Antenna Matching for the TRF7960 RFID Reader

Application Report SLOA135 May 2009 Antenna Matching for the TRF7960 RFID Reader John Schillinger... 1 Introduction This paper describes the design method for determining an antenna matching circuit. While there are an infinite number of possible impedance matching networks, this application example focuses on a 50-Ω three element match. A three element match is recommended as it allows the designer to select the required antenna quality factor, Q, for the application. The PCB design used in this application is based on FR4 material, Er = 4.5, with a material thickness of 0.62 inches. The board design is 2 layer design (layer 1 top, & layer 2 bottom), with 2 oz copper. 2 Antenna Fabrication A schematic of a typical antenna circuit is given in Figure 1. The antenna trace width together with the trace spacing, will determine the antenna impedance and Q respectively. 1.5 inches C1 C2 R1 2.125 inches C3 C4 50 ohms R + jx Ckt Feed 50 ohms Trace Width == 0.050 inches Trace Spacing == 0.020 inches 2.0 inches (approximately) Read Range == 4.0 inches Figure 1. RFID Antenna (Typical) SLOA135 May 2009 Antenna Matching for the TRF7960 RFID Reader 1

Antenna Fabrication www.ti.com In this application, the antenna is fabricated on the top side of PCB. Some antenna designs will have antenna traces on both top and bottom layers. In either case, it is important to keep ground planes away from antenna traces or elements. Notice that the antenna drawing (Figure 1) shows a diagonal measurement of approximately 2.0 inches. A rule of thumb is that the expected read range is twice the antenna diagonal measurement. Figure 2. Antenna Bottom Layer Figure 3. Antenna Top Layer 2 Antenna Matching for the TRF7960 RFID Reader SLOA135 May 2009

www.ti.com 3 Antenna Impedance Matching Antenna Impedance Matching A three element match is used as it has the added advantage of allowing the circuit Q to be a chosen value. Q = F O /BW = 13.56 MHz/2 MHz = 6.78 Where the required operating bandwidth is chosen at 2 MHz. The first step in impedance matching is calibrating the network analyzer. This is done by connecting a RF test connector on three blank circuit boards, one with a 50 load, second with a short (0-Ω resistor), and third with an open. By using the application board in the calibration, PCB parasitic effects are accounted for. Following the calibration step, the antenna trace is connected to the test connector as required by 0-Ω resistors. The antenna trace is measured as shown in Figure 4; the resulting measurement is the starting impedance which will be matched to 50 Ω. Figure 4. Antenna Impedance = = (0.4648 Ω + j107.42) = = 1.2608 µh The measured antenna impedance (0.4648 + j107.42) at 13.56 MHz is shown in Figure 4. Note that in this application the antenna impedance is at the Smith Chart s outer limit; or other wise stated the starting impedance is up against the rail. This makes the impedance matching a little more difficult. A minimum bandwidth (BW) of 2 MHz is chosen in order to accommodate the upper and lower RFID sidebands for various data rates given in ISO15693 & ISO14443 A/B. Hence the approximate resistor value needed is determined as follows: Q = F O /BW = 13.56 MHz/2 MHz = 6.78 Q = R P /X L = R/2 x π x 13.56 MHz x 1.2608 µh = R/107.42 Note that the X L value is the same as the measured Smith Chart value (107.42) from Figure 4. R P = Q x X L = 6.78 x 107.42 = 728 Ω Figure 5 is a Smith Chart impedance matching simulation. SLOA135 May 2009 Antenna Matching for the TRF7960 RFID Reader 3

Antenna Impedance Matching www.ti.com Imp 2 56 pf Shunt 10 25 50 Figure 5. Smith Chart Impedance Match Simulation The measured impedance from Figure 4 is shown in Figure 5 as Imp 1. A shunt 56-pF capacitor rotates the impedance to Imp 2. Next a shut resistor (which sets the antenna Q or BW), rotates the impedance from Imp 2 to Imp 3; where the impedance is now on the 50-Ω circle. The final matching element is a 56-pF capacitor, which rotates the impedance from Imp 3 to Imp 4 (46.6 + j0.3). This capacitor is split into two parallel caps (47 pf + 10 pf) to allow fine tuning of the antenna frequency while also reducing component parasites. Note earlier the shut resistor was calculated to be 728 Ω whereby it is now rounded to up to 1.0 KΩ in order to yield a match with standard capacitor values. Final circuit values for the antenna circuit given in Figure 1 are as follows: C1 = = 56 pf C2 = = DNP (Do Not Place) R1 = = 1.0 KΩ C3 = = 47 pf C4 = = 10 pf 4 Antenna Matching for the TRF7960 RFID Reader SLOA135 May 2009

www.ti.com 4 Antenna Performace Antenna Performace Figure 6. Smith Chart Figure 7. Return Loss (50.635 - j804.67) = = 14.586 nf (RL = 40 db at 13.56 MHz) Note that Figure 7 shows a 10 db return loss over a 1.0124-MHz BW (Mkr 3-2). A rule of thumb is that the antenna 3-dB BW is twice the 10-dB return loss bandwidth. Applying the rule to this application would yield an antenna bandwidth of 2.248 MHz. Figure 8. Polar Plot Figure 9. VSWR Plot SLOA135 May 2009 Antenna Matching for the TRF7960 RFID Reader 5

Antenna Performace www.ti.com Figure 10. Linear Plot Figure 11. Phase Plot Figure 12. Real Part Figure 13. Imaginary Part 6 Antenna Matching for the TRF7960 RFID Reader SLOA135 May 2009

www.ti.com Antenna Performace Figure 14. Measured Antenna Bandwidth and Q Figure 15. Antenna Transmission Test As a final test to ensure the antenna elements will radiate, a test set up was established. A 0-dBm CW signal is applied to the Tx antenna; the Rx antenna is placed 8.5 inches from the Tx antenna (the width of a standard piece of office paper). The output from the Rx antenna is measured on a spectrum analyzer which shows a 49.5-dBm signal level. Both Tx & Rx antenna are PCB RFID reader antennas. SLOA135 May 2009 Antenna Matching for the TRF7960 RFID Reader 7

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