Measuring Common-Mode Chokes with the Y21 Method

Updated October 2025 — RF.Guru Technical Series

All the math may look intimidating at first… but modern tools handle most of it for you. Programs like VNWA Tools, nanoVNA-Saver, Keysight ADS, MATLAB RF Toolbox, and Python’s scikit-rf can automatically calculate Y-parameters and Z_DUT. Some VNAs even perform the math internally so you can watch results update live. The real challenge isn’t the math — it’s building a proper test fixture and performing a solid SOLT calibration right at the coax connectors.

This article walks through the on-line method — a simplified practical approach that minimizes math and focuses on good measurement practice. You only need a dual-port VNA, a calibrated fixture, and your choke or ferrite to start getting accurate, repeatable results.

What You’re Trying to Get

Treat the setup as a π-network:

• Y₁ – Shunt admittance at Port 1 (fixtures and cable parasitics to ground)
• Y₃ – Series admittance between Port 1 and Port 2 (the choke under test)
• Y₂ – Shunt admittance at Port 2 (parasitics to ground)

The two-port Y-matrix is:

[[Y₁ + Y₃, −Y₃], [−Y₃, Y₂ + Y₃]]

The key identity is Y₂₁ = −Y₃ ⇒ Z_DUT = −1 ⁄ Y₂₁

The shunt parasitics follow directly:

Zshunt1 = 1 ⁄ (Y₁₁ + Y₂₁), Zshunt2 = 1 ⁄ (Y₂₂ + Y₁₂)

Building the Fixture

Your fixture should present the choke under test as the series element between the two VNA ports, with minimal stray inductance and well-defined ground references. Use short, rigid coax leads and solder directly to the connectors if possible. Keep loop area small and symmetrical.

Each port’s shield must see a small, predictable capacitance to ground — this defines the shunt legs of the π-network. Typically, 47 pF to 220 pF NP0/C0G capacitors from each port shield to the common ground plane work well. The exact value is not critical as long as both sides are equal and stable over frequency. Use your VNA to confirm that with no DUT installed, the fixture shows a shallow capacitive response — this ensures your parasitics are well behaved and can later be subtracted mathematically if desired.

Tip: Mount the capacitors physically close to the coax connector shells to minimize lead inductance. Avoid ceramic dielectrics with high temperature or voltage coefficients.

What You Need

• A 2-port VNA (50 Ω system) capable of S₁₁, S₂₁, S₁₂, S₂₂ measurement
• A series fixture placing the DUT between ports 1 and 2
• SOLT calibration at the coax ends using a female–female barrel THRU

Avoid using a bare wire THRU — it changes reference planes and adds delay/parasitics.

What You Measure

Sweep your band of interest and export the complex S₁₁, S₂₁, S₁₂, S₂₂ values at each frequency point. That’s all you need — the rest is math your software will handle automatically.

How to Calculate — Simplified Path

Modern software or even spreadsheet scripts can handle the conversions, but conceptually the Y21 path is simple:

1. Convert S → Y using Y = (I − S) / (I + S) × 1/Z₀
2. Take the element Y₂₁
3. Compute the choke’s impedance as ZDUT = −1 / Y₂₁

For reciprocal passive devices, −1/Y₂₁ and −1/Y₁₂ should coincide within a few percent.

Pocket Recipe

1. Perform SOLT calibration at cable ends (barrel THRU).
2. Connect the fixture and sweep the band.
3. Export S₂P data or let the VNA compute Y₂₁ directly.
4. Compute ZDUT = −1 / Y₂₁.
5. Plot magnitude |Z|, resistive R, and reactive X versus frequency.

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