So how do we do it? There are three primary factors: 1) Minimizing Ferrite Loss, 2) Minimizing Waveguide Loss, and 3) Precision Fabrication & Alignment.
Minimizing Ferrite Loss – The traditional method used to tune Faraday rotation isolators is to use ferrites that are substantially longer than the minimum required length and then tune the magnetic bias field to achieve optimal performance. This approach works very well at the lower frequencies. But at the higher mm-wave frequencies ferrites become very lossy. To minimize loss in mm-wave isolators it is essential that the ferrite length be reduced as much as possible.
At Micro Harmonics we use a saturating magnetic bias field and the minimum possible ferrite length to achieve 45° rotation. We measure our magnetic bias fields to ensure the ferrites are saturated. We use magnetic armatures to achieve a focused, uniform bias field in the ferrite. The graph to the right shows the measured magnetic bias field near the surface of the ferrite core. The peak measured value of 2000 Oe is substantially more than what is required for saturation.