GB2242803A

GB2242803A - Microwave alarm sensor

Info

Publication number: GB2242803A
Application number: GB9002106A
Authority: GB
Inventors: Jeremy Kenneth Arthur Everard; John Philip Chambrie Vigurs
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-01-31
Filing date: 1990-01-31
Publication date: 1991-10-09
Also published as: GB9002106D0

Abstract

A microwave alarm sensor modulates the output of an oscillator 1 with a coded signal and mixes the transmitted and received signals. Back reflected signals occuring within one bit time interval of the code are detected and filtered and spatial movement can be detected by measuring the doppler shift frequency. Signals reflected outside one bit time interval produce a low level signal as on average the bit pattern averages to near zero, thus limiting the range of the sensor. The coded signal may be a single pulse, a pulse train, a pseudo-random code, a Golag code, any noise like code or a noise signal. The modulation may be amplitude, frequency or phase. Bursts of code are transmitted to reduce power. <IMAGE>

Description

MICROWAVE ALARM SENSOR Specification This invention relates to microwave alarm sensors with a well defined range profile. The system can be used to detect signals within a defined spatial range and ignore signals from targets outside this range. This has the advantage of reducing the rate of false alarms caused by for example moving vehicles (cars and lorries) lifts and lift doors and liquids in pipes.

Large metal targets have higher gains than human beings who have gains of order 40 dB therefore simple threshold detection does not give accurate range information.

A microwave alarm sensor which incorporates electronic modulation and signal processing techniques is described where an oscillator which forms the transmitter and also drives the receiver mixer is modulated with a pseudo random or digital noise like code where the modulation can be amplitude modulation or phase modulation or frequency modulation or any combination of these modulation types. Back reflected signals occuring within one bit time interval of the code are detected and filtered whereby spatial movement is detected by using the dopplar shift. Signals reflected outside this time window produce a low level signal as on average the bit pattern coming out of the mixer averages to near zero.

Any form of digital code can be used including pseudo random bit sequences or Golay codes or bursts of codes.

To reduce the power requirements code bursts can be sent out where each burst should usually contain at least one complete sequence. Thus power saving is achieved by keeping the sensor off for most of the time. For example the system may transmit for 1 ms and be off for 20 ms where if the repeat frequency is greater than the maximum dopplar shift then the dopplar shift can be detected directly by band pass filtering. If the repeat frequency is less than than the dopplar freqeuncy then sample and hold detectors can be used.

A block schematic of the system is shown in Figure 1. An oscillator (1) (which could be a Gunn diode or a FET oscillator or a dielectric oscillator) is modulated with a pseudo random bit sequence code herafter called a PRBS code using a code generator (2). The modulated signal is passed through a power splitter with one output going to the aerial (3) to be transmitted and the other path goes to the receiver mixer (4). The return signal caused by a reflecting target is mixed with part of the transmitted signal and passed through a bandpass filter (5) and detector(6). Moving targets will produce offset frequencies due to the Dopplar shift. Signals within a specified velocity range will produce an alarm signal.

The use of spread spectrum code enables high sensitivity with a well defined spatial range profile.

For example a sensor using a code generated by a 10 MHz clock (bit length 100ns) will have a range of approximately 15 metres whereas a 20 MHz clock will produce a range of 7.5 metres. The range is therefore related to the clock frequency by the equation: Range in metres = 150/fin MHz.

The modulation can be switched cyclically between frequencies to check the range of the target.

It is also possible to use the same oscillator diode or FET for the oscillator and mixer at the same time.

Similar techniques could be used for ultrasonic and optical intruder alarms.

The system works because the autocorrelation function of a pseudo random maximal length sequence is shown in Figure 2. The autocorrelation function is 1 for zero delay and -1/m for delays greater than one bit where m is the number of bits in a sequence before it repeats.

The pseudo random bit sequences can be built using a series of shift registers with feedback from certain points using exclusive OR or NOR gates. The full effect of this system is only obtained if the signal is averaged or integrated over one or an integer number of sequences.

Note that the field propagation characteristics superimpose a P 1/P characteristic onto the autocorrelation function or I V 1/r2 where P is the power r is the distance between transmitter and the target and V is the voltage and I is the current out of the mixer.

Note other codes can also be used including Golay codes where the sum of the correlation of two code bursts give 1 for zero delay and zero for delays greater than one bit.

The oscillator can also be modulated with an amplified noise source where the spectrum is shaped using filters. In fact a very noisy oscillator could also be used where a 20 MHz noise bandwidth would give a range of approximately 15 metres.

Where modulation is refered to this can be amplitude modulation including amplitude shift keying or on/off keying or phase modulation or phase shift keying or frequency modulation or frequency shift keying where phase or frequency modulation can usually be achived by incorporating a varactor or varying the power supply voltage. A combination of modulation techniques could also be used.

A working proto-type system is shown in Figure 3 where discrete components are used. A commercial system would often contain many of the components on an integrated circuit. IC's 1 and 2 (74HC164; 8 bit shift registers) and IC 3 (74HC7266; exclusive NOR gates) form a 21 1-1 bit shift register (where other sequence lengths can be produced using different taps). These are clocked by circuit 4.The output of the sequence generator (pin 3 of IC1) is fed into a 14 bit ripple counter (5) where the 210 and 211 and 212 and 213 are anded with pin 3 from IC 1 to produce 1 ouput sequence every 16 sequences. (The system can also be run in CW mode by taking the output (pin 3 of IC 1) directly to the pulse amplifier.) This signal then drives pulse amplifier Q1 and Q2 to drive a microwave module containing either a FET or Gunn oscillator. The microwave mixer diode output which is also driven by the oscillator within the microwave module is fed into Q3 and amplified in opampl and filtered using a fourth order Butterworth Filter in opamp 2 and 3. The output frequency is then dependant on targets moving within the 1 bit range of the alarm sensor. The DC voltage from opamp 1 is also monitored to check for pieces of metal being placed over the alarm sensor to evade detection. Further power can also be saved by switching the complete sequence generators off when they are not required.

Shorter sequences for example of bit length 26-1 or 27-1 or 28-1 or 29-1 or 21 -1 may also provide adequate performance as the gain of a person is approximately 40 dB and the gain of a vehicle is approximately 60 dB therefore a power dynamic range of 20 - 40 dB (100 to 10,000) would probably be adequate where a current/voltage dynamic range of 10 to 100 would therefore probably be adequate.

A single pulse radar where the pulse width defines the range may also be used if the chirp on the oscillator is sufficiently small.

Claims

MICROWAVE ALARM SENSOR

CLAIMS 1. A microwave alarm sensor comprising an oscillator means for modulating the oscillator with a coded signal where the output from the oscillator is split in two where one part is applied to a means for multiplication and correlation usually termed a mixer where the other part is transmitted via an antenna where a back reflected signal from an object is also incident on the mixer via means for reception of back reflected signals where the output of the mixer after passing through means for filtering produces a signal which indicates if there is a reflector within a defined range such that targets within a defmed movement range produce a signal of frequency equal to the Doppler shift and targets within a defined distance range produce a highly correlated large low frequency signal which is detected to show an alarm condition where targets outside the defined range produce low level signals with low correlation which are not detected.
2. A microwave alarm sensor as claimed in claim 1 where the modulation applied to the oscillator can be amplitude or phase or frequency or any combination thereof.
3. A microwave alarm sensor as claimed in claim 1 where the modulated code can be a single pulse a pulse train a pseudo-random code a Golay code or any noise like code.
4. A microwave alarm sensor as claimed in claim 1 in which only bursts of code are transmitted to reduce the current drain on the power supply.
5. A microwave alarm sensor as claimed in claim 1 which includes monitoring of the output signal such that covers placed over the sensor also trigger the alarm.
6. A microwave alarm sensor substantially as described with reference to the accompanying drawings.