Linewidth measurements of Josephson flux-flow oscillators in the band 280–330 GHz

TLDR

An integrated receiver comprising two Josephson flux‑flow oscillators coupled to an SIS mixer via center‑fed interdigital capacitors was used to bias the oscillators on velocity‑matching steps, mix their 280–330 GHz outputs in the SIS mixer, and detect the amplified difference frequency on a spectrum analyzer. The oscillators exhibited a combined linewidth of 2.1 MHz across the band, with 430 nW of available power at 320 GHz, sufficient to optimally pump the SIS mixer.

Abstract

An integrated receiver consisting of two Josephson flux-flow oscillators and a superconductor-insulator-superconductor (SIS) mixer has been used to measure the linewidth of the oscillators. The oscillators are coupled to the SIS mixer using two centerfed interdigital capacitors, one of which with a fairly weak (−20 dB) coupling. By biasing the two oscillators (a few microvolt apart) on their velocity matching steps, the two resulting oscillations (280–330 GHz) were mixed in the SIS mixer. The difference frequency was amplified and detected on a spectrum analyzer. The combined linewidth of the two oscillators was 2.1 MHz throughout the band. The calculated available power incident on the SIS mixer was 430 nW at 320 GHz. The rf power from the oscillators, which could be tuned by changing the current bias, is high enough to pump the SIS mixer optimally.