Development of cmos sub-terahertz receivers for spectrometers

Sub-terahertz (THz) frequency bands, covering approximately 100 GHz - 600 GHz of the electromagnetic spectrum, and many uses such as in short range radars, spectrometers for detection of chemicals, electromagnetic imaging for medical applications (T-Ray) and detection of concealed weapons. This region which lies in between the microwave region and the infrared region has been in the focus amongst the scientists and researchers for quite a long time due to its wide range of applications. The idea to use this spectrum in silicon was very much appreciated in the industry as well as in academia. We have systems that do exist for such applications but at a very discrete level and far from everyday use. So, the idea that we introduced through this project is to bring such a system (Gas Spectrometer) on complementary metal oxide semiconductor (CMOS) chip level, which can later be integrated in a device such as our mobile handset. Being a very complex project, it was divided into three main focus areas of research, the signal generation, the transmitter, the antenna and the receiver. We focused on the design of the receiver part. Spectrometers working in the sub-THz frequency can be used to identify different chemicals based on their response to incident electromagnetic waves. Different chemical species respond to electromagnetic waves uniquely, undergoing rotational and vibrational transitions at different frequencies. A spectrometer can detect these transitions by transmitting a series of tones at incremental frequency steps and analyzing the transmitted waveform. In the case of absorption spectroscopy, typical at sub-THz regimes, the chemical absorbs at certain frequencies and the transmitted waveform has the spectral fingerprint of the chemical. In our portion of the project, it is these fingerprints that we have tried to detect and implement the system through a CMOS based receiver. Spectrometer receiver system analysis and the sub-harmonic mixer design were the primary focus of our research. Due to its very wide band frequency requirement, we faced quite a challenge with the design of the sub-harmonic mixer. To prove the concept of sub-harmonic mixer, we taped-out our design first as a test chip and then implemented the idea in a receiver chain. The test chip results show mixer conversion loss of almost 26 dB in a 20 GHz of bandwidth. This is reported to be the first working CMOS sub-harmonic mixer at millimeter wave frequencies. An effort was made to improve its performance in 65nm CMOS process and a complete on chip receiver solution is provided.