Principal Investigator Ruonan Han
here is a growing interest in pushing the frequency of beam-steering systems towards terahertz range, in which case narrow-beam response can be realized at chip scale. However, this calls for disruptive changes to traditional terahertz receiver architectures, e.g., square-law direct detector arrays (low sensitivity and no phase information preserved) and small heterodyne mixer arrays (bulky and not scalable). In the latter case, corporate feed for generating and distributing the local oscillation signals (LO) -- typically a necessary component -- can be very lossy at large scale. Here, we report a highly scalable 240-GHz 4×8 heterodyne array achieved by replacing the LO corporate feed with a net- work that couples LOs generated locally at each unit. A major challenge for this architecture is that each unit should fit into a tight λ/2×λ/2 area to suppress side lobes in beamforming, making the integration of the mixer, local oscillator, and antenna into a unit ex- tremely difficult. This challenge is well-addressed in our design. We have built highly-compact units, which ultimately enables the integration of two interleaved 4×4 phase-locked sub-arrays in 1.2-mm2.
The schematic of the circuit of one unit core component is a self-oscillating harmonic mixer (SOHM), which can simultaneously (1) generate high-power LO signal and (2) down-mix the radio frequency (RF) signal. The SOHM is connected to both an intra-unit slot antenna (TL4 and TL4’) for RF receiving and a co-planar waveguide (CPW)/slotline mesh (TL3) for strong LO coupling with neighboring SOHMs. Owing to the coupling, LOs generated in each unit can be all locked to an external reference signal so that the array is coherent. Measured spectrum of 4.6-MHz (below the noise corner frequency) baseband signal is shown, from which we obtain a sensitivity (required incident RF power to achieve SNR=1 at baseband) over 1-kHz detection bandwidth of 38.8pW -- more than 6× improvement over state-of-the-art large-scale homodyne arrays.