blog about Guide to Accurate Sparameter Measurements with Network Analyzers

In the field of electronic engineering, accurately evaluating and optimizing circuit network performance is crucial. Without precise measurements of circuit characteristics, designing high-performance electronic devices would be akin to navigating blindfolded. Network analyzers serve as the fundamental solution to this challenge by measuring impedance and attenuation in circuit networks, providing engineers with critical insights.

Network analyzers are specialized instruments designed to analyze electronic circuit networks, with core capabilities focused on measuring impedance and attenuation. Technological advancements have continuously expanded their frequency range, now covering millimeter-wave bands up to 110 GHz, significantly broadening their application scope.

There are two primary types of network analyzers:

• Scalar Network Analyzers: Measure only signal amplitude for determining frequency characteristics. Their simpler architecture facilitates easier implementation of high-frequency measurements.
• Vector Network Analyzers (VNAs): Measure both signal amplitude and phase, providing comprehensive circuit network information. VNAs offer superior measurement precision and have become indispensable in modern electronic measurement applications.

Given the critical role of VNAs in contemporary electronics, this article focuses specifically on vector network analyzer principles and applications.

While various parameters can describe circuit networks (including V-parameters, Z-parameters, and H-parameters), network analyzers primarily utilize S-parameters (scattering parameters) due to their power-based nature, making them particularly suitable for high-frequency circuit characterization.

The fundamental architecture of a network analyzer comprises several key components: a signal source, signal splitter (power divider), directional coupler, and at least three receivers. These elements must cover the instrument's entire operational frequency range.

The measurement process begins with the signal source generating a test signal that splits into two paths: one connects to a reference (R) receiver as a baseline, while the other serves as the incident signal entering the device under test (DUT) through the directional coupler. The A receiver captures reflected signals, while the B receiver measures transmitted signals.

S-parameters are determined by calculating the ratios A/R and B/R. The received signals undergo conversion to intermediate frequencies, followed by synchronous detection to extract real and imaginary components. Digital processing then presents this data in various formats including Smith charts, logarithmic magnitude, phase, and group delay.

S-parameters quantitatively describe a DUT's transmission and reflection characteristics. For a two-port network, the key S-parameters are:

• S11: Signal reflected from port 1 when excited at port 1
• S21: Signal transmitted to port 2 when excited at port 1
• S12: Signal transmitted to port 1 when excited at port 2
• S22: Signal reflected from port 2 when excited at port 2

Each S-parameter is a complex number with real (magnitude) and imaginary (phase) components. Transmission characteristics (S21/S12) relate to gain, loss, isolation, group delay, and transmission coefficients, while reflection characteristics (S11/S22) correspond to impedance, return loss, voltage standing wave ratio (VSWR), and reflection coefficient.

The exceptional measurement precision of network analyzers stems from sophisticated calibration processes that eliminate inherent system errors. By measuring known standards (open, short, load), the system characterizes its own S-parameters, then mathematically removes these errors from DUT measurements during post-processing.

• SOLT (Short-Open-Load-Thru): Standard for coaxial systems
• Offset Short: Preferred for waveguide applications
• LRL/TRL/LRM (Line-Reflect-Line variants): Ideal for microstrip and coplanar waveguide (CPW) structures

1. Establish measurement reference planes at coaxial cable ends (port 1 & 2)
2. Measure open (∞ resistance) on port 1 and short (0 resistance) on port 2, storing forward reflection responses
3. Reverse the standards and measure reverse reflection responses
4. Connect matched loads (50Ω) to both ports for directionality/isolation measurements
5. Perform thru connection between ports for transmission response characterization

Calibration standards typically trace to national measurement standards, enabling error correction through vector mathematics. Full two-port calibration (12-term error correction) addresses directional, source/load match, frequency response, and isolation errors.

While calibration removes systematic errors, several non-repeatable error sources remain:

• Connector repeatability variations
• Receiver residual noise
• Environmental fluctuations (temperature, humidity, vibration)
• Frequency stability impacts on phase measurements
• Calibration process repeatability

Optimal measurement practices include:

• Maintaining consistent connector torque
• Controlling environmental temperature
• Using high-stability signal sources
• Minimizing coaxial cable movement during measurements

These precautions help minimize unstable error contributions and ensure measurement accuracy.

Network analyzers represent indispensable tools in modern electronic measurement. Through comprehensive understanding of S-parameter theory, meticulous calibration practices, and careful error management, engineers can precisely characterize and optimize circuit network performance. Mastering network analyzer techniques remains essential for professionals working in RF, microwave, and high-speed digital circuit design.