Ultra-wide tuning range signal generation in CMOS for millimeter wave rotational spectroscopy

Electromagnetic waves in the millimeter and submillimeter wave frequency range are being utilized in fast-scan rotational spectroscopy for detection and identification of gas molecules. This technique can be used for monitoring indoor air quality, gas leaks, human breath, and others for a wide variety of safety, security and medical applications. Advances of the high frequency capability of complementary metal-oxide semiconductor (CMOS) have made it an affordable means for implementing the electronics for these spectroscopy systems. A signal generation circuit operating at ~100 GHz and higher with an ultra-wide frequency tuning range (~50%) is a key component for the systems. This research focuses on developing key techniques for ultra-wide tuning millimeter wave signal generation in CMOS.;First, the applicability of using NMOS switched variable inductors in millimeter wave frequencies for wide frequency tuning is demonstrated via design of an LC voltage-controlled oscillator (LC VCO) incorporating NMOS switched variable inductors and tunable output buffers. The prototype fabricated in a TI 65-nm bulk CMOS process demonstrated a wide frequency tuning of 21.5 to 33.4 GHz without a frequency tuning gap. The phase noise at 10-MHz offset of VCO varies from -117 to -109 dBc/Hz. The oscillator core consumes 4 or 6 mA from a 1.2-V power supply. These represent a record 43.3% tuning range with FOMT ranging from -191.7 to -181.9 dBc/Hz. With tunable output buffers, the measured signal output power is above -15 dBm across the entire frequency range.;Second, a passive quadrature coupling and broadband harmonic combining network is demonstrated. This network has 3-dB lower theoretical fundamental-to-4 th order harmonic conversion loss than that of the linear superposition technique, with no extra DC power consumption, lower phase noise, while bypassing the need for a broadband on-chip bias-tee. A quadrature LC VCO incorporating this novel passive coupling and broadband harmonic combining for frequency multiplication by 4 is fabricated to generate signals over a wide frequency range above 90 GHz. The prototype fabricated in TI 65-nm bulk CMOS demonstrated a continuous frequency tuning from 85 to 127 GHz. This ~40% frequency tuning range is at least 4X higher than the other CMOS implementations with center frequency over 90 GHz. At power consumption of 30~45 mW from a 1.5-V power supply, the measured output power varies from -15 to -23 dBm and phase noise at 10-MHz offset varies from -108 to -102 dBc/Hz over the output frequency range. These are sufficient for use in millimeter wave rotational spectroscopy. This work paves the way for implementing a single-chip CMOS transmitter for rotational spectroscopy at 180-300 GHz.