Constant-Phase Electrically Controllable rf Attenuator using Permalloy Films at Resonance

Abstract

A constant-phase, electrically controllable attenuator using Permalloy films has been investigated both analytically and experimentally. The well known behavior of the permeability as a function of bias field near resonance is utilized. The permeability of a Permalloy film can be expressed as μ=μ1−jμ2. In a film-loaded strip line, the attenuation constant is strongly affected by μ2 while the phase constant depends on the magnitude and sign of μ1. Controllable attenuation with net zero phase change can be realized with the following arrangement: Two film strips, identical in properties and geometry are used in a strip line and both are biased at H0, the resonance bias field, resulting in a maximum attenuation and zero phase shift. Super-imposed is a control field of opposite polarity on the two film strips. If the control field is set at ΔH, then the net bias field is H0−ΔH on one film strip and H0+ΔH on the other, yielding a permeability, −|μ1|−j|μ2| on one and +|μ1|−j|μ2| on the other. As a result, the phase change due to the reactive permeability components cancel each other out and only the attenuation due to the loss component remains. Such an attenuator has been built and tested successfully over a frequency range from 1 to 3 Gc/sec.