The Square Kilometre Array (SKA)

Abstract:

Efficiently mapping large areas of the sky with high spectral resolution at mm and sub-mm wavelengths will require a new generation of heterodyne focal-plane array receivers. The number of pixels in such arrays has not increased particularly rapidly in the last two decades, with maximum achieved pixel numbers between 16 and 64 (e.g. [1] and [2]). Thus new approaches are needed to address such problems as local oscillator (LO) injection, feed horn fabrication and SIS mixer design, fabrication and repeatability.

Here we describe a prototype focal-plane array unit based on unilateral finline SIS mixers, fed with smooth-walled feed horns. LO injection power diplexing is achieved by a combination of directly machined waveguide Y-power splitters and bow-tie cross waveguide couplers. The 1×4 prototype array, currently under construction, will demonstrate several technologies relevant to the construction of large format arrays.

Abstract:

The presence of spurious frequency tones emanating from a harmonically generated local oscillator (LO) signal can significantly degrade the performance of a SuperconductorInsulator-Superconductor (SIS) tunnel junction mixer [1]. The existence of harmonics is often revealed by distorted I-V curves obtained when pumping the SIS device with the LO. We have investigated this effect by using a commercially available source and correlating the photon step induced I-V curve structure with a direct frequency measurement made via a Fourier Transform Spectrometer (FTS) and a spectrum analyser. Our results suggest that intermixing frequency products are generated within the first, low frequency, stage of the LO chain, and these unwanted signals are converted to high frequency tones through subsequent stages of LO amplification and frequency up-conversion. The experimental work has been performed by using measurement facilities available at the University of Oxford and the Rutherford Appleton Laboratory (RAL), UK.