Passive RFID sensor tag

1. A passive radio frequency identification (RFID) transponder, comprising:

an antenna configured to receive radio frequency (RF) interrogation signals from an RFID reader and send RF backscattering signals to the RFID reader,

a rectifier connected to the antenna and configured to power up the passive RFID transponder from the received RF interrogation signals,

a backscatter modulator configured to modulate the RF backscattering signals with a modulation frequency and connected to the antenna for communication to the RFID reader with a backscattering principle to provide radio frequency identification (RFID) features,

an oscillator having an oscillation frequency, and

a sensing element configured to measure a predetermined variable external to the RFID transponder, the sensing element having an electrical property that varies based on variation in value of the measured predetermined variable,

wherein the sensing element is connected to the oscillator such that the variation in the electrical property of the sensing element changes the oscillation frequency of the oscillator, the oscillator frequency being dependent on the value of the predetermined variable measured by the sensing element, and

wherein the oscillation frequency of the oscillator is a harmonic multiple N of the modulation frequency of the backscatter modulator, and wherein a frequency divider is arranged between the oscillator and the backscatter modulator to derive the modulation frequency from the oscillation frequency.

2. A transponder as claimed in claim 1 , wherein the sensing element comprises a capacitive sensing element, a resistive sensing element or an inductive sensing element.

3. A transponder as claimed in claim 1 , wherein the frequency divider comprises an analog frequency divider or a digital frequency divider.

4. A transponder as claimed in claim 1 , wherein a division ratio N of the frequency divider is controllable or programmable.

5. A transponder as claimed in claim 1 , wherein a control unit of the transponder is arranged control or program the frequency divider.

6. A transponder as claimed in claim 1 wherein the sensing element can be read or interrogated at two or more modulation frequencies, and wherein a division ratio N of the frequency divider is arranged to be changed depending on the modulation frequency selected by a reader or interrogator for reading or interrogating the specific sensing element.

7. A transponder as claimed in claim 1 , comprising a plurality of sensing elements arranged so that the oscillation frequency is affected by only one sensing element at a time.

8. A transponder as claimed in claim 7 , comprising a selector or digitally controlled switches are provided to connect one of the plurality of sensing elements at time to the oscillator.

9. A transponder as claimed in claim 1 , wherein the oscillator comprises one of an RC oscillator, a ring oscillator, an LC-oscillator, an RLC-oscillator, and a MEMS oscillator.

10. A transponder as claimed in claim 1 , wherein the oscillator is designed and dimensioned operate at the harmonic oscillation frequency such that a high sensitivity is obtained, and the desired modulation frequency is derived from the harmonic oscillation frequency by a division ratio N, wherein N is an integer.

11. A transponder as claimed in claim 1 , wherein the modulation frequency is within range of approximately 40-640 kHz.

12. A transponder as claimed in claim 1 , wherein N is an integer equal to or greater than 10.

13. A transponder as claimed in claim 1 , wherein the oscillation frequency is at least 5 MHz.