Abstract
Conventional image-reject mixing based on Hartley and Weaver architectures cannot deal with the mixing spurs generated by the nonlinearity of the mixer, leading to a fundamental restriction on the instantaneous bandwidth and frequency tuning range. This paper proposes and demonstrates a new architecture for broadband image-reject mixing enabled uniquely by photonics. The RF signal and the electrical LO signal are converted into the optical domain and are then launched into the signal and LO ports of a 90-degree optical hybrid, respectively. The optical hybrid coupler introduces 0, π, π/2 and 3π/2 phase shifts to the input signals, so a new dimension for additional balanced detection is enabled, which can dramatically remove the undesirable mixing spurs and the common-mode noises. As a result, the proposed architecture suppresses the downconverted image and the nonlinear mixing spurs simultaneously, enabling a truly wideband microwave frequency mixer. A theoretical and experimental investigation is performed. More than 60-dB image rejection and mixing spur suppression is achieved over a 40-GHz working frequency range.
Highlights
Frequency downconverter is an essential part in almost all the RF information systems [1,2,3], by which a radio frequency (RF) signal at ωRF is mixed with a local oscillator (LO) signal at ωLO to generate an intermediate frequency (IF) signal at ωIF = |ωRF-ωLO| for further processing in low-frequency receivers
To suppress the image interference without employing a pre-filter, image-reject mixers (IRMs) based on phase-cancellation technique known as the Hartley architecture and the Weaver architecture were proposed in the 1980s [4]
The key is to introduce a pair of quadrature LO signals, which mix with the RF signals to generate two quadrature IF outputs
Summary
In order to break the electronic bottleneck [5], many photonics-based approaches were previously reported to extend the working frequency range and bandwidth [5,6,7,8,9,10,11,12,13,14,15], but almost all of them were implemented based on the Hartley architecture Both the Hartley and Weaver architectures, can only eliminate the downconverted image, which are incapable to cope with the RF/LO leakages and mixing spurs generated by the nonlinearity of the mixer (for example, the mixing component between the RF and image) [4]. Performance of the mixing spur rejection ratio versus the LO power is investigated
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