Radar Basics

YIG - Filter

Figure 1: Principle of a YIG filter

The YIG- filter is an application of the different permeability numbers µ^±_r.

A YIG- filter is a high-precision spherical ferrite made of the single crystalline material YIG. YIG stands for Yttrium Iron Garnet, which is chemically Y₃Fe₅O₁₂. Garnet means a specific crystal structure. With a YIG- sphere between orthogonal coupling loops, a pass-through resonator with coaxial connections and tuning via a magnetic field H₀ can be built in a simple filter arrangement. YIG is a ferrite material that resonates at frequencies in microwave range when immersed in a DC magnetic field. This resonance frequency is directly proportional to the strength of the applied magnetic field and has very linear tuning over several octaves.

The linearly polarized alternating magnetic field of the input coupling loop does not generate a signal in the orthogonally arranged output coupling loop without any magnetization.

When a magnetic field H₀ is applied, a clockwise circularly polarized magnetization is excited strongly in a resonance-like manner. The counterclockwise circular polarization is hardly excited.

(1)

The saturation magnetization M_s (about 142 kA/m) of YIG is strongly temperature-dependent. By the above equation, the rotating magnetization produces an output signal that is 90° out of phase with the input signal. The arrangement acts as a linearly tunable bandpass filter with a small relative bandwidth corresponding to a resonator’s quality factor Q of about 10⁴ since YIG has a very low damping constant.

The tuning range is essentially limited upward by the magnetic field H₀ that can be generated. The lower cutoff frequency results from the condition H_i>0 and is about 1.67 GHz for a spherical shape.

With such YIG resonators, broadband tunable filters can thus be constructed, which can be tuned with the regulated direct current through an electromagnet. They are also used in broadband tunable oscillators.