GB2298718A - Electromagnetic field detector - Google Patents

Description

ELECTROMAGNETIC FIELD DETECTOR This invention relates to a device for detecting electromagentic radiation, particularly radiation in the microwave region.

Devices using microwaves as an energy source have come into common use in domestic and industrial environments particularly for heating aqueous compositions. Unless such energy sources are carefully screened or surrounded by absorbent material there their use leads to leakage of microwave energy into the environment. When such microwave energy is absorbed by mammalian tissue it may lead to undesirable effects. Strict standards are set for acceptable leakage from microwave energy sources. However in use the leakage prevention systems and interlocks in such sources may deteriorate resulting in the release of unaccpetable levels of radiation.

In GB-B-2 236 593 an electrical signal detector is described which is capable of receiving electromagnetic radiation and providing a warning signal in the event of the received radiation signal exceeding a pre-determined limit. In its preferred form the detector included a metallic screen which enclosed the main functional items apart from the antenna used to receive the radiation.

Electronics Today International, vol. 22, no. 7 (July 1993) describes a field strength monitor for use particularly in the microwave region of the electromagnetic spectrum. The monitor is only capable of detecting such fields and providing a meter reading proportional to the signal strength. It is incapable of providing an alarm signal or for continuously monitoring areas to detect the occurrence of excessive radiation levels.

The present invention provides an electrical detector for receiving electromagnetic radiation in the frequency band 2.2 to 2.6 GHz, especially the region 2.45 GHz, which avoids the use of screening, is tolerant of variations in its detection diode characteristics, has an enhanced audio output and has an integral function test facility.

According to the present invention there is provided a detector for providing a warning of the presence of electromagnetic radiation above a pre-determined power threshold having a signal receiving antenna, a received signal detector, a detected signal amplifier, a signal level detector adapted to provide an alarm signal when the detected signal exceeds a pre-determined level, characterised in that the signal detector comprises a biased diode having a series resistor having a value at least 100 times more than the forward resistance of said diode, a test facility consisting a means to supply sufficient voltage to the received signal detector to cause an alarm signal to be generated, an alarm signal amplifier adapted to supply an amplified alarm signal and that the components are substantially free of metallic screening.

The signal receiving antenna in the preferred embodiment comprises a folded dipole which may be a discrete element or, more preferably, formed in the wiring pattern of a printed circuit board. The dipole, as shown in GB-B-2 236 593, is coupled through a capacitor to a diode which acts as the received signal detector. The diode carries a small forward bias current to improve its operation as a detector. In order to overcome the batch to batch variation among commercially available diodes a resistor is placed in series with the diode. The value of the resistor is high compared with the forward resistance of the diode by at least 100 times and preferably lies in the range 1 kohm to 250 kohm.

As described in GB-B-2 236 593, the output from the signal detector is amplified by a detected signal amplifier whose output is passed to a signal level detector adapted to provide an alarm signal when the detected signal exceeds a pre-determined level. A test facility is provided by means of a switch which supplies an input from the positive power supply rail to the positive going input of the amplifier of sufficient amplitude to ensure that the signal level detector provides an alarm signal. The value of the test voltage can be set to a value that not only confirms functional operation of the circuit elements following the detector but also confirms that supply source, normally a battery, remains at an adequate level.

In the device shown in GB-B-2 236 593 the preferred embodiment includes an enclosure formed from a metallic screening material. It has now been found that detectors of this type function with greater sensitivity if such screening is avoided and the circuits and their associated components are substantially free from such a screening enclosure.

The alarm signal amplifier provides an amplified alarm signal which, in prior devices, has been used to operate an alarm such as a sound generator and/or a visual indicator showing the presence of leakage of electromagnetic radiation. In preferred embodiments the alarm amplifier has been one amplifier in an integrated ciruit containing a plurality of amplifiers. The power available from such a device has been found to be limited and unable to generate the necessary amplitude of alarm signal. In the detector according to the present invention the output from the alarm signal amplifier is used to switch on a normally non-conducting transistor. The transistor power dissipation capability can be selected to provide a high operating current, and hence high volume output, audio generator such as a piezo electric sounder.

In order that the invention may be clearly understood it will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of an electromagnetic radiation detector according to the invention, and Figure 2 is a circuit diagram of a preferred embodiment of the invention.

A detector for providing a warning of the presence of electromagnetic radiation, see Figure 1, consists of a signal receiving antenna 1 whose output is supplied to a received signal detector 2. The output from the detector 2 is passed to a detected signal amplifier 3 followed by a signal level detector 4. The signal level detector 4 provides an alarm signal when the detected signal exceeds a pre-determined level for a preset time. The alarm signal is further amplified by an alarm signal amplifier 5 which operates an audible warning device 6. The detector is supplied with power from a battery 7 which supplies power continuously when in use as a radiation monitor. A test switch 8 allows sufficient voltage from the battery to be supplied to the received signal detector to cause an alarm signal to be generated.Operation of the switch 8 confirms that the circuit elements 3, 4, 5 and 6 are functioning and checks that the battery 7 is capable of providing sufficient power to operate the alarm.

A detailed circuit diagram of a detector according to the invention, see Figure 2, shows a folded dipole 10 whose dimensions are designed, in known manner, to ensure maximum sensitivity in the appropriate region of the microwave band. This is centered on 2.45 GHz in the case of microwave ovens. The received signal is fed via a dc blocking capacitor 11 to a signal detector formed by a diode 12 and a series resistor 13. The detector output is fed to an amplifier 14 having an appropriate feedback resistor network to ensure high gain and to provide suitable biasing of the differential inputs and the diode 12. The output of the amplifier 14 is further amplified by a lower gain amplifier 15. The two amplifiers 14 and 15 form the detected signal amplifier.The output from the amplifier 15 is fed to an amplifier 16 which acts as a driver for a charge pump formed by a series diode and a reservoir capacitor. Microwave energy escaping from ovens occurs in the form of pulses. The charge pump averages such pulses and prevents the emission of alarm signals in the event of single pulses or short trains of pulses generated by lightning or switching transients. The voltage on the reservoir capacitor forms one input to a high gain differential amplifier 17 whose other input is a reference voltage formed by a resistive divider across the power supply. Such a circuit has a virtually dichotomous output whose value depends on whether the input is above or below a threshold value determined by the reference voltage.

The output from the differential amplifier 17 is fed to the base of a normally non-conducting transistor 18. The amplifiers 16 and 17 form the signal level detector. When the output of the amplifier 17 is in its high state, following reception of a train of pulses of electromagnetic radiation, the transistor 18 conducts and power is supplied to a high efficiency piezo sounder 19 which provides a loud alarm signal. The amplifiers 14, 15, 17 and 18 are contained in a single quad op amp integrated circuit element.

The detector is powered by a PP3 type dry battery 20 which provides an output of 9 volts. A test facility is provided by a switch 21 which connects the positive supply rail to one input of the amplifier 14. This has a similar effect to the dipole 10 being in a high field strength radiation area so that the alarm system operates. This test facility provides indication that the amplifiers 14, 15, 16 and 17 are functioning together with the transistor 18 and the piezo sounder 19. The test also indicates that the battery 20 has sufficient power to operate the system.

The connections between the circuit elements and the dipole 10 are alll performed by conducting strips on a printed circuit board. Unlike the system shown in GB-B-2 236 593, there is no radiation screen and the populated board is contained in a small box formed from a synthetic polymer composition.

Claims (9)

1. A detector for providing a warning of the presence of electromagnetic radiation above a pre-determined power threshold having a signal receiving antenna. a received signal detector. a detected signal amplifier. a signal level detector adapted to provide an alarm signal when the detected signal exceeds a pre-determined level. characterised in that the signal detector comprises a biased diode having a series resistor having a value at least 100 times more than the forward resistance of said diode. a test facility consisting a means to supply sufficient voltage to the received signal detector to cause an alarm signal to be generated. an alarm signal amplifier adapted to supply an amplified alarm signal and that the components are substantially free of metallic screening.

2. A detector as claimed in claim 1. characterised in that the resistor placed in series with the signal detector diode lies in the range range 1 kohm to 250 kohm.

3 A detector as claimed in claim 1 or claim 2. characterised in that it includes a switch which supplies a test signal input from the positive side of a power supply to the positive going input of the detected signal amplifier and having sufficient amplitude to ensure that the signal level detector provides an alarm signal.

4. A detector as claimed in claim 3. characterised in that the amplitude of the test signal input is set to a value that not only confirms functional operation of the circuit elements following the detector but also confirms that the power supply is at an adequate level.

5. A detector as claimed in any of the preceding claims.

characterised in that the alarm signal amplifier is a transistor directly connected to the signal level detector and which switches a power supply to a piezo sounder.

6. A detector as claimed in any of the preceding claims.

characterised in that the detected signal amplifier and the signal level detector are each formed from a pair of integrated circuit amplifiers and that the two pairs comprised a quad op amp integrated circuit.

7. A detector as claimed in any of the preceding claims.

characterised in that all the connections between the ciruit elements and the receiving antenna are formed as metal strips on a printed circuit board which is contained in a box formed from a synthetic polymer composition and is free of any radiation screen.

8. Detectors for providing a warning of the presence of electromagnetic radiation as claimed in claim 1 and as herein described.

9. Detectors for providing a warning of the presence of electromagnetic radiation as herein described with reference to the accompanying drawings.