Linewidth of submillimeter wave flux-flow oscillators

Abstract

A reliable technique for wide band measurements of the spectral linewidth of superconducting oscillators integrated on-chip with superconductor-insulator-superconductor ͑SIS͒ detectors has been developed. The spectral linewidth of flux-flow oscillators ͑FFO͒ based on the unidirectional and viscous flow of magnetic vortices in a long overdamped Josephson tunnel junction was measured in the frequency range 250-580 GHz, and a linewidth as low as 200 kHz was obtained at 450 GHz. Also stable frequency locking of a FFO to very high ͑р60th͒ harmonics of an external microwave reference source has been demonstrated. The proposed technique may improve the sensitivity, frequency resolution, and stability of the fully superconducting integrated submillimeter wave receiver.

Phase locked 270–440 GHz local oscillator based on flux flow in long Josephson tunnel junctions

“…For spectral radioastronomy applications besides the noise temperature and the antenna beam pattern, the frequency resolution of the receiver, which is determined by both the instant linewidth of the local oscillator and its long-time stability, should be much less than 1 ppm of the center frequency. Recently a reliable technique for linewidth measurements was developed 4 and a free-running FFO linewidth as low as a few hundred kHz has been observed. 4,5 The reduction of the linewidth obtained by phase locking described below significantly improves the spectral resolution of the sub-mm receiver.…”

“…Recently a reliable technique for linewidth measurements was developed 4 and a free-running FFO linewidth as low as a few hundred kHz has been observed. 4,5 The reduction of the linewidth obtained by phase locking described below significantly improves the spectral resolution of the sub-mm receiver. Equally important is that the coexisting wide band lock-in tunability of the FFO enables a large spectral coverage.…”

Towards a phase-locked superconducting integrated receiver: prospects and limitations

“…i) A receiver DSB noise temperature below 100 K has been achieved for SIR with the internal FFO operated in the frequency range 480 -520 GHz [7]; it means that the performance of the SIS mixer is close to the quantum limit. ii) A free-running FFO linewidth considerably below 1 MHz has been measured near 450 GHz [8,9]. Furthermore, phase locking the Josephson FFO to an external oscillator has recently been demonstrated experimentally [9].…”

“…The absence of the resonance structure makes it possible to provide a detailed analysis of the free-running FFO spectrum, which has been measured using the integrated harmonic mixer technique [8]. A frequency detector system with a relatively low loop gain has been used to frequency lock the FFO.…”

“…Note that the observed FFO linewidth is almost one order of magnitude wider [4,10] than predicted by the theory for a lumped Josephson tunnel junction [11,12]. The FFO linewidth was measured in a wide frequency range up to 600 GHz using a novel experimental technique [8]. A specially designed integrated circuit comprising the FFO, the SIS mixer, and the microwave circuit elements needed for the rf coupling is used for linewidth measurements.…”