1. 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

2. 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

3. 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

4. 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

5. 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

6. 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