VNA
Understanding the VNA
The VNA measures how electrical networks modify signals in terms of both amplitude and phase. Unlike a scalar network analyzer (which measures only amplitude), the VNA captures the complete complex transfer function, enabling computation of impedance, group delay, and time-domain response through inverse FFT.
VNA Measurements
- S11 (Reflection): Input impedance match, plotted on Smith Chart. Return loss and VSWR are derived from S11 magnitude.
- S21 (Forward Transmission): Gain or loss through the device. Phase of S21 determines group delay.
- S12 (Reverse Transmission): Isolation or reverse gain. Important for amplifier stability analysis.
- S22 (Output Reflection): Output impedance match.
Calibration
VNA accuracy depends on proper calibration. Standard calibration techniques include SOLT (Short-Open-Load-Thru), TRL (Thru-Reflect-Line), and electronic calibration (ECal). Calibration removes systematic errors from cables, adapters, and the VNA itself.
Modern VNA Features
- Time-domain reflectometry (TDR) through inverse FFT
- Mixed-mode (differential) S-parameter measurement
- Noise figure measurement capability
- Active device characterization with bias and power sweep
Frequently Asked Questions
What does a VNA measure?
A VNA measures the complex S-parameters of RF devices: reflection (S11, S22) and transmission (S21, S12) as a function of frequency. From these, it derives impedance, gain, loss, phase, group delay, VSWR, return loss, and Smith Chart displays.
Why is VNA calibration important?
Calibration removes systematic errors from the measurement setup (cables, adapters, VNA internal paths). Without calibration, these errors are indistinguishable from the device response. Proper calibration moves the measurement reference plane to the device terminals for accurate results.
What is the difference between a VNA and a spectrum analyzer?
A VNA measures how a device modifies a known stimulus signal (S-parameters). A spectrum analyzer measures the power spectrum of an unknown signal. The VNA is for device characterization; the spectrum analyzer is for signal analysis. The VNA generates its own test signal; the spectrum analyzer does not.