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Transponder's sensitivity - a new way to specify UHF transponders?
It is widely being recognised that in situations where operating
range and manufacturing costs are the issue, the frequency of
operation of the transponders in the future is going to be
in the UHF frequncy band, somewhere between 800 to 950MHz.
The physics of radio frequency propogation mean that energy from a reader to power the transponder can more easily be delivered at these frequencies, bearing in mind the compromise of antenna sizes, than at other frequencies.
Below 100 Mhz the mode of propagation is magnetic coupling and range becomes an issue as it is difficult to project the energising field meters from the reader coils.
When the mode of propogation is electric field coupling, the projection of the energising field is no longer an issue as range relates to transmitter power. However, the collecting area of the transponder (called the transponder antenna aperture) where the energy from the reader is intercepted relates to the operating frequency squared, meaning that a 2.45GHz system would need 7 times more transmitter power than a 900MHz system for the same range, while still having an antenna system that is 36% of the size of the 900Mhz version.
The EAN/UCC are lobbying for a frequency in the 860-930 MHz band, the motor manufacturers who require all tyres to now be labelled are specifying this band, and whereas a few years ago there were very few manufacturers providing transponders at this frequency more and more of the established players are announcing that they have UHF transponder systems under development.
A further contributing issue for the frequency band is that the transponders can be so simple, just one chip and a foil antenna strip, that the manufactured costs can become so low that mass acceptance of RFID becomes a reality.
The staff of Trolley Scan have been leaders in the field of UHF transponders since 1990 and now Trolley Scan has taken the developments in this frequency band one step further, namely in the issue of transponder sensitivity.
Passive transponders, that is transponders that have no onboard battery but receive their operating energy from the reader's energising field, collect their energy from the reader, in an area called the transponder antenna aperture and convert it to operating power. In the case of a 915MHz transponder using a dipole antenna, this is an area of 134 sq cms. Trolley Scan have developed a simple form of comparing developments by specifying how much RF energy needs to be collected in the aperture to operate the transponder- called the transponder sensitivity.
RF passive transponder systems are very cruel to electronic designers as although developments in electronic circuitry mean that operating voltages and currents are becoming very low, the reality of transponder design means that only two parameters are really dominant, namely the impedance of the antenna and the operating voltage of the transponder circuit. Even if developments of technology in future meant that the electronic circuits operating current dropped a further 3 fold for example, it would have no impact on the sensitivity of the transponder - a 3 volt logic circuit would still need 23 milliwatts of power to be collected in its aperture.
A 3 volt circuit using schottky diodes in a voltage doubler configuration would need 3.6 volts across the diodes, 1.8 volts RF before the doubler, 1.27 volts RMS RF across the antenna and with a 72 ohm dipole antenna, 23mW RF power. (Even a 0 volt logic circuit - if such a thing was possible - would need 0.6mW to operate!!!)
Trolley Scan in its EcoTag technology is now delivering 1 milliwatt transponders and a 100uW version of the RFIDmodule is on the horizon. This means that the reader's power can currently be reduced by 23 times compared to the 3 volt circuit for the same performance, and 230 times for the future versions. Hence readers become smaller and instead of running down a car battery in minutes, a cell phone battery in an EcoTag reader can last for hours.
Transponder sensitivity can become a simple parameter to allow users to compare different UHF RFID systems in the future for implications on range and reader requirements.
Below is a graph showing how sensitivity impacts range and energising power. WERP(watts) is the product of the power from the transmitter in the reader multiplied by the gain of the transmit antenna. A matched dipole has a gain of 1.6 and a 6 element Yagi antenna has a gain of 15.
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