The invention concerns a device for limiting unwanted electromagnetic radiation from a transponder, according to the introductory part of Claim 1. In so called transponders, a code is superimposed on an incoming, high frequency signal and reflected. One way to superimpose the code is to mix in an information signal. In this respect, harmonic components can occur in the reflected signal. Since the band width in radio communication is a limited resource, it is important to eliminate problems with unwanted radiation beyond the specified radio channel. Previously it has been known to transmit information from a transponder by connecting a nonlinear element (e.g. a diode) to an antenna. Different biasing of the nonlinear element implies that the electromagnetic waves captured by the antenna are reflected with different amplitude and phase. To use the information signal for biasing the nonlinear element, a mixer is provided where the information signal is the local oscillator signal (gives the nonlinear element alternating different biasing), the input signal is fetched form the antenna and the intermediate frequency signal is conneted to an antenna. It may be one common antenna.

The principle is described in GB-patent publications 1 534 750, 2 259 210 and 2 273 422, and US-patent publications 5 173 705 and 5 311 186. Here the technique of using a diode or a FET-transistor biased to two levels is described. By turning the bias on and off, the reflection coeffcient is changed so that the incoming electromagnetic signal is modulated.

A disadvantage with the technology where a nonlinear element is biased in the on/off-position is that a number of harmonic frequency components in addition to that wanted is produced. This can be solved by using a sine information signal, and bias the nonlinear element as described in GB-patent application 1 339 608. This means that the reflection coefficient moves in a linear area and the radiation of harmonics is reduced. This is, however, a complex and expensive solution. Furthermore, the prior art makes high dynamic demands to in order to handle trans- ponders situated very near the interrogator. A transponder which is situated very near the interrogator will of course reflect and radiate a lot of effect, which also places

with respect to the smallest physical distance to an interrogator that can use the same frequency without interference occuring.

It is therefore an object of the present invention to provide a device which limits the side band radiation at transponders, and which is simple in production and use. The object of the invention is achieved with a device having features as stated in the characterizing part of Claim 1. Further features are clear from the dependent claims.

In the following the invention will be described in more detail, by means of an example of embodiment and with referance to teh accompanying drawings, where

Fig. 1 shows a setup for measuring signal reflection in the context of the present invention, and

Fig. 2 shows an example of the embodiment of present invention.

In Fig. 1 there is shown a signal generator 1 which supplies a carrier wave of e.g. 5,8 GHz to a circuit according to present invention. The signal generator 1 generate a pure sine carrier wave, and the output effect from the signal generator is varied during the measurements. The signal is passed through a transmision line 2 to a circulator 3. The signal is further passed through a transmision line 4 to a nonlinear element 5, which in this case is a diode.

A signal generator 8 represents an information signal and generates a sine signal having a DC-component. In this case the generator has a frequency of 1 MHz. The signal is connected to the nonlinear element 5 through resistors 6 and 7. Earth is denoted with referance numeral 9. The information signal is mixed with the incoming signal and the resulting side bands are reflected and transmitted back through transmision line 4 via the circulator, through a transmission line 10 to a spectral analyser 11.

Measured results at a high frequency signal with an effect from -41 dBm to -11 dBm are listed in table 1. In the table there is shown the effect of the wanted side bands of f ₀ ±lMHz, and the unwanted harmonic signals at f ₀ ±2MHz, f ₀ ±3 MHz, f ₀ ±4 MHz and f ₀ ±5 MHz

Table 1. Measurement of side band and harmonics

* HF fo±IMHz fo ±2MHz fo ± 3MHz fo ± 4MHz f ₀ ± 5 MHz

(dBm] fdBml fdBml fdBml fdBml fdBml

-41 -51 -73 -74 -86 < -85

-31 -41 -64 -64 -78 -85

-21 -32 -58 -58 -80 -80

-11 -28 -54 -71 -82 < -85

At the first measurement (-41dBm) the side bands are at -51 dBm, i.e. a loss of 10 dB relative to the incoming signal. When the incoming high frequency signal is -21 dBm, the effect is shown by the high frequency signal biasing the diode. The level of the unwanted harmonic signals is no longer proportional with the level of the incoming signal. The unwanted harmonic signals are here weaker than for the measurement at -31 dBm. At

-11 dBm the harmonic components have reached a final maximum, and the components of f ₀ ±5 MHz has actually disappeared. It is further clear from the measurement that the radiated effect at f ₀ ± 1 MHz has reached a maximum at -28 dBm. An example of a practical embodiment of the present invention is shown in Fig. 2. The circuit is resized on a microstrip substrate 20, comprising a dielectric plate where a metal pattern as shown is imprinted on one side, while the other side is completely covered with metal. The circuit is connect to the antenna (not shown) via a terminal 21. A nonlinear element 29 functions both as a modulator and a demodulator. In this embodiment a Schottky diode is used where two similar diodes 28 and 29 are capsuled in the same component package 27. A stub 22 and a line 34 constitue an alignment network for the nonlinear element 29. The two diodes 28 and 29 are connected through lines 24 and 25. To the lines 24 and 25 is connected a first earth fan 23. At the frequency in question, the earth fan 23 is dimensioned so that signal earth is located in a point 30 between the lines 24 and 25. This is realized by the two

A second earth fan 26 is connected to opposite side of the diodes 28 and 29, and rep¬ resents signal earth in a point 32. Since signal earth is present in the points 30 and 32, supply lines 31 and 33 for earth and bias (in demodulator modus) or information signal (in modulator modus), respectively, can be connected to these points 30, 32, without in- fluencing the impedance, or the high frequency signal effect is output through these lines. In practical terms, the alignment network 22 will be a compromise between a perfect alignment and a misalignment. At such an optimum alignment, a lot of the effect will be absorbed, and this will give a very sensitive demodulator. When a small effect is supplied to the alignment network 22, the reflection coefficient will be very sensitive for informa- tion signals, as a consequence of the alignment. Furthermore, the alignment means that a high effect will give a net current in the Schottky diode 29. This will limit the radiation and the reflection coefficient will move in a more linear area.

The alignment network 22 is aligned in the frequency band in question, so that the Schottky diode 29 both become a sensitive demodulator and an effective modulator, so that strength on the modulating signal may be reduced.

The Schottky diode 29 is biased for demodulation by a standing current set up through the diode 29. At modulation, the diode is biased with a voltage corresponding to the threshold voltage for the diode, and a pure sine voltage having an amplitude similar to the threshold voltage is added. The embodiment of the invention is optimized for a number of situations. Firstly, less loss is achieved by frequency conversion. The Schottky diode that is used is protected against destruction by static electricity and electromagnetic pulses by connection in paral¬ lel with a reversed Schottky diode 28 through the lines 24 and 25. By using a weak modulating signal, the incoming electromagnetic effect is used for biasing a nonlinear element. Furthermore, the maximum reflection from the transponder is limited to a final upper limit.

By using impedance alignment, the modulation sensitivity is increased so that biasing occurs at a lower effect in the incoming electromagnetic field. Additionally, this impe¬ dance alignment makes the modulator into an effective demodulator, so that two-way communication may be realized with only one diode.

By using impedance alignment, the modulation sensitivity is increased so that biasing occurs at a lower effect in the incoming electromagnetic field. Additionally, this impedance alignment makes the modulator into an effective demodulator, so that two-way communication may be realized with only one diode. The wanted reflected signal is no longer propertional with incoming electromagnetic field effect, but is limited to a final limit. This solves the problem with dynamics in the interrogator. Additionally, this opens up the possibility of the physical distance between two interrogators using the same radio channel being reduced. This is important in view of the large expansion of such systems in highly populated areas, with parking buildings in each block, etc.

Radiation of unwanted side bands is limited by the modulator being more linear the stronger the incoming electromagnetic field is.

The circuit will be very simple, inexpensive and adapted for volume production, as individual trimming is not necessary. The modulator can also be used as a demodulator having a higher sensitivity than if the nonlinear element is connected directly to the antenna. This means that both the demodulator and the modulator can be achieved in the transponder with only one non¬ linear element.

The invention applies mainly to the use of a modulating signal which is continuously (sine shaped) and which has such small amplitude that the effects with limitation in the radiation and linearization occurs.