A New Approach To YIG-Based Band-Reject Filters

Oscillators and filters rely on resonant behavior to function, and at higher frequencies, the size of the crystal and cavity dictate the frequency of resonance. Yttrium iron garnet (YIG) has an unusual property when used as a resonator, as its resonant properties do not depend on size but on the strength of the magnetic field passing through it. This has some downsides but also some major benefits.

YIG resonators have been in use for decades in oscillators, bandpass filters, and band-reject filters, although they are rarely the first choice when small size or low power are critical metrics. However, they remain valuable in test equipment such as spectrum analyzers, clock recovery units, and military systems because they have benefits not achievable with any other technology. The advantages and disadvantages of YIG technology are shown in Table 1.

The Venerable Band-Reject (Notch) Filter

Test equipment and some military applications require extremely sensitive receivers located next to high-power transmitters, and a continuing issue is how to stop the receiver from being overwhelmed by the transmitter signal that can obscure signals of interest. A ship on which surveillance equipment sits near very high-power radar is a good example. For interference that is either predictable in frequency or dwells long enough to be measured and responded to, it is possible to notch out the unwanted signal.