VNA Measurements & Calibration
A vector network analyser (VNA) measures the complex S-parameters of a device under test (DUT) — both magnitude and phase — across a swept frequency range. Understanding what it measures, and how calibration moves the reference plane to your device, is essential for making accurate RF measurements.
What a VNA Measures
A 2-port VNA applies a stimulus at port 1 and measures the signals at both ports simultaneously. It computes:
| Parameter | Physical meaning | Typical display |
|---|---|---|
| S₁₁ | Input reflection coefficient. How much is reflected from port 1. | Smith Chart, log mag (RL) |
| S₂₁ | Forward transmission. Gain or insertion loss from port 1 to port 2. | Log magnitude (dB) |
| S₁₂ | Reverse transmission. Isolation. | Log magnitude (dB) |
| S₂₂ | Output reflection coefficient. | Smith Chart, log mag |
Calibration: Moving the Reference Plane
Without calibration, a VNA measures its own cables, connectors, and switch imperfections. Calibration removes these systematic errors by measuring known standards and computing correction factors. The most common calibration is SOL (Short-Open-Load) or SOLT (Short-Open-Load-Through):
- Short: Γ = −1 (all reflected, 180° phase). Defines the reference plane position.
- Open: Γ = +1 (all reflected, 0° phase, adjusted for fringing capacitance).
- Load: Γ = 0 (50 Ω termination, no reflection). Sets the amplitude reference.
- Through: Connects port 1 to port 2 directly. Characterises transmission path.
Always calibrate at the ends of the test cables, not at the VNA ports — calibration moves the reference plane to wherever you connect the standards.
Port Extension
If you cannot calibrate at the device terminals (e.g. a connector is on a PCB and you calibrate at the cable end), port extension electrically "moves" the reference plane by adding a known electrical delay. It rotates the phase of S₁₁ and S₂₂ to compensate for the cable length. It is an approximation — it assumes the line is lossless and dispersionless — but is adequate for many applications.
Time-Domain Gating
The VNA can apply an inverse Fourier transform to its frequency-domain data to show the time-domain response (like a TDR). This reveals reflections from connectors and discontinuities separated in time (i.e. distance). Gating in the time domain — windowing to select only a specific time region — then transforming back to frequency allows you to isolate the response of a specific section of the DUT and remove connector effects. This is particularly useful for antenna measurements and filter characterisation.
Interpreting S₁₁ on a Smith Chart
A single frequency point on the Smith Chart shows the impedance at that frequency. As frequency sweeps:
- A resistor traces a point on the real axis.
- A capacitor traces clockwise along the X = 0 boundary toward the short-circuit point.
- A series LC traces a loop: inductive (upper half), resonant (real axis), capacitive (lower half).
- A transmission line section traces a clockwise circle centred on the origin.
- An antenna shows a small loop near the edge of the chart, touching or nearly touching the real axis at resonance.
Practical Measurement Tips
- Torque connectors to spec (SMA: 0.9 N·m / 8 in·lb). Under/over-torquing causes repeatable but incorrect results.
- Let the VNA warm up for 15–30 minutes before calibrating for best stability.
- Calibrate with the cable attached in its final position — bending it after calibration introduces errors.
- Use at least 201 frequency points across your band; use 1601+ for time-domain measurements to maximise time resolution.
- When measuring low-loss filters (< 0.5 dB), use a full 4-port SOLR or TRL calibration for best accuracy.
- Check calibration by measuring a known open or short — it should show a perfect circle on the Smith Chart rim.