GB2273422A - Communication system - Google Patents

Abstract

A communication system for interrogating a pseudo-passive transponder (P.P.T.) employs an interrogator transmitting signals (22, 23) at two different frequencies simultaneously, the relative frequencies of the two signals being controlled so that the difference frequency always has one of two values ( DELTA f0, DELTA f1), each value corresponding to a digital one or a digital zero, and is varied to carry digital data. The two signals are mixed in a diode within the P.P.T. to produce the difference signal and the frequency of the difference signal is then used to obtain the digital data. <IMAGE>

Description

COMMUNICATION SYSTEM This invention relates to a communication system and particularly to a communication system comprising pseudo-passive transponders interrogatable with a frequency modulated signal.

Pseudo-passive transponders are transponders which are interrogated with an R.F. signal and transmit information back to the interrogator by altering the characteristics of the portion of the interrogating signal which the PPT reflects back to the interrogator.

In the past this interrogation has generally been carried out using an amplitude modulated radio frequency (R.F.) signal because a signal of this type can be detected by the PPT using only an antenna and a signal diode as a detector, allowing the PPT to be made very cheaply.

Legislation is now being enacted or proposed in many countries intended to eliminate the problem of electromagnetic interference between different electronic equipment and this legislation generally places limits on the power per unit frequency which equipment can emit. In some instances such legislation makes systems employing pseudo-passive transponders interrogated by a fixed frequency amplitude modulated signal illegal because in order to give a useful interrogation range the power at the A.M. interrogation frequency must be relatively large.

The obvious way to comply with this legislation is to employ a frequency modulated interrogation signal, however this has the drawback that in order to identify the interrogating signal the PPT will need a reference frequency and the cost, power consumption and bulk of a local oscillator to provide such a reference frequency increases the cost and size of the PPTs upwards until their use is no longer economically viable in many applications.

This invention was intended to provide a pseudo-passive transponder interrogated with a frequency modulated signal but not requiring a local oscillator.

This invention provides a frequency modulated communication system comprising a transmitter and a pseudo-passive transponder, the transmitter being arranged to transmit a first and second signal at two different frequencies simultaneously and the pseudo-passive transponder being arranged to receive the first and second signals and to mix them in a non-linear element to generate sum and difference frequencies, the relative frequencies of the first and second signals being such that the generated frequencies carry information.

Systems embodying the invention will now be described by way of example only with reference to the accompanying diagrammatic figures in which; Figure 1 shows the circuitry of a pseudo-passive transponder (PPT) employing the invention; Figure 2 shows the changes in frequency over time of an interrogation signal suitable for use with the PPT of Figure 1; Figure 3 shows a further alternative interrogation signal, identical features having the same reference numerals throughout.

Referring to Figure 1 a pseudo-passive transponder (PPT) 1 is shown. The PPT 1 comprises an antenna 2 linked to an antenna matching network formed by an inductor 3 and two diodes 4 and 5. In use signals received by the antenna 2 pass through the antenna matching network and are applied to a diode 6.

The frequency modulated interrogating signal sent out by an interrogator unit intended to co-operate with the PPT 1 is shown in Figure 2. The interrogator transmits a first reference signal 20 and a second data signal 21, bbth of the signals 20 and 21 are transmitted at a series of different frequencies but the frequency of the reference signal 20 always remains within a first, reference frequency band 22 while the frequency of the data signal 21 always remains within a second, data frequency band 23. The reference and data frequency bands 22 and 23 are both in the radio frequency (R.F.) part of the electromagnetic spectrum.

When an interrogator is near the PPT 1 the antenna 2 picks up the reference and data signals 20 and 21 and these pass through the antenna matching network 3, 4, 5 to the diode 6. In the diode 6 the two signals 20 and 21 mix and generate signals at their sum and difference frequencies because the diode 6 is a non linear device. Thus the output from the diode 6 comprises the signals 20 and 21 and the sum and difference signals generated by the signals 20 and 21 an d this output is supplied to a first band pass filter 7 which passes only the difference signal to a first amplifier 8 which amplifies the difference signal and supplies it to the frequency discriminator 9.The output from the diode 6 is also supplied to a second bandpass filter 10 which has a passband corresponding to the reference frequency band so that it passes only the reference signal 20 to a second amplifier 11 which amplifies the reference signal and supplies it to a clock pulse generator 12. The combinations of frequencies used for the reference and data signals 20 and 21 are arranged so that their difference signal is always at a frequency within the pass band of the band pass filter 7 while the individual frequencies of the reference and data signals 10 and 11 never fall within the pass band of the band pass filter 7. Thus the only signals supplied to the frequency discriminator 9 is the difference frequency between the reference and data signals 10 and 11.

The interrogator alters the frequency of the reference signal 10 ever T, where T is a short period of time, typically T would be a few ten thousandths of a second giving a frequency change rate of a few kilohertz. The value of the difference frequency F between the reference and data signals 20 and 21 is used to carry a digital interrogation code to the PPT 1. Each frequency change of the reference signal 20 corresponds to a separate bit of the signal, the value of this bit being given by the frequency difference F between the reference and data signals 20 and 21. In order to do this, each time the frequency of the reference signal 20 is changed the corresponding frequency of the data signal 21 required to give the correct difference frequency F is calculated and transmitted.

In Figure 2 the frequency difference Fo corresponds to a digital zero in the interrogation code and the frequency difference F1 corresponds to a digital one in the interrogation code. As can be seen the frequency of the data signal 21 does not necessarily change each time the frequency of the reference signal 20 changes because it is possible for a change in the digital value of the interrogation code from one bit to the next to cause a change in the difference frequency equal and opposite to the change in the frequency of the reference signal 20, for example as shown in Figure 2 where the frequency of the reference signal 20 changes as time equals 2T but due to the change in the difference frequency from FO to F1 the data signal 21 stays at a constant frequency from the time T to time 3T.

The frequency discriminator 9 senses whether the difference frequency applied to it is at frequency Fo or frequency F1 and generates a corresponding digital zero or one and supplies it to a processor 13. The clock pulse generator 12 is supplied with the reference signal 20 and each time the reference signal 20 changes value the clock pulse generator 12 supplies a clock pulse to the processor 13, these clock pulses are used to ensure that the clocking into the processor 13 of the digital values of the frequency discriminator 9 is synchronised with the transmission of the digital interrogation code from the interrogator, if there was no such synchronisation the digital interrogation code would be corrupted.

When a pre-arranged number of bits of the digital interrogation code have been clocked into the processor 13 it compares this digital interrogation code with codes stored in a digital memory 14.

The interrogator transmits the digital interrogation code repetitively and the processor 13 has no way of knowing at what point in.the code it has begun receiving it. As a result, when comparing the received digital code with the digital codes stored in the memory 14 the processor 13 compares the two code sequences separately for each of the bits of the received digital code sequence being the first bit in the sequence. If the two code sequences agree for any start bit the processor takes the two codes to be the same. In a system where it is desired that some PPTs will react to certain digital code sequences while other PPTs will not it will be necessary for all of these code sequences to be different whichever of their bits is chosen to be the starting digit in the sequence.

An alternative procedure would be for each interrogation code sequence to be divided into two sections, the first section being common to all of the possible interrogation codes and identifying the start digit of the code sequence and a second section unique for each different code sequence which will be compared with the stored code sequences to determine whether or not individual PPTs should respond to the interrogation code. In systems of this type the processor 13 would have to identify the first section of the code sequence and use this to identify where the first bit of the second section of the code sequence lay and then compare the second section of the code sequence starting at this bit with the code sequence stored in memory 14.

Once the interrogation code sequence being transmitted has been identified as being identical with the interrogation code sequence held in the memory 14 the processor 13 instructs a modulator 15 to begin switching the diodes 4 and 5. This alters the impedance of the antenna matching network and so modulates the signal reflected back to the interrogator by the antenna 2. The processor 13 causes the modulator 15 to modulate the reflected signal so as to transmit a digital value held in the memory 14.

Alternatively values supplied to the processor 13 by sensors and kept in the processor 13, or any other data held in or supplied to the PPT 1, could be transmitted to the interrogating unit.

In order to minimise the power consumption of the PPT 1, the PPT 1 would normally remain in a quiescent mode in which only the amplifier 8 and frequency discriminator 9 were powered. When the frequency discriminator 9 sensed signals at one of the difference frequencies F it would switch on the rest of the electronics.

The frequency discriminator 9 need not be particularly sensitive because the difference between the difference signals F0 and F1 corresponding to digital zero and digital one can be arranged to be as large as necessary.

Although the system described compares the received digital code sequence with only one digital code sequence in the memory 14 it is of course possible there could be a plurality of digital code signals stored in the memory 14 and the processor 13 could compare the received digital code sequence with all of them and respond if any of them agreed, alternatively the response made could be different for different interrogation code sequences.

Although it is convenient to change the reference frequency 20 for each bit of the digital code sequence because it allows the detector to be clocked by the changes in the frequency of the reference signal 20 in order to ensure synchronisation, it would be possible to have the frequency of the reference signal 20 changed more often or less often than the bit rate of the digital code sequence.

An example of the reference and data frequency sequences employed by a system in which the reference frequency 20 changes less often than the bit rate of the interrogation code sequence is shown in Figure 3.

In this system the reference frequency 20 changes every 4T, where T is the bit rate of the interrogation code.

The frequency of the data signal 21 is selected to give the appropriate difference frequency Fo, F1 for each bit of the interrogation code sequence. The frequency of the interrogation signal 21 must be recalculated for each bit of the interrogation code sequence but does not necessarily change with each bit, for instance where two sequential bits of the interrogation code are identical and the reference frequency 20 does not change clearly the frequency of the data signal 21 also will not change from one bit to the next. It is of course also possible, as before, that the change in the difference frequency F could be equal and opposite to the change in the reference frequency 20 again resulting in no change in the data frequency 21 from one bit to the next.

Referring to Figure 4 a further system is shown in which the reference frequency 20 is changed more often than the bit rate of the digital code. The reference frequency 20 changes four times for each bit of the digital code, so for each sequential group of four frequency shifts by the reference signal 20 the frequency of the data signal 21 must move by the same amount as the reference signal 20 so as the keep the difference frequency F constant.

A system employing a reference frequency of this type could be employed using approximately the same PPT as that shown in Figure 1. Although the changes in the reference frequency 20 could be used to clock the processor 13 the comparison of the digital interrogation code and the stored digital code would have to allow for the fact that each bit of the digital code is in effect transmitted four times. A system of this type should be highly resistant to corruption by interference because each bit of the interrogation signal is transmitted four times using different pairs of frequencies each time.

In the systems of both Figure 3 and 4 it would of course be possible to vary the ratios between the bit rate and the changes in the frequency of the reference signal 20, the ratios of 4 to 1 and 1 to 4 used in the examples shown are merely chosen for convenience and any other ratio could be selected depending on the characteristics and requirements of a specific system.

Although all the systems described operate using the difference frequency F between the reference signal 20 and data signal 21 to carry information it would of course be possible to use the same principles to operate a system where the sum of the frequencies of the reference and data signals 20 and 21 was used to carry information.

Claims (10)

1. A frequency modulated communication system comprising a transmitter and a pseudo-passive transponder, the transmitter being arranged to transmit a first and second signal at two different frequencies simultaneously and the pseudo-passive transponder being arranged to receive the first and second signals and to mix them in a non-linear element to generate sum and difference frequencies, the relative frequencies of the first and second signals being such that the generated frequencies carry information.

2. A communication system as claimed in claim 1 in which the difference frequency carries information.

3. A communication system as claimed in claim 2 in which the frequencies of the first and second signals are varied such that the difference frequency always has one of two pre-arranged values.

4. A communication system as claimed in claim 3 in which the information carried is digital and one of the pre-arranged values corresponds to a digital one and the other corresponds to a digital zero.

5. A communication system as claimed in claim 4 in which the frequency of the first signal changes between each bit of the digital information carried by the system.

6. A communication system as claimed in claim 5 in which the changes in the frequency of the first signal are used to clock a digital electronic device forming a part of the pseudo-passive transponder.

7. A communication system as claimed in any preceding claim in which the non-linear element is a diode.

8. A communication system substantially as shown in or as described with reference to figures 1 and 2 or the accompanying drawings.

9. A communication system substantially as shown in or as described with reference to figure 3 of the accompanying drawing.

10. A communication system substantially as shown in or as described with reference to figure 4 of the accompanying drawing.