter antenna via amplifier means and first filter means, which first filter means serves to discriminate against the second harmonic of the carrier frequency in favour of the carrier frequency, and which transceiver further includes a direct conversion receiver having a harmonic mixer to which received signals are fed from a receiver antenna via second filter means, which second filter means discriminates against the carrier frequency in favour of the second harmonic thereof, and to which a harmonic mixer a local oscillator signal is fed from the carrier frequency source, thereby to provide in the presence of a signal at the receiving antenna corresponding to the second harmonic of the carrier frequency an output signal at the audio frequency which is fed via audio frequency filter and amplifier means to detection indicator means.

2. A system as claimed in Claim 1. wherein the semi-conductor diode of the tag is a germanium diode.

3. A system as claimed in Claim 1 or Claim 2. wherein the tag comprises a single diode and two elongate dipole elements which comprise the antenna and which are connected to the anode and cathode respectively of the diode.

4. A system as claimed in any preceding claim, wherein the antennas of the transceiver each comprise a pair of conductors arranged side by side in spaced apart relationship. so as to define a spiral, which spiral is supported on a planar insulative substrate.

5. A system as claimed in any preceding claim, wherein the tag is supported on a self adhesive flexible insulative planar substrate whereby it is easily affixed to an article to be detected.

6. A system as claimed in any preceding claim, wherein the harmonic mixer comprises a pair of diodes connected mutually back-to-back in parallel, a choke connected to the parallel diode combination, and a capacitor connected at a junction between the diodes and the choke, the local oscillator signal being applied to the said mixer via the capacitor the diodes being fed at one end thereof with received signals from the second filter means and being arranged to feed at the other end thereof the audio frequency filter and amplifier via the choke.

7. A system as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.