- Buyers Guide
February 2010 Expert Advice
On the surface, choosing an RF inductor for a new circuit design seems to be an easy task, as only a small number of key specifications are involved. However, parameters such as inductance and Q factor vary greatly with frequency. This makes it challenging to compare different inductor styles and sizes from the basic specifications provided on most data sheets. As anyone who has had to switch back and forth between pages of L and Q graphs can attest, the selection process can be tedious and time consuming.
Designers face increasing pressure to shorten development cycles and get to market quicker. They need a tool that will help them select RF inductors for critical circuits without a lot of repetitive modeling and prototyping.
The Old Way
Consider an engineer designing a 900 MHz circuit, looking for a 10 nH inductor with a body size of 0603 or smaller. The process often begins by examining Q factor versus frequency charts, as shown in Figure 1. In order to compare the performance of different part numbers, the designer must consult several sets of curves from multiple data sheets or web pages.
Figure 1 Printed Q vs Frequency curves rarely show all the inductance values in a product family. Both 0402 families actually have a 10 nH value, but they are not plotted because of space constraints.
In this example, the problems begin when the designer is only able to find Q curves for 10 nH inductors on the 0603 family datasheets, but not for the 0402 sizes. The manufacturer actually has 10 nH parts in the smaller size; the curves just aren’t published on the data sheet.
Another complication can arise when the curves are tightly spaced. This makes it difficult to distinguish the curve of interest and pick out the proper Q value. Finally, each graph may use a different vertical and/or horizontal scale, so it’s difficult to make a quick visual determination of which component has the highest Q value.
Because of space constraints on a printed datasheet, most inductor manufacturers only provide curves for a small fraction of the part numbers in a product family. While they may include tabular Q data, it’s typically provided at only one or two frequencies, leaving the engineer to try to extrapolate data at his particular operating frequency.
These are typical difficulties facing engineers who try to compare different inductor models using printed datasheets or web pages. While careful examination of the data might lead to choosing the optimal inductor, this manual method is time consuming and frustrating, especially when the designer is trying to compare three or four alternate parts. Moreover, tedious methods like this can lead to a lack of accuracy, or even worse, outright errors.
New Timesaving Method
To overcome these problems, Coilcraft engineers developed a unique new interactive online comparison tool for RF inductors. It replaces frustrating manual methods and helps prevent false starts in the design of new microwave and RF circuits. By increasing the precision of the inductor selection process, the odds of achieving acceptable circuit performance in the first prototype are much better.
The tool is located at www.coilcraft.com/compare. The user enters his/her operating frequency between 1 MHz and 3 GHz, then selects up to four inductors to compare using pull-down lists to pick the product series and inductance value.
In seconds, the results screen appears (Figure 2). At the top is a table showing specifications for each part at the user’s own operating frequency: inductance, Q factor, impedance and equivalent series resistance (ESR). Below that, these same parameters are graphed to show performance over the 1 MHz to 3 GHz frequency range. Each part is plotted in a different color to make it easy to distinguish. And because they all appear side-by side on the same grid, it’s easy to quickly spot the best performer at any frequency.
Figure 2 Coilcraft’s RF Inductor Comparison Tool shows data for up to 4 parts. A table gives specifications at any frequency specified by the user and side-by-side graphs make it easy to spot the best performer.
If the user wishes to do further simulation modeling, there are links to each part’s SPICE model and S-parameter file. Or if they’re ready to build a prototype, they can request free evaluation samples with a single click.
A “Print to PDF” button allows the comparison results to be printed or saved to a project file on the user’s local hard drive.
While a few other component manufacturers have begun to offer tools that produce on-demand graphs, these typically show only one part at a time and are based on SPICE models. Or they have to be downloaded and installed on the user’s computer with component databases that must be continually updated by the user in order to keep up with new product introductions.
In contrast, the Coilcraft tool compares up to four parts simultaneously and uses actual performance data measured using an impedance analyzer over a range of 1MHz to 3GHz. No simulation or equations are used to represent performance. And because it is web-based, the user is always sure that they have access to data on the newest and best performing inductor families.
The RF Inductor Comparison Tool is just one of the unique design tools on Coilcraft’s web site. There are applications that find parts with the highest Q factor or impedance at a given frequency. Another identifies parts with the desired actual inductance (as opposed to nominal L) at the user’s frequency. These tools demonstrate how manufacturers have begun to use the power of the web to help engineers make more accurate component choices in far less time.