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To better understand how S-parameters are measured, and what they represent, let’s use the analogy of a light wave impacting a lens. As illustrated in Figure 1, most of the light wave incident upon a lens is transmitted through the lens, but some is reflected back from the lens. The index of refraction, analogous to impedance mismatch, determines how much light is reflected back from the lens.

When measuring S-parameters, the instrument sends a power wave down the measurement cable at a specified frequency. When the wave encounters the device under test (DUT) any impedance mismatch creates some reflected energy. The instrument — typically a vector network analyzer — records the incident, transmitted and reflected energy, and converts the amplitude and phase information to S-parameters. Network analyzers can sweep S-parameters across a very wide range of frequencies. The results can be converted back to impedance and phase characteristics of the DUT over the measured bandwidth using simulation programs (or by hand if you enjoy doing complex matrix math).

Impedance analyzer measurements of inductors are made to an upper frequency of 3 GHz. When using an accurate phase calibration standard and good calibration and fixture compensation techniques, impedance analyzer measurements of inductance (L) and Q factor (Q) are the most accurate possible. Impedance analyzer measurements are essentially 1-port measurements, typically in the form of L and Q factor or impedance (Z) and phase values. Most RF and microwave inductors have L and Q specified at frequencies ≤250 MHz.

Many RF and microwave inductors have a self-resonant frequency (SRF) above the 3 GHz limit of impedance analyzers. When performance above 3 GHz is required, a vector network analyzer can be used to measure S-parameters at frequencies as high as 40 GHz. Network analyzers use the transmission-reflection measurement method and report results in the form of S-parameters.

Many RF and microwave inductors have a self-resonant frequency (SRF) above the 3 GHz limit of impedance analyzers. When performance above 3 GHz is required, a vector network analyzer can be used to measure S-parameters at frequencies as high as 40 GHz. Network analyzers use the transmission-reflection measurement method and report results in the form of S-parameters.

As the frequency of an electrical wave increases, the wavelength decreases. Therefore, in order to have high measurement precision as the frequency increases, we need higher physical precision for the measurements, calibration standards, and fixtures. In effect, a small error in the physical length of a fixture or calibration trace will lead to larger measurement error as frequency increases.

At the high RF and microwave frequencies at which S-parameters are measured, printed circuit board (PCB) parasitics become increasingly involved. If a test fixture is not representative of the intended circuit at those higher frequencies, the measured performance will not agree with that of the intended circuit.

Adding PCB parasitics to fixture de-embedded S-parameter measurements for simulation requires care and experience. The most accurate method of obtaining circuit-specific S-parameters is to measure them using the intended PCB and layout. The next best method is to use global substrate-scalable models, such as those developed by Modelithics.

S-parameters are small-signal measurements (or based on small-signal models) that have no bias current applied. Therefore, S-parameters should not be used for large-signal loss or saturation modeling of inductors, chokes, or transformers. Saturation is typically not a concern with open-magnetic-constructed (non-magnetically shielded) RF and microwave inductors and chokes, or with wideband RF transformers when used within the published current rating.

At the high RF and microwave frequencies at which S-parameters are measured, printed circuit board (PCB) parasitics become increasingly involved. If a test fixture is not representative of the intended circuit at those higher frequencies, the measured performance will not agree with that of the intended circuit.

Adding PCB parasitics to fixture de-embedded S-parameter measurements for simulation requires care and experience. The most accurate method of obtaining circuit-specific S-parameters is to measure them using the intended PCB and layout. The next best method is to use global substrate-scalable models, such as those developed by Modelithics.

S-parameters are small-signal measurements (or based on small-signal models) that have no bias current applied. Therefore, S-parameters should not be used for large-signal loss or saturation modeling of inductors, chokes, or transformers. Saturation is typically not a concern with open-magnetic-constructed (non-magnetically shielded) RF and microwave inductors and chokes, or with wideband RF transformers when used within the published current rating.

Coilcraft has published measurement-based S-parameters for our RF- and microwave-frequency inductors (chip inductors and air core inductors), RF chokes, wideband RF transformers, and high-speed common mode chokes. The S-parameter data files represent de-embedded 50 Ohm measurements unless otherwise indicated. Because a variety of customer-chosen PCB materials, thicknesses, and board layouts are possible, effects due to circuit board traces, board materials, ground planes, or interactions with other components are necessarily not included.

Our**RF inductor S-parameters **are produced by creating a lumped element (SPICE) model that best matches a representative sample of measured inductors. The S-parameters are then generated from the model using software. Because our RF inductor S-parameters are generated from our models, they offer the advantage of ease of use vs. SPICE models. A “readme” file is included with each zip file for a series, describing the measurement conditions.

The simulated SRF of our S-parameter inductor models will be higher than that of a measurement of the inductor mounted on a circuit board, due to fixture de-embedding. The parasitic reactive elements of a circuit board or fixture will lower the effective in-circuit resonant frequency, especially for very small inductance values. These can be added to the model, if known.

Because our inductor models are generated from a SPICE model that is typically valid up to near the SRF, they capture the 1st resonance and no higher-order parasitic effects. Meaningful simulations of higher-order effects require substrate interactions, which are best captured by a measurement on the intended PCB or a Modelithics substrate-dependent model using the intended PCB characteristics.

S-parameters for**Coilcraft wideband RF transformers and high-speed common mode chokes** are made by measuring a typical part from a representative sample. The published S-parameters represent de-embedded measurements as described in the individual “readme” file for the applicable series.

Our

The simulated SRF of our S-parameter inductor models will be higher than that of a measurement of the inductor mounted on a circuit board, due to fixture de-embedding. The parasitic reactive elements of a circuit board or fixture will lower the effective in-circuit resonant frequency, especially for very small inductance values. These can be added to the model, if known.

Because our inductor models are generated from a SPICE model that is typically valid up to near the SRF, they capture the 1st resonance and no higher-order parasitic effects. Meaningful simulations of higher-order effects require substrate interactions, which are best captured by a measurement on the intended PCB or a Modelithics substrate-dependent model using the intended PCB characteristics.

S-parameters for

Understanding the meaning of S-parameters, how they are measured, and their limitations can lead to more meaningful simulations of RF- and microwave-frequency inductors, chokes, wideband RF transformers, and high-speed common mode chokes. This document describes how Coilcraft S-parameters are generated, and how to best apply them to your simulations.

Keysight, S-Parameter Measurements (Basics for High Speed Digital Engineers), White Paper

- Assistance with Safety Agency Approvals
- Basics of Inductor Selection (from Electronic Design magazine)
- Calibration, Compensation, and Correlation
- Current and Temperature Ratings
- Getting Started: An Introduction to Inductor Specifications
- Hipot Testing of Magnetic Components
- How Current and Power Relates to Losses and Temperature Rise
- Measuring Self Resonant Frequency
- Operating Voltage for Inductors
- Selecting Current Sensors and Transformers
- Simulation Model Considerations: Part I
- Simulation Model Considerations: Part II
- Testing Inductors at Application Frequencies
- Working Voltage Ratings Applied to Inductors

- PCB Washing and Coilcraft Parts
- Selecting Flux for Soldering Coilcraft Components
- Soldering Surface Mount Components

- Broadband Chokes for Bias Tee Applications
- Inductors as RF Chokes
- Key Parameters for Selecting RF Inductors

- Beyond the Data Sheet: The Need for Smarter Power Inductor Specification Tools
- Choosing Inductors for Energy Efficient Power Applications
- Current Sense Transformers for Switched-mode Power Supplies
- Determining Inductor Power Losses
- Ferrite Vs Pressed Powder-core Inductors
- Forward or Flyback? Which is Better?
- Notes on Thermal Aging in Inductor Cores
- Selecting Coupled Inductors for SEPIC Applications
- Selecting Inductors to Drive LEDs
- Selecting the Best Inductor for Your DC-DC Converter
- Structured Design of Switching Power Transformers
- Transformers for SiC FETs

- Coilcraft LC Filter Reference Design
- Common Mode Filter Design Guide
- Common Mode Filter Inductor Analysis
- Data Line Filtering
- Fundamentals of Electromagnetic Compliance
- Passive LC Filter Design and Analysis
- Selecting Common Mode Filter Chokes for High Speed Data Interfaces

- Applying Statistical Techniques to the Design of Custom Magnetics
- Choosing Power Inductors for LiDAR Systems
- Coilcraft Conical Inductors
- Designing a 9th Order Elliptical Filter for MoCA® Applications
- Measuring Sensitivity of Transponder Coils
- Power-handling Capabilities of Inductors
- Signal Transformer Application
- Transponder Coils in an RFID System
- Using Baluns and RF Components for Impedance Matching
- Using Standard Transformers in Multiple Applications