How to use Network Analyzer to measure resonant

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How to use Network Analyzer to measure resonant frequency of HF (13. 56 MHz)

How to use Network Analyzer to measure resonant frequency of HF (13. 56 MHz) passive transponders Josh Wyatt Texas Instruments Embedded RF Applications/Systems 02/16/2011

Method Steps • Fabricate transmit/pickup coil fixture Slide 3 • Frequency Settings, Calibration and

Method Steps • Fabricate transmit/pickup coil fixture Slide 3 • Frequency Settings, Calibration and Display Settings for Vector Network Analyzer (VNA) Slide 4 • Measure transponder f. RES and BW Slide 5 • Calculate XL and L of tag from resonance measurement and known tag IC input capacitance Slide 6

Transmit/Pickup Coil • • • Example is made from 24 mm bobbin, 22 mm,

Transmit/Pickup Coil • • • Example is made from 24 mm bobbin, 22 mm, 10 mm bobbin, some SMA coaxial connectors and cable. This can be used with VNA and also spectrum analyzer with tracking generator. Dimensions are not super critical, this is just one example that does work. Single turn coil to Port 1(blue wire) Two turn coil to Port 2 (red wire) Dimensioned Drawing of Example Actual Example

VNA Settings & Calibration • • Set VNA for: – 13. 56 MHz center

VNA Settings & Calibration • • Set VNA for: – 13. 56 MHz center frequency – 5 MHz span Calibrate for 2 port Transmission Measurement (just need thru) Attach Transmit/Pickup Coil Assembly Set Scale for: 3 d. B/div Reference Value = -25 d. B Reference Position = 10 divisions Log Mag View Example Measurement with properly tuned tag f. C = 13. 72 MHz, BW = 683 k. Hz, Q = 20

Resonant Frequency and Bandwidth Measurement • • Put Tag on pickup coil fixture Activate

Resonant Frequency and Bandwidth Measurement • • Put Tag on pickup coil fixture Activate 3 markers – – – • • Marker 1 = f. CENTER Marker 2 = f 1 Marker 3 = f 2 BW = f 2 - f 1 Use bandwidth captured to calculate Q value, where: Kiloway based tag being measured on VNA fixture FCI based tag on fixture f. C = 14. 4 MHz, BW = 610 k. Hz, Q = 23. 6

Calculate L, then XL based on measured value • • Using transponder resonant frequency

Calculate L, then XL based on measured value • • Using transponder resonant frequency measurement and known input capacitance of transponder IC, L then XL can be calculated without having to break the tag IC/coil connection to measure directly. Using known TI input capacitance (23. 5 p. F) and measurement previously shown on slide 4, calculation examples are below. Complex impedance would be: ~ 1+ j 488 to 1+j 493