Jamie Leech

Abstract:

In this paper, we will present the design and the simulation of a 230GHz finline Ultra-wide IF Bandwidth SIS mixer. This mixer will be used in a novel millimeter-wave heterodyne interferometer: GUBBINS. GUBBINS is designed to demonstrate high surface brightness mm-wave interferometry at modest spatial and spectrum resolution. Its observational targets are the spectrum of the Sunyaev-Zel'dovich effect in the brightness galaxy cluster[3]. The archetype of the mixer design described here is an antipodal finline SIS mixer designed by Paul Grimes in Oxford Experimental Cosmology group in 2008[1]. Here several improvement and modification are made to simplify the design and fabrication, and also enhance the IF bandwidth. An unilateral finline replaces the complicated antipodal finline. No RF bandpass filter is needed after finline. The tuning circuit design presented here aims to achieve wider RF coupling bandwidth, even though only a single junction is used. A multi-stage IF transformer follows the IF bonding pad matching the IF output of the mixer to the input of the IF amplifier, as well as reducing the impact of the parasitical capacitance introduced by the RF finline and RF radial stub.

Abstract:

We present the first, complete 230 GHz feedhorn array manufactured by direct drilling of smooth-walled horns into a single plate of aluminium. The horn design process, based on a genetic algorithm, is described and the fabrication process, via direct drilling using shaped drill bits, is presented. We present cross coupling and beam pattern measurements of a close-packed pair of the smooth-walled horns fabricated in a single block of aluminium. We also present a prototype 37 horn array, again fabricated by drilling into a single block. Our measurements show that our designs and fabrication techniques will be robust when applied to large focal arrays of horns consisting of hundreds or thousands of feedhorns. We expect our smooth-walled horn designs and novel manufacturing techniques will offer an attractive, low-cost alternate to traditional horn arrays consisting of electroformed corrugated horns.

Abstract:

We present the design of a broadband superconductor-insulator- superconductor (SIS) mixer operating near 700 GHz. A key feature of our design is the utilisation of a new type of waveguide to planar circuit transition comprising a unilateral finline taper. This transition is markedly easier to design, simulate and fabricate than the antipodal finline we employed previously. The finline taper and the superconducting circuitry are deposited on a 15 μm thick silicon substrate. The employment of the very thin substrate, achieved using Silicon-On-Insulator (SOI) technology, makes it easy to match the incoming signal to the loaded waveguide. The lightweight mixer chip is held in the E-plane of the waveguide using gold beam leads, avoiding the need for deep grooves in the waveguide wall. This new design yields a significantly shorter chip, free of serrations and a wider RF bandwidth. Since tuning and all other circuits are integrated on the mixer chip, the mixer block is extremely simple, comprising a feed horn and a waveguide section without any complicated mechanical features. We employ a new type of smooth-walled horn which exhibits excellent beam circularity and low cross polarisation, comparable to the conventional corrugated horn, and yet is easier to fabricate. The horn is machined by standard milling with a drill tool shaped into the horn profile. In this paper, we describe the detailed design of the mixer chip including electromagnetic simulations, and the mixer performance obtained with SuperMix simulations. We also present the preliminary measurements of the smooth-walled horn radiation patterns near the mixer operating frequencies. © 2010 SPIE.