Achieving Impedance Matching for TRF79xxA Single Receive Channel Using L Network: Calculations and Simulation
This content discusses the process of achieving impedance matching for a single receive channel in TRF79xxA devices, which operate with a 50-ohm antenna. The calculations involve determining the Q values of the legs, calculating reactances, and deriving the required values for L and C components. Simulation on a Smith Chart validates the impedance transformation from 4 ohms to 50 ohms. Detailed steps and formulas for achieving the impedance match are provided.
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Presentation Transcript
Output Impedance Matching for TRF79xxA (single and dual channel receive examples, with potential improvement to existing match) Texas Instruments S2 MCU NFC/RFID Applications/Systems Team
SINGLE RECEIVE CHANNEL 50 IMPEDANCE MATCH FOR TRF79xxA
Background / Problem being solved Need for impedance match for TRF79xxA devices which are utilizing only one of the RX channels and still operates with 50 antenna. This impedance match must transform a source resistance of 4 to a 50 load.
Calculations Math for L Network low pass filter impedance match follows. We start by calculating the Q of the legs, using the known values of the load and source. ?? ?? ?? = ?? = 1 Where: QS= Q of the series leg QP= Q of the parallel (shunt) leg RL= Resistance of the load, in this case = 50 RS= Resistance of the source, in this case Pin 5 of TRF79xxA (RDSON = ~4 )
Calculations ?? ?? ?? = ?? = 1 50 4 1 ?? = ?? = ?? = ?? = 12.5 1 ?? = ?? = 11.5 ?? = ?? = ?.???????
Calculations (cont.) Now we calculate XS(series reactance) and XP(parallel (shunt) reactance) ??= ?? ?? ??= 3.391165 4 ??= ??.?????? ????????? ??=?? ?? 50 ??= 3.391165 ??= ??.?????? (??????????)
Calculations (cont.) Using XSand XP, we can now calculate L and C values needed for the impedance match circuit. 1 ? =?? ? = ?? 1 13.56466 85.1568?6 ? = ? = 85.1568?6 14.74209 1 ? = ???.???? ? = 1.255389?9 ? = ???.??????
Calculated circuit values populated and plotted on Smith Chart Using L and C values calculated, we can simulate (using Smith chart) and see that the result produces the desired transformation from 4 to 50
How do we get there with math? Single RX Channel Matching Calc Qs Qp Rp (antenna connection) Rs (TRF79xxA Pin 5) 3.391164992 3.391164992 50 4 Qs = Qp = sqrt of (Rp/Rs - 1) Xs = Qs * Rs 3.391164992 13.56465997 Xp = Rp / Qp 14.74419562 L C 159.3E-9 796.5E-12 move to standard value of 1500pF + 100pF (in parallel), in series with same for voltage divider presentation to RXIN_1 move to standard value of 150nH
Standard component value circuit populated and simulated on Smith Chart Using standard L and C values, closest to what was calculated, we can simulate again (using Smith chart) and see that the result produces similar desired transformation from 4 to 50
Results! Increase in read range with ISO15693 (+1cm), ISO14443A & B (+0.5cm) versus standard USB Dual RX EVM.
DUAL RECEIVE CHANNEL 50 IMPEDANCE MATCH FOR TRF79xxA
Background TRF79xxA devices have single transmit (TX) out and two receive (RX) channels. Impedance match from TX_OUT pin of TRF79xxA device to a single ended 50 13.56MHz resonant antenna circuit is desired. Dual receive inputs need consideration for both voltage input and phase relationship to each other. The following slides will explain the basic theory and practical implementation of these requirements.
Reference Schematic Section of Impedance Match
Simulated Ideal Match from 4 to 50 @ 13.56MHz Note: Ideal circuit, there are no DC blocking caps and non-standard component values are shown
Simulated Practical Match from 4 to 50 @ 13.56MHz Note: Practical circuit, as we inserted DC blocking caps and used standard component values
Simulation of actual (also practical) match from 4 to 50 @ 13.56MHz Note: Actual circuit populated on EVM, as we inserted DC blocking caps and used standard component values
Improved Matching circuit using standard values 680pF in first L section changes to 560pF, this allows us to get to 50 point later. 100pF + 27pF change to 82pF (reduction in BOM), this allows us to hit 50 and not over shoot the target impedance.
How do we get there with math? Dual RX Channel Matching Calc 2.061552813 2.061552813 21 4 Qs Qp Rp (antenna connection) Rs (TRF79xxA Pin 5) to midpoint from TRF79xxA 2.061552813 8.246211251 Qs = Qp = sqrt of (Rp/Rs - 1) Xs = Qs * Rs 10.18649625 Xp = Rp / Qp L C 96.8E-9 1152.8E-12 use 560pF in parallel with existing 1200pF in series to get close enough because of DC block, need to move to 150nH 1.175139303 1.175139303 50 21 Qs Qp Rp (antenna connection) Rs (midpoint) to antenna connection midpoint 1.175139303 24.67792536 Qs = Qp = sqrt of (Rp/Rs - 1) Xs = Qs * Rs 42.54814717 Xp = Rp / Qp L C 289.8E-9 276.0E-12 use 82pF in parallel with existing 220pF, 680pF in series with each other move to 330nH (standard value) NOTE: the final C value can be adjusted for the actual PCB circuit implementation
