NEC-LIST:Backscatter modulation on RFID tags

I am doing some work on RFÏD tag backscatter modulation in the 900MHz region.

Some of the earlier work was undertaken by Jeremy Landt at Amtech.

The conventional way of doing backscatter modulation is to provide a good
impedance match between antenna and transponder input circuitry. The input
circuit is generally a full wave rectifier or voltage doubler which provides
the DC supply for the transponder. A switch, usually a diode or Mos transistor
is connected across the antenna port and controlled by the data stream to
deliberately mismatch the antenna, thereby causing reflection in time with the
data and hence backscattering the tags data to the reader receiver. The
problem with this method is that one needs a large storage capacitor on board
the chip to hold up the DC while the modulator is in the shorted condition.

A second problem is that if one uses a high antenna output impedance to
maximise the recovered voltage, the rise time of the DC storage capacitor is
lengthened because the radiation resistance is now in series with the
relatively low impedance of the storage capacitor.

The tag's antenna will generally have a fairly low Q, in the order of 2 to 5.

If the phase of the incident wave could be varied on reflection without having
to short the antenna, then the modulation would not cause gaps in the
rectified DC signal.

Can one use phase modulation for backscatter? If so how would one go about
phase modulating the backscatter signal without having a large amplitude
component?

Any ideas?

Chris Turner IEng MIIE
tel/fax: +44 1494 784547
mob: +44 7812 174 133
email: turner.c_at_virgin.net

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