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we are able to propose some design improvements to increase the tag’s performances. Set up test bench The setup test bench – see figure 5 - is composed of a PC (used to proceed the query then record the tag’s response demodulated and verify this is an RN16 re- sponse - 16-bit random number), an RF signal generator com- Fig. 4: RFID tag reader architecture. Graph 1: Benchmark results - frequency bined with a modulator I/Q low range. frequency, a circulator to transmit the signal to the antenna (Laird Technologies S8658W- 3.6% is one of the best among all the P12sMM) and at least a spectrum analyser tags tested and the minimum power used to measure and down convert the signal to detect an RN16 response (wake up response from the tag. power) is 8dBm. Tag 7 presents lesser the distance between the antenna and the performances. this tag only works from tag is 60 cm. the « query » signal is gener- 820 to 910MHz. ated using the following parameters: modula- The modulation depth of only 1% is very tion FM0 - BLF= 180 kHz - Tari=25 us. The small which means the reader will need signal generator RF is used from 800MHz to be very sensitive. the minimum power to 1000MHz, with an output power from Graph 2: Benchmark results – wake-up to detect a RN16 response (wake up 0dBm to 16dBm. Our measurements yielded power and modulation depth. power) is 11dBm. the minimum and maximum frequency range within which we could detect a RN16 Identifying and solving the response as shown on graph 1 and the mini- issues mum input power needed to detect a RN16 Why doesn’t tag 7 work within the proper response over the 860-960MHz frequency frequency range? Graphs 4 and 5 show range as shown on graph 2. the minimum power applied to detect Graph 3 illustrates the measured average an RN16 between 825 and 985MHz for modulation depth in % detected on the RN16 tags 7 and 9, respectively. Tag 9 works response over the working frequency range. from 830 to 985MHz while tag 7 is able We name “modulation depth” the ratio: delta to answer but only for a smaller range of Graph 3: Set up RN16 response versus a/ a *100 average of the signal response 830 to 915MHz. versus the reader signal transmitted – see frequency and power. graph 3. the tag’s response over frequency depends on the matching antenna with the combined input stage of the receiver, rectifier Data results analysis and transmitter of the tag. The antenna from tag 7 should be Overall, 15 tags were tested using the test bench described re-centred to 910MHz instead of 845MHz to cover the proper above. Among the 15 tags, only nine gave us a RN16 response. the nine valid tags can be put into two categories as shown in table 1. From these measurements, it comes out that tag 9 presents the best performances. It works across all the fre- quency range of the EPC standard, its modulation depth of Fig. 5: Set up RN16 response versus frequency and power. www.electronics-eetimes.com Electronic Engineering Times Europe January 2013 41


EETE JANUARY 2013
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