Digital Signal Processing Reference
In-Depth Information
The reflected parts of the wave travel back to the interdigital transducer, where
they are converted into a high frequency pulse sequence and are emitted by the dipole
antenna. This pulse sequence can be received by the reader. The number of pulses
received corresponds with the number of reflective strips on the substrate. Likewise,
the delay between the individual pulses is proportional to the spatial distance between
the reflector strips on the substrate, and so the spatial layout of the reflector strips can
represent a binary sequence of digits.
Due to the slow dispersion speed of the surface waves on the substrate the first
response pulse is only received by the reader after a dead time of around 1.5 ms after
the transmission of the scanning pulse. This gives decisive advantages for the reception
of the pulse.
Reflections of the scanning pulse on the metal surfaces of the environment travel
back to the antenna of the reader at the speed of light. A reflection over a distance of
100 m to the reader would arrive at the reader 0.6 ms after emission from the reader's
antenna (travel time there and back, the signal is damped by
>
160 dB). Therefore,
when the transponder signal returns after 1.5 ms all reflections from the environment
of the reader have long since died away, so they cannot lead to errors in the pulse
sequence (Dziggel, 1997).
The data storage capacity and data transfer speed of a surface wave transponder
depend upon the size of the substrate and the realisable minimum distance between
the reflector strips on the substrate. In practice, around 16 - 32 bits are transferred at a
data transfer rate of 500 kbit/s (Siemens, n.d.).
The range of a surface wave system depends mainly upon the transmission power
of the scanning pulse and can be estimated using the radar equation (Chapter 4). At
the permissible transmission power in the 2.45 GHz ISM frequency range a range of
1 - 2 m can be expected.
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