Thanks to their non-coherent nature and narrow bandwidth limit, super regenerative receivers (SRRs) are well suited for terahertz imaging applications using continuous-wave excitation.
To fully leverage silicon CMOS for imaging detectors, these devices should be designed for small chip areas with low power consumption—thus making array integration possible. At the University of California, a pair of engineers recently presented intermodulated regenerative receivers (IRRs) as a technique that allows for low noise reception beyond an active device’s maximum frequency of oscillation, fmax. In doing so, their approach overcomes the fundamental tradeoff between pre-amplified and direct-detection approaches. The receivers’ logarithmic mode, for example, is preferred for terahertz applications, as the soft compression improves pixel contrast while extending available dynamic range.
Specifically, Adrian Tang and Mau-Chung Frank Chang have introduced an ultra-high-frequency IRR. To successfully operate beyond the maximum oscillation frequency of active devices for terahertz imaging applications, the fundamental oscillator is intermodulated in a conventional super-regenerative receiver (SRR). A second oscillator is used to boost the reception frequency.
The engineers note that a low-noise amplifier (LNA) does not need to be present for the circuit to operate effectively. While wireless data communications engineers typically characterize a receiver’s performance by noise figure and sensitivity, the terahertz community focuses on noise-equivalent power (NEP) to determine how well small signals will be detected. With NEP, noise performance is quantified with integration time considered. Basically, NEP describes detector noise by specifying the input power required by an identical but noiseless detector to match the output voltage noise of the detector under consideration. An integration time of exactly 1 s is specified.
When the receiver is implemented in 65-nm CMOS (fmax = 280 GHz), reception frequency to 349 GHz is achieved. In 40-nm CMOS (fmax = 350 GHz), maximum reception frequency increases to 495 GHz. At the alternate intermodulation frequencies, multiple received bands are generated. They enable the possibility of false-color terahertz imaging. The prototype IRR consumes 18.2 mW/pixel in 0.021 mm2 when implemented in 65-nm CMOS. In 40-nm CMOS, it consumes 5.6 mW in a 0.11-mm2 footprint. See “Inter-Modulated Regenerative CMOS Receivers Operating at 349 and 495 GHz for THz Imaging Applications,” IEEE Transactions On Terahertz Science And Technology, March 2013, p. 134.