FREQUENCY DETECTION

Abstract: In a bandpass filter, a first cyclical analogue signal V3 at a centre frequency divided by integer N in divider (44) and the divided signal supplied to a digital filter (48) as its system clock; a second cyclical analogue signal at a frequency to be measured, differing by a small amount from the centre frequency, is delayed by N cycles by a dynamic store (54); a subtractor (56) subtracts the second signal from the delayed second signal and supplies any difference to the digital filter (48); the output of the digital filter is related to the frequency of the second signal, but the centre frequency is suppressed. The filter may be used in an interrogation for an electronic detection system in which an interrogation signal is modified in frequency by a small amount, characteristic of a device to be detected.

Description: FREQUENCY DETECTION This invention relates to a frequency detection system, especially a system capable of measuring frequency shift in a signal which may have undergone a phase shift. One example of the application of such a detection system is in an electronic J 5 identification system comprising an interrogator and one or more non-contact transponders. A signal at a first frequency is radiated by the interrogator, and the non-contact transponder returns in response a second signal at a slightly different frequency; the frequency difference may be small in magnitude and must be measured very accurately. The frequency difference identifies a particular transponder. 10 In some electronic identification systems, for example a system used to control a locked door, or in a supermarket check-out to allow automatic billing of selected goods, the distance between the interrogator and the transponder can be controlled, and the system designed to operate accordingly. Often the transponders are arranged to alter the Q factor ofthe system in a manner characteristic of each transponder, and the change in Q factor is 15 measured. Often the change in Q factor is effected by shorting a coil in the transponder, and such a system is described in our co-pending UK Patent Application No. 9505350.0 filed on 16th March 1995. If however the distance between the transponder and the interrogator is increased, then the influence of a shorted coil may be too small to give a significant change in 20 Q factor. To increase the operational distance of the system, a high frequency back-signalling arrangement is required, as set out in our UK Patent Applications 9508600.5 filed on the 27th April 1995 and 9511085.4 filed on the 1st June 1995. A disadvantage of the arrangement set out in these applications is that the transponder requires an increased energy level which may not be supplied easily. 25 It is a first object ofthe invention to provide a frequency detection system based on a digital filter having a novel input arrangement. It is a second object ofthe invention to provide an electrical identification system, , based on a shorted-coil transponder system, and capable of operating over substantial 4 distances without the need for a high power source. According to the invention a bandpass filter comprises: first supply means to supply a first cyclical analogue signal at a centre frequency fi ; divider means to divide the centre frequency by an integer N and supply the frequency divided signal to a digital filter as the system clock of said digital filter; second supply means to supply a second cyclical analogue signal at a frequency to be measured 2 which differs from the centre frequency by a small amount; delay means to delay the second signal by N cycles; subtraction means to receive the current second signal S c and the delayed second signal Sp and to provide any difference to the digital filter as a difference signal; whereby the digital filter provides an output signal in which the centre frequency is suppressed and which is related to the frequency ofthe second signal. The delay means typically comprises a dynamic store, and the integer N is typically 20. Also according to the invention an interrogator for an electronic detection system comprises a first oscillator arranged to transmit a cyclical analogue interrogation signal at a centre frequency; antenna means to receive a second signal at a frequency differing by a small amount from the centre frequency; delay means to delay the second signal by N cycles where N is an integer; subtraction means to receive the second signal and the delayed second signal and to provide any difference as a difference signal to a digital filter; and frequency division means to divide the centre frequency signal by an integer N and supply the divided frequency to the digital filter as the system clock of said digital filter; whereby the output ofthe digital filter has the centre frequency suppressed and is related to the frequency ofthe second signal. Optionally the divider means is a phase-locked loop comprising a phase discriminator supplied by the first oscillator, a voltage controlled oscillator, and a divide unit. The invention will now be described by way of example only with reference to the accompanying drawings in which :- Figure 1 is illustrates in principle an electronic identification circuit; Figure 2 illustrates in partial block diagram form an interrogator according to the invention; Figures 3a and 3b illustrate changes in current and voltage of the interrogator's antenna; Figures 4a and 4b illustrate respectively the voltage and frequency output ofthe Colpitts oscillator, and Figure 5 illustrates the current and previous signals supplied to the subtraction device. In Figure 1 an electronic identification circuit comprises an interrogator 10 and a passive transponder 12. The interrogator transmits power to the transponder, as indicated at 14, for example at frequency f \ = 150 to 256 kHz, and the transponder utilises the power to operate the circuit, and to reply with an identification signal 16, modulated in amplitude, frequency or phase by known techniques. For example the transponder modifies the frequency ofthe signal by changing the total Q factor ofthe system to frequency {2- Tne interrogator 10 has a transmit antenna 18 and a receive antenna 20. As disclosed in our co-pending Patent Application No. 9505350.0 filed on 16th March 1995, the transponder 12 has a data transmit antenna 22 which is modulated by shorting of its coil, so that the Q factor ofthe interrogator-transponder system varies. The construction ofthe antenna is such that the Q factor is broad and flat, so that detection of a small frequency change cannot be achieved by known techniques. Figure 2 illustrates an interrogator circuit capable of detecting a small frequency change in such circumstances and incorporating a bandpass filter according to the present invention. The primary coil ofthe transmit antenna 18 ofthe interrogator 10 is connected across an inverter circuit comprising two MOSFET transistors 30,32 and each end ofthe coil 18 is also connected respectively to one of two capacitors 36,38. The circuit constitutes a Colpitts oscillator indicated by reference 31 , and the values are chosen so that, contrary to conventional design principles, the oscillator is unstable. The inverter circuit has a large drive capability and therefore a large energy transmission. A signal representing oscillator output, which is referenced f^, V3 in Figure 2, is connected to a phase discriminator 40 the output of which is connected to a voltage controlled oscillator VCO 42. The VCO 42 is connected to a divider 44 which provides the second input to the phase discriminator 40, and also to a digital filter 48. The filter 48 has high internal precision; it may be an IEEE 32-bit floating point filter. The interrogator receive antenna 20 is connected across an anti-alias filter 50 whose output is connected through A/D converter 52 to the input circuit 46 of the filter 48, providing a signal at frequency f2- Figures 3 a and 3 b are, respectively, plots of current through and voltage across the primary coil of antenna 18 on start-up of the circuit; it will be seen that a standard frequency is quickly reached, and currents may be as high as ±50 milliamps. The VCO 42 generates a frequency which is eight times the frequency of signal V3 from the Colpitts oscillator and supplies it to the digital filter 48 as a clock signal. The divider 44 divides this frequency by N for supply to the phase discriminator 40, as indicated at reference V5. The phase discriminator 40 and VCO 42 form a phase-locked loop. On start-up of the Colpitts oscillator 31, the VCO quickly reaches and is then locked to the same frequency. Suppose now that the transponder 12 is modulated to change the Q factor of the system. The modulation is caused by closing and opening a switch in the transponder 12 to short a part of its coil 22, and the effect is illustrated in Figure 4a which shows the voltage across the coil ofthe receive antenna 20; as the switch is closed, the peak-to-peak voltage is unchanged, although the absolute signal magnitude changes; this is in marked contrast to prior art interrogator circuits which detect an absorption of energy and therefore suffer a considerable change in peak-to-peak signals. The effect of one switch closure and opening is shown, i.e. one data bit ofthe transponder. Figure 4b shows the output of a Colpitts oscillator; because it has been made unstable, its frequency follows the frequency transmitted by the transponder antenna 18 and received by the receive antenna 20; there is in fact a small change in frequency, not easily visible in this figure. The VCO 42 typically comprises a capacitor and current injector, and, having no coil, is not sensitive to changes in frequency ofthe signal received from the transponder 10. The VCO operating frequency changes only through the phase-locked loop. The input circuit 46 of the digital filter 48 comprises a dynamic store 54 and a subtract unit 56 whose output is supplied to the filter 48. The dynamic store 54 supplies to the subtract unit 56 both the current value S c of the signal supplied by the A/D converter 52, and also the value of that signal N cycles previously Sp, where N is the integral associated with the divider 44. Typically N is 20. The effect is to provide a bandpass filter, centred on the frequency ofthe signals Sp and S c ; this effect is illustrated in Figure 5. The two signals Sp and S c are identical in frequency but differ in phase. Since a division by 20 has been applied by the divider 44 to the filter clock signal, the signal cycle is repeated every 20 samples. The vector values of the current and previous signals Sp, S c for each pair of samples are given in Table 1. Table 1 Sample Signal S c Signal Sp 0 0.00 0.96 1 0.31 1.00 2 0.59 0.94 3 0.81 0.78 4 0.95 0.55 5 1.00 0.27 6 0.95 -0.04 7 0.81 -0.35 8 0.59 -0.62 9 0.31 -0.83 10 0.00 -0.96 11 -0.31 -1.00 12 -0.59 -0.94 13 -0.81 -0.78 14 -0.95 -0.55 15 -1.00 -0.27 16 -0.95 0.04 17 -0.81 0.35 18 -0.59 0.62 19 -0.31 0.83 20 0.00 0.96 21 0.31 1.00 22 0.59 0.94 23 0.81 0.78 24 0.95 0.55 25 1.00 0.27 It can be seen that the signal relationship is repeated every 20 cycles, so if the subtractor subtracts a signal delayed by a time corresponding to 20 cycles from the undelayed signal, the result is zero. This applies no matter which pair of delayed and undelayed signals are subtracted, and no matter which part ofthe cyclical signal is used. If the signal received from the antenna 20 is identical in frequency to that radiated by antenna 18, the output ofthe subtract unit 56 is zero. In effect, this frequency has been suppressed. Suppose however the transponder 12 sends its characteristic signal, which differs slightly in frequency from the interrogation signal. The signals Sp and S c now slowly drift apart, and the subtract unit signal every 20 samples is non-zero; this signal is supplied to the digital filter 48, which outputs a signal indicating the frequency received by antenna 20, i.e. the frequency associated with the particular transponder, but with suppression of the radiated frequency. The output ofthe digital filter 48 can be connected to conventional signal analysis means (not shown) to indicate the identity ofthe transponder. For example, if the system is used in a supermarket check-out the identity signal may be the related price of an article to which the transponder is attached. To permit identification of a second transponder, the circuit must be reset by conventional means (not shown). The Colpitts oscillator 31 and the VCO 42 lock once more to a transmitted interrogation frequency, and the interrogation, reception and identification process is repeated. In practice the loop filter in the discriminator is disconnected at the same time as an interrogation signal is emitted by the antenna 18. The frequency measurement is made, and the loop filter is reconnected, thus resetting the system. During a measurement of frequency, the VCO 42 remains locked to the original frequency of the Colpitts oscillator 31. The frequency ofthe unstable Colpitts oscillator may in fact change, but the VCO 42 continues to supply a clock signal to the digital filter 48 at the original divider frequency. In effect, the frequency component ofthe VCO oscillator is used and the phase of the signal does not affect the behaviour ofthe digital filter. The electronic identification according to the invention allows rapid identification ofa large number of different transponders. When applied to a check-out ofa supermarket, an entire trolley load of goods may be identified in a relatively short time. In a second application of a bandpass filter according to the invention, the signal emitted by antenna 18 is a radar signal operating on the Doppler principle, for example in a known design of burglar alarm system. Such radars often operate at 120 kHz, and an analogue to digital converter is too slow in operation for detection of a reflected radar signal. A bandpass filter according to the invention is ideal for such an application. The apparatus described in detail above can be directly applicable, with adjustment to operate at appropriate frequencies.

Claims: CLAIMS 1. A bandpass filter circuit characterised by comprising: first supply means (31) to supply a first cyclical analogue signal at a centre frequency (f j ) ; divider means (44) to divide the centre frequency by an integer N and supply the divided signal to a digital filter (48) as the system clock of said filter; second supply means (20, 50, 52) to supply a second cyclical analogue signal at a frequency to be measured (f^) which differs from the centre frequency by a small amount; delay means (54) to delay the second signal by N cycles; subtraction means (56) to receive the current second signal S c and the delayed second signal Sp and to provide any difference to the digital filter as a difference signal, whereby the digital filter provides an output signal in which the centre frequency is suppressed and which is related to the frequency ofthe second signal. 2. A bandpass filter according to Claim 1 in which the delay means comprises a dynamic store (54). 3. A bandpass filter according to Claim lor Claim 2 in which the integer N is 20. 4. An interrogator for an electronic detection system characterised by comprising: a first oscillator (31) arranged to transmit a cyclical analogue interrogation signal at a centre frequency; antenna means (20) to receive a second signal at a frequency differing by a small amount from the centre frequency; delay means (54) to delay the second signal by N cycles where N is an integer; subtraction means (56) to receive the second signal and the delayed second signal and to provide any difference as a difference signal to a digital filter (48); frequency division means (40, 42, 44) to divide the centre frequency signal by an integer N and to supply the divided signal to the digital filter as the system clock of said filter; whereby the output of the digital filter has the centre frequency suppressed and is related to the frequency ofthe second signal. 5. An interrogator according to Claim 4 in which the divide means comprises a phase- locked loop comprising a phase discriminator (40) supplied by the first oscillator (41), a voltage controlled oscillator (42) and a divider unit (44).

Inventors: SCHEELEN JOS, SCHEELEN, JOS
Applicants: BRITISH TECH GROUP INT, SCHEELEN JOS, BRITISH TECHNOLOGY GROUP INTER-CORPORATE LICENSING, SCHEELEN, JOS
Application year: 1996
Main classification: G06K7
IPC codes: G01S13, G06K7, G06K17, G08B13, H04B1