A Vector Beam Measurement System for 211-275 GHz

Paper in proceeding, 2006

We propose a novel design for a vector measurement system for the characterization of mm-wave receiver optics alignment across the 211-275 GHz band. Previous published work on vector mm and submm beam measurements employs phase-locked loops (PLLs) with Gunn-oscillators and/or multiple frequency sources, [1], [2]. We are developing a measurement set-up without any PLLs and employ the combination of a single frequency source, comb-generator and direct multiplication LO unit. The design takes advantage of different harmonics used to generate the RF and LO signals and to create the desired IF. Importantly, at the same time it allows obtaining a perfect phase-coherence and initial phase-noise cancellation. The signal source is a vector network analyzer (VNA). In the suggested scheme the VNA generates a signal, fsource, which is fed into a comb-generator that generates number of frequencies ∆fsource apart. When fsource is low, this results in a large number of closely spaced, phase-coherent frequencies. Since any selection among the generated frequencies is phase-coherent, any two of them can be used to produce phase-coherent RF and LO signals by filtering and multiplication. As a test source we use a harmonic mixer mounted on a xyz-scanner. Its absolute position with respect to the receiver optics is predefined by means of triangulation system comprising lasers and position sensitive detectors. The IF from the SIS mixer is down-converted using suitable reference from the comb generator to produce an IF equal to fsource. This signal is fed into Port 2 of the VNA and amplitude and phase are measured. The advantage of using comb generator is to have closely spaced frequencies to choose from and therefore ease the production of the desired IF-frequency and obtaining initial-phase-noise cancellation. Most of the phase-noise is cancelled in the down-conversion in the SIS-mixer and the remaining is cancelled in the second down-conversion before the measurements in the VNA. The cancellation of the remaining phase-noise present at IF is obtained by selecting also the LO, for the second down-conversion, from the harmonics generated by the comb-generator. We also propose to use the same measurement set-up for the frequency bands 275-370 GHz and 385-500 GHz. We believe that our system design has the potential to cover all three bands by only replacing two filters and the LO multiplication unit. Scalar beam measurements up to 320 GHz have already demonstrated a dynamic range of about 20 dB with the harmonic mixer as the RF transmitting source [3]. We expect that vector measurements provide even greater dynamic range, indicating that we can push the frequency even higher with this harmonic mixer as the RF comb source. At the moment we are in the assembly phase of the system design and intend to present results from measurements of the Band 1 of the Facility receiver for Atacama Pathfinder EXperiment (APEX) at the time for the Conference.