Embedded RF Applications/Systems How to use Network Analyzer to measure resonant frequency of HF (13.56MHz) 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 Display Settings for Vector Network Analyzer (VNA) Slide 4 Measure transponder fRES and BW Slide 5 Calculate XL and L of tag from resonance measurement and known tag IC input capacitance Slide 6

Dimensioned Drawing of Example Transmit/Pickup Coil Example is made from 24mm bobbin, 22mm, 10mm 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: 13.56MHz center frequency 5MHz span Calibrate for 2 port Transmission Measurement (just need thru) Attach Transmit/Pickup Coil Assembly Set Scale for: 3dB/div Reference Value = -25dB Reference Position = 10 divisions Log Mag View Example Measurement with properly tuned tag fC = 13.72MHz, BW = 683kHz, Q = 20

Resonant Frequency and Bandwidth Measurement Put Tag on pickup coil fixture Activate 3 markers Marker 1 = fCENTER Marker 2 = f1 Marker 3 = f2 BW = f2 - f1 Use bandwidth captured to calculate Q value, where: Kiloway based tag being measured on VNA fixture fC = 14.4MHz, BW = 610kHz, Q = 23.6 FCI based tag on fixture

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 (23.5pF) and measurement previously shown on slide 4, calculation examples are below. Complex impedance would be: ~ 1+ j488 to 1+j493