VSWR classification and non-resonant encoding of RFID tags using a near-field encoder
In one embodiment, a near-field RFID encoder is provided that includes a pair of capacitive elements formed from an arrangement of stripline conductors. The near-field encoder may non-resonantly excite RFID tags. In addition, the near-field encoder may characterize RFID tag quality using a VSWR measurement.
1 . A capacitive RFID tag encoder, comprising:
a substrate;
a ground plane on a first surface of the substrate;
a first plurality of serially-connected stripline conductors on a second surface of the substrate, the serially-connected stripline conductors in the first plurality being arranged within a first area of the second surface,
a second plurality of serially-connected stripline conductors on the second surface of the substrate, the serially-connected stripline conductors in the second plurality being arranged within a second area of the second surface, the encoder being configured to drive the first plurality of serially-connected stripline conductors with an RF signal and to drive the second plurality of serially-connected stripline conductors with a phase-shifted version of the RF signal, wherein the RFID tag encoder is configured to drive the RF signal into the stripline conductors so as to encode an RFID tag at a frequency outside of a resonant operating bandwidth for the RFID tag.
2 . The capacitive encoder of claim 1 , wherein each of the stripline conductors in the first and second plurality is arranged in parallel with the remaining stripline conductors.
3 . The capacitive encoder of claim 1 , wherein the first and second plurality of stripline conductors are each arranged in a fractal pattern.
4 . The capacitive encoder of claim 1 , further comprising:
a stripline feed on the second surface for receiving the RF signal;
a first connector stripline connecting the stripline feed to the first plurality of stripline conductors so that the first plurality of stripline conductors is driven with the RF signal; and
a second connector stripline connecting the stripline feed to the second plurality of stripline conductors, wherein the second connector stripline has a different length than the first connector stripline so that the second plurality of stripline conductors is driven with the phase-shifted version of the RF signal.
5 . The capacitive encoder of claim 4 , wherein the length difference of the second connector stripline is such that the phase-shifted version is approximately 180 degrees out of phase to the RF signal.
6 . The capacitive encoder of claim 1 , wherein a spacing between each of the stripline conductors in the first plurality is at least as large as a thickness of the substrate, and wherein a spacing between each of the stripline conductors in the second plurality is at least as large as the thickness of the substrate.
7 . The capacitive encoder of claim 1 , wherein a characteristic impedance for the stripline conductors in the first and second plurality is at least 50Ω.
8 . A method, comprising:
near field exciting an RFID tag with RFID encoder, the RFID encoder near field exciting the RFID tab by driving an RF signal into an RF feed;
varying a frequency for the RF signal during the near field excitation;
during the varying of the frequencies, measuring a VSWR on the RF feed at various ones of the varied frequencies to determine a VSWR behavior of the RF tag as a function of frequency; and
based upon the determined VSWR behavior, characterizing the RFID tag.
10 . The method of claim 8 , wherein the characterization comprises comparing the determined VSWR behavior with an expected VSWR behavior.
11 . The method of claim 9 , wherein the comparing comprises determining if the determined VSWR behavior is within an acceptable tolerance of the expected VSWR behavior.
12 . The method of claim 9 , wherein the characterization comprises determining if the RFID tag is suitable for a desired application.
13 . The method of claim 9 , wherein the RFID encoder includes a plurality of stripline conductors connected to the RR feed.
14 . The method of claim 13 , wherein the plurality of stripline conductors are organized into:
a first plurality of serially-connected stripline conductors on a second surface of a substrate, the serially-connected stripline conductors in the first plurality being arranged within a first area of the second surface; and
a second plurality of serially-connected stripline conductors on the second surface of the substrate, the serially-connected stripline conductors in the second plurality being arranged within a second area of the second surface.
15 . The capacitive encoder of claim 1 , wherein the first and second plurality of stripline conductors are each arranged in a fractal pattern.
16 . A method of encoding an RFID tag, the RFID tag having a resonant operating bandwidth, comprising:
providing a near field RFID encoder having a plurality of stripline conductors connected to an RF feed; and
driving the RF feed with an encoding RF signal outside of the resonant operating bandwidth to encode the RFID tag.