AU605354B2

AU605354B2 - Signal locating apparatus

Info

Publication number: AU605354B2
Application number: AU22113/88A
Authority: AU
Inventors: Christopher James Gamgee; Alan Nickols
Original assignee: Amskan Ltd
Current assignee: Amskan Ltd
Priority date: 1987-09-17
Filing date: 1988-09-12
Publication date: 1991-01-10
Anticipated expiration: 2008-09-12
Also published as: AU2211388A

Description

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RALI1G()o9 PATENTS ACT COMPLETE SPECIFICATION O R I G I NA L (FOR OFFICE USE) CIa s s Int Class 0@

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#11*t I I I I I I I LI I-I I I I I I II I I 14 I It I I I IA 1111 1 1 Application Number: Lodged Complei:e Specification Lodged: Accepted: Publi shed: Pri ori ty: Related Art: This documentl contains the amend 'ments made unde ri Section 49 and is correct for printing. Name of Applicant(s): S, AN..L .MI Addres of pplicnt Cs: MOO2#-A~ 4 e ss j SE /-bcy kqcE PATENT ATTORNEY SERVICES 26 Ellingworth Parade, Box Hill, Victoria 3128 Complete specification for the invention entitled: "SIGNAL LOCATING APPARATUS" The following statement is a full description of this invention, including the best method of performing i t known to us r SIGNAL LOCATING APPARATUS This invention relates to apparatus for locating an input signal within a frequency band. The invention relates particularly to the location of an input signal from a radio transmitter and it will be generally convenient to describe the invention in relation to this particular field of use, however the invention is not limited to this particular type of signal.

The apparatus of the present invention has been developed for use in relation to electronic apparatus for identification of objects including people, animals, and objects e.g. as shown 0 at I0 in EP 0108643. Such an identification system includes interrogator means for generating an interrogation signal. The

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.2a2 apparatus also includes one or more transponders, each of which a, includes an interrogation signal receiver for receiving the interrogation signal and operative in response to transmit a *t radio frequency (RF) reply signal containing coded information I I to enable identification of the particular transponder and its bearer. The apparatus includes an RF receiver for receiving the reply signal to enable decoding of the information.

In the case of such an identification apparatus, a number of transponders would normally be provided and the apparatus is operative to read data transmitted by the transponders. The transponders may generate the coded information signals within a nominal frequency band, e.g. around 400 MHz. As the transmitter frequency of such transponders is controlled for example by tolerances of the components of the transponder which are relatively inexpensive devices manufactured to low tolerances, there is some variation to be expected in the transmitter frequency. For, example variations up to perhaps 0.5 MHz may be expected, la.

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To enable the receiver to identify signals within a band of frequencies, it is possible to make the receiver in such a way as to have a wideband response. However this reduces the sensitivity of the receiver to low power signals which are likely to be encountered in an identification apparatus having low power transponders.

Because low power transponders should be designed to have a maximum useful life, the transponder should not be designed to transmit information signals for relatively long periods of time in order to ensure that the signal receiver detects and decodes the signal.

It is an object of the present invention to provide a signal 1 locating apparatus for locating an input signal within any Sfrequency band.

A preferred object of the present invention is to provide a signal locating apparatus which can relatively rapidly locate an input signal within a frequency band, but with minimum bardwidth to maximise sensitivity. This will enable a relatively short duration signal of unknown Frequency (within a band) to be '0 located.

A A- 4 inva+ ion there is Orovided a

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signal locating apparatus for locating an input sig of a first frequency within a frequency band, the gnal locating apparatus including a signal receiver fE receiving signals in the frequency band including e input signal, the signal receiver being charac ised by having a relatively narrow bandwidth res e. of variable centre frequency, the signal receiv including a detector operative to generate a detected ^<Tlgna1 on a -signalin whe 8n.th cent re frequeny-is- According to the present invention there is provided a signal locating apparatus for locating an input signal of a first frequency within a frequency band, the signal locating apparatus including a signal receiver for receiving signals in -the frequency band including the input signal, the signal receiver being characterised by having a narrow bandwidth response of variable centre frequency within said frequency band, the signal receiver including a detector operative to generate a detected signal on a signal line when the centre frequency is substantially equal to the first frequency, the signal locating apparatus further including search means for Svarying the centre frequency of the signal receiver and o00 operative to cause the signal receiver to search for the presence of the input signal within the frequency band, the search means including an integrator which, after the application of an initialisation signal to the search means and oin the absence of the detected signal, is operative to commence S:to generate an integrator output signal, the integrator output signal having a progressively varying characteristic, the signal receiver being responsive to the progressively varying integrator output signal to vary the centre frequency thereof, the search means being operable in response to occurrence of the detected signal on the signal line to stop the variation in the integrator output signal and hence stop the variation of the centre frequency and nence stop the search for the presence of the input signal.

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substantially equal to the first frequency, the signal locating apparatus further including search means for varying the cen e frequency of the signal receiver at a relatively high ra so as to cause the signal receiver to search for the pre nce of the input signal within the frequency band, t> search means including an integrator operative to ge rate an integrator output signal, the integrator output signal having a varying characteristic after the applicati of an initialisation signal thereto and in the absence o the detected signal, the signal receiver being responsiv eto the varying integrator output signal to vary the ce re frequency thereof, the signal receiver being operable response to occurrence of the detected signal 0 on the sign line to stop the variation in the integrator output gnal and hence stop the variation of the centre fre ency and hence stop the search for the presence of the put signal.

Possible and preferred features of the present invention are illustrated in the accompanying drawings. However it is to be understood that the features illustrated in and described with reference to the drawings are not to be construed as limiting on the scope of the invention.

Fig. 1 shows a block schematic diagram of a signal locating apparatus according to one possible embodiment of the present invention, and Fig. 2 shows typical waveforms for the demodulator and integrator outputs.

The signal locating apparatus illustrated in Fig. 1 is for locating within a frequency band an input signal of a first frequency. The apparatus includes a signal receiver 10 for receiving an input 11 within the frequency band and which includes the input signal sought, the signal receiver 10 having a relatively narrow bandwidth response of variable centre frequency. The first part of Fig. 2 shows the signal receiver output if the centre frequency is swept through the entire band. The signal receiver 10 generates a detected signal 32 (Fig. 2) when the centre frequency is substantially equal to the first frequency. The signal locating apparatus also includes search means 20 for varying the centre frequency at a relatively -4 6, high rate so as to cause the signal receiver 10 to search for the presence of the input signal within a frequency band, the search means 20 being operable in response to generation of the detected signal 32 to stop the searcl for the presence of the input signal. In particular the search means 20 stops varying the centre frequency in response to generation of the detected signal 32.

tt The signal receiver 10 includes an oscillator 13 of variable frequency which determines the centre frequency. The signal receiver 10 further includes a mixer 14 receiving the input 11 ,0 and operative to generate an intermediate frequency (IF) signal 15 if the oscillator frequency is correctly offset from the input signal frequency. The IF signal 15 is then applied to a demodulator 16 (or FM detector), the demodulator 16 generating the detected signal 32 upon receipt of the IF signal 15. If the input signal is absent, the input 11 comprises a background noise signal which is input to the demodulator 16 and this in turn produces at the output of the demodulator 16 a signal 31 (Fig. 2) related to the noise level, this output for example being a voltage level.

The search means 20 is operable to vary the oscillator frequency in order to control the centre frequency. The search means 20 is operable to cause the oscillator frequency to conti nuously vary from one extreme of the frequency band towards the other. For example, the oscillator 13 may be caused to scan for an input frequency from 399.5 MHz towards 400.5 MHz.

The search means 20 includes a n integrator 21 which controls scanning of the centre frequency of the signal receiver 10, the integrator 21 being connected to demodulator output line 12 so A D that the occurrence of the detected signal 32 on the demodulator -output line 12, after an initialisation signal 30 is applied to 8 °the integrator 21 at the beginning of the sweep or scan of the f- 8 efrequency band, causes an abrupt change in the integrator output o 88o(Fig. If the receiver centre frequency is not equal to the first frequency, the output of the demodulator 16 is a first 0 V voltage level 31 resulting from background signal noise, this 8 0 8 8°voltage level 31 at the output of the demodulator 16 being a R applied to the integrator 21 which in turn provides an output rampA 36 on line 23 s9-RigA! 3- which is used to control sweeping 2,q :or scanning of the oscillator frequency. This is achieved by 0 0 applying a reference voltage 29 related to an expected or predetermined level of the detected signal 32. The integrator 21 determines the function: J (VI Vref) dt where Vl is the level of the demodulator output 31 and Vref is the level of the reference voltage 29. The value of this function, and hence the output signal 36 ceLases to ramp as shown

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T in the lower part of Fig. 2 when the detected signal 32 reaches the level of the reference signal 29. This stops the sweeping or scanning of the centre frequency of the receiver 10. Up to the time that the detected signal 32 appears, the integrator output signal 36 is a ramp which causes continuous scanning of the centre frequency. When the integrator output signal 36 stops ramping this can then effectively "lock" the oscillator frequency to the desired first frequency. Any "overshoot" of the centre frequency causes a reversal of the integrator output A0,. 36 which will reduce the centre frequency hack to the input f t first frequency. The demodulator output signal 31 can be passed to output 25 for enabling processing, such as extraction of t• coded information.

The signal locating apparatus in one possible embodiment has an integrator 21 which has a selectively variable time constant so as to be operable as a sweep rate controller operable in response to an initial indication of the presence of the detected signal 32 to vary the time constant of t'he integrator 21 so as to reduce the ramp rate and hence reduce the rate of search. That is, in response to commencement of the detected signal 32, the rate of sweeping of the oscillator frequency is reduced to provide slower more precise "locking" of the signal receiver centre frequency to the first frequency. This can occur in response to the rise of the demodulator output 31 during time irterval At as shown in Fig, 2.

The signal locating apparatus further includes frequency logging means ?2 operable preferably before an input signal locating operation is carried out (although possibly after such an operation) to scan the frequency range and log the background signal data. In this embodiment the apparatus further includes background compensating me an s operable duriny the input locating operation to use the logged background signal data to compensate f or background signal effects and reduce the chance of a spurious background signal being detected and identified as the input signal sought.

Preferably the frequency logging operation is initiated substantially immediately before a time window within which an i nput s ig nal1 i s to be expected and identified. For example in an object identification apparatus, an interrogation signal may be generated and d irected towards a transponder wh ich responds a af ter a known predetermined delay period. In this environment, the l o gg ing operating i s preferably carri ed out immediately t, before the response time window of tetasodr The freq;uency logging means 22 and background compensating 11tmeans may be comprised by a microprocessor 27. T h e microprocessor 27 is arranged to be responsive to the output 4 of the integrator 21 and controls the scan rate of the os,-cillator 13. During the logging operation the microprocessor 23 27 monitors and stores in memory the output -34s of the integrator VI. During th.: input signal search operation, the microprocessor 27 is operative to continuously compare the 23 integrator output -4 to the logging operation data in memory to thereby e na bl1e an increase in selectivity of the apparatus to a g e n uine in p ut si gnal and reduce the risk of a background signal e from some RF source nearby) being identified as the input signal sought.

It will1 be seen that the preferred embodiment of the signal locating apparatus described herein and illustrated in the ,A drawing will relatively quickly scan through a frequency band within which an input signal of a relatively constant but unknown frequency is to be encountered, Thus the sensitivity of the system within which the apparatus is incorporated can be relatively high by enabling a specific frequency to be locked onto. This also enables the system to be used with relatively low power input signals. These features make the signal locating apparatus particularly suitable for use with inexpensive transponders used in an object identification Ssystem.

Claims

1. A signal locating apparatus for locating an input signal of a first frequency within a frequency band, the signal locating apparatus including a signal receiver fuo. r';_-eiving signals in the frequency band including the input signal, the signal receiver being characterised by having a narrow bandwidth response of variable centre frequency within said frequency band', the signal receiver including a detector operative to generate a detected signal on a signal line when the centre frequency is substantially equal to the first frequency, the signal locating apparatus further including search means for Svarying the centre frequency of the signal receiver and operative to cause the signal receiver to search for the presence of the input signal within the frequency band, the search means including an integrator which, after tho application of an initialisation signal to the search means and in the absence of the detected signal, is operative to oommence to generate an integrator output signal, the integrator output Ssignal having a progressively varying characteristic, the signal receiver being responsive to -the progressively varying integrator output signal to vary the centre frequency thereof, the search means being operable in response to occurrence of the detected signal on the signal line to stop the variation in the integrator output signal and hence stop the variation of the centre frequency and hence stop the search for the presence of the input signal.

2. A signal locating apparatus as claimed in Claim 1 wherein, if the receiver centre frequency is not equal to the f irst, frequency, the output of the detector~ on the signal line comprises a first voltage level which is applied to the integrator so that the int 3rator output signal comprises a ramp signal which is used to control sweeping or scanning of the centre frequency of the signal receiver.

3. A signal locating apparatus as claimed in Claim 2 wherein the integrator receives a reference signal related to a level of the detected signal, and the integrator output signal ceases to ramp when the detected signal reaches the level of the reference signal hence stopping sweeping or scanning of the centre frequency of the signal receiver.

4. A signal locating apparatus as claimed in any one of Claims 1 to 3 wherein the integrator has a variable time constant so as to be operable as a sweep rate controller, the integrator being operative in response to an initial indicatio3n of the presence of the detected signal to vary the time constant of the integrator so as to reduce the ramp rate and hence reduce the rate of chsango of the centre frequency.

S. A si-jnal locating apparati.2 aa claimed in any one of the preceding claims and further including frequency logging means operable to scan the frequency band and to log background sighal data, the signal locating apparatus further including background compensating means operable, during an operation of the apparatus for the purpose of locating an input signale to use the logged background $signal data to compensate for background signal effects and reduce the chance of a spurious background signal being detected and identified as the input signal sought.

6, A signal locating apparatus as claimed in claim 5 wherein the apparatus is associated with a trans~ponder which resp~onds to an interrogation siLgnal after a predetermineri delay period., the U fr; 'ency logging means being operable to log the background Sig-, data immediately before a time window occurring after said delay period.

7. A signal locating apparatus as claimed in Claim 6 wherein the frequency logging means and background compensating means are comprised by a processing means connected and operative in response to the output of the integrator, the processing means controlling in use the Scan rate of the signal receiver, the processing means being operative during a background logging operation to monitor and store in a memory as logged data the output of the integrator, the processing means being operative, during an operation of the apparatus to search for the presence lo i of an input signal to compare the integrator output to the logged data stored in memory during the background logging operation and to compensate for variations in the integrator output resulting from background signals so as to thereby enable an increase in selectivity of the apparatus to a genuine input signal and reduce the risk of a background signal being identified as the input signal sought.

8. A signal locating apparatus as claimed in any one of the' preceding claims wherein the signal receiver includes an oscillator of variable frequency which determines the centre frequency, the signal receiver further including a mixer receiving the signals in the frequency band including the input signal and operative to generate an intermediate frequency signal if the oscillator frequency has a predetermined relationship to the input signal frequency, the detector comprising a demodulator connected to receive -the intermediate frequency signal and operative to generate the detected signal /,,upon receipt of the intermediate frequency signal. r i

9. A signal locating apparatus as claimed in Claim 8 wherein if the input signal is absent, a background noise signal is input to the demodulator, the demodulator as a result producing a noise related signal output related to the background noise signal.

A signal locating apparatus as claimed in Claim 8 or 9 wherein the oscillator frequency is variable under the control of the integrator so as to cause the frequency of the oscillator to continuously vary from one extreme of the frequency band 10 a.19 ct a 4 O a a a: 9* 4 t a a, t t at towards the other.

11. A signal locating apparatus substantially as herein before described with particular reference to the accompanying drawings. Dated this 28th day of August, 1990 PATENT ATTORNEY SERVICES Attorneys for AMSKAN LIMITED A, 4 Si I~ IP MyT c!