Extension of Coincidence Limiting Frequency Range in Ferrimagnets

Power limiting using the first-order spin-wave instability has been observed in a ferrimagnetically coupled filter-limiter at higher frequencies than had previously been believed possible. For example, a filter-limiter, using orthogonal strip-line resonators coupled by the dipolar fields of a YIG sphere, was constructed at 4000 Mc. Output power was limited with the sharp break at the threshold and flat limiting curve characteristic of coincidence limiters. This result is in contrast with the operation of similar limiters at this frequency which use the second-order spin-wave instability in which the limiting characteristic has a much more gradual slope transition above the threshold. Coincidence limiters have previously been believed to have a maximum operating frequency ωc=2ωMNT (3.27 kMc for a YIG sphere) since, with an isolated sample, coincidence of the uniform precession resonance and the subsidiary absorption occurs only for ω<ωc. These results are observed for a coupled mode resonance in these ferrimagnetically coupled circuits which occurs for dc magnetic field values in the subsidiary absorption region below the uncoupled uniform precession value (ω/γ). It is found that a minimum value of the circuit-sample coupling is required to observe this extension of coincidence type limiting. Limiting thresholds as low as −10 dBm have been observed with the strip-line circuit and lower levels are believed to be possible. Wide low-level bandwidths (>10%) are also obtained with these circuits, but with an increase in the limiting threshold. The above results are explained by computing the normal modes of the microwave-ferrimagnetic resonator circuit. It is found for tight enough coupling that one of the branches of the normal mode spectrum is in coincidence with the subsidiary absorption at a higher frequency than the uniform precession resonance. The usual coincidence limiter operation (ω<ωc) is thus the small coupling case of the more general type of operation.