The input power was controlled with an attentuator and the
effective distance was calculated using the Friis transmission formula. The horizontal line with no markers shows
the WISP voltage supervisor threshold of 1.9 V. ??????Uplink packet errors??™??™ is the number of failed uplink packets
divided by attempted uplink packets, expressed as a percentage. ??????Responses per query??™??™ is the percentage of
issued reader queries that return a packet with a valid CRC.
WISP: A Passively Powered UHF RFID Tag with Sensing and Computation 271
consisted of an EPC Gen 1 RFID reader driving a 6 dBi circularly polarized patch antenna.
The reader??™s antenna and WISP were placed 1 m apart and 1 m off the ground to minimize
multipath effects. An adjustable attenuator was used to vary the power delivered from the
reader to the patch antenna. Then the Friis path loss equation was used to calculate the loss
of the RF signal from the reader to the tag over the 1 m. Thus, the WISP received power is
de?¬?ned as reader transmit power (1 Watt), minus variable attenuator, minus transmission
path loss. It should be noted that the 1 Watt source represents peak output power of the
RFID reader, while the average output power (not considered here) is highly dependent on
reader transmission rate and the speci?¬?c implantation of the EPC Gen 1 protocol.
To measure recti?¬?ed output voltage, the WISP is placed in its low power state, and
voltage is averaged over a 10 s interval using an oscilloscope.
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