II. VARIABLE INDUCTOR TYPES
A. Moving Core
Perhaps the most recognizable variable inductor technology is the moving core. Most of us are familiar with the cylindrical slug moving through a hollow cylindrical winding commonly found in analog radios. Tunable inductors that move a core using a threaded screw are also quite common.
Modern linear actuation stages can provide very precise, repeatable positioning and they are quite reliable. This made the moving core technology an obvious choice that we explored. The moving core prototype met our requirements in terms of inductance precision, range and resolution. The drawback was that it was rather slow due to the need to mechanically ramp up and then settle after each inductance change. A switch point test on a modern PSEU requires many thousands of discrete inductance settings and the moving core technology did not meet our needs in terms of PSEU automated test throughput.
B. Switched Inductance Decade Boxes
Switched inductance decade boxes are an attractive solution because they are available “off the shelf” from a number of vendors. The switched inductance decade boxes use relays to put a number of inductors in series and/or parallel to produce a desired inductance. This is another technology we explored but discarded. We found issues with monotonicity and “holes” in the inductance range when small changes in inductance required switching in and out relatively large inductors. Though these effects were small, they prevented the switched inductance decade box technology from meeting our needs in terms in precision and resolution.