200-280GHz CMOS RF Front-End of Transmitter for Rotational Spectroscopy*

A 200-280 GHz RF front-end of transmitter is demonstrated in 65-nm CMOS. Saturated EIRP is greater than -5dBm over a frequency range of 60GHz. When the input power is -20dBm, EIRP is greater than -10dBm for most of the frequency range, and achieves 3-dB and 6-dB bandwidths of 24% and 33%. The front-end was integrated with a fractional-N synthesizer to form a transmitter operating at 208-255GHz with EIRP of -18 to -11dBm. The transmitter and a CMOS receiver are used for rotational spectroscopy and to detect ethanol in human breath. Index Terms — CMOS, millimeter-wave, RF front-end, rotational spectroscopy, transmitter, breath analysis Introduction Rotational spectroscopy enables detection of gas molecules with absolute specificity and excellent sensitivity, as well as determination of concentration. Presently, rotational spectrometers are implemented with compound semiconductor devices that are bulky and costly. A transmitter for the spectrometer should generate an FM signal that can be scanned over ~100-GHz frequency range with a 10-kHz step. The transmitted power level should be -30 to -10dBm to avoid the saturation of gas molecules in a sample [1]. This paper reports a 200-280GHz RF front-end consisting of a wideband amplifier, a mixer and an on-chip dipole antenna that amplifies and mixes an input signal at 85-145GHz with a local oscillator (LO) at 110-140GHz to generate the RF signal. Use of an up-conversion mixer instead of a frequency doubler alleviates the problem of providing broadband high input power (~5dBm) to doublers. The RF front-end was integrated with a fractional-N synthesizer with quadrature phase combining similar to [2] that provides a signal at 83-120GHz and a frequency quadrupler to provide a local oscillator signal (Fig. 1). This transmitter with the highest level of integration for rotational spectroscopy was fabricated in 65-nm CMOS and used for rotational spectroscopy at 220-250GHz. Furthermore, a CMOS receiver [3] and the transmitter were used in a rotational spectrometer to detect ethanol in human breath. RF Front-End of Transmitter Design The power and frequency plans for the design of transmitter are shown in Fig. 1. As the output power requirement is not high and wideband amplification at 90-150GHz is challenging, it is better to use a mixer rather than a frequency multiplier as proposed in [2]. Compared to self-mixing in a doubler [4] one of the mixing terms (LO), can be generated at higher power over a narrower bandwidth, resulting in a higher conversion gain and power efficiency, especially when the IF input power is low. The mixer based system also helps in scanning, as coarse and fine steps can be split between LO and IF, as well as, filling in frequency gaps sometimes present in broadband VCO’s by using LO frequency tuning. To compensate for the low input power of ~-20dBm from a broadband synthesizer [2], a 4-stage stagger tuned amplifier shown in Fig. 2 is inserted before the IF input port of mixer. A Marchand balun [4] is used for conversion of the single ended input or PLL output to differential. A differential topology enabled the use of neutralization and transformers avoiding coupling capacitors [5, 6]. The neutralization capacitor used to cancel Cgd of the transistor is designed using a top-to-bottom metal routing for lower resistive losses. The transformers use a turning ratio of ~1 to keep the self-resonant frequency higher than the operating frequency. Differential transmission lines are also used for inter-stage matching. This also increases the bandwidth of match as it effectively folds the impedance response of the following stage on a Smith chart. The amplifier simulation results are shown in Fig. 3(a). It achieves 20-dB gain from the single ended input to the differential output. The mixer (Fig. 4(a)) is implemented using a fully balanced structure with a differential RF port driving an on-chip dipole antenna [3]. The amplifier output and LO input are coupled to the mixer using transformers. The RF port is matched with a shunt stub created by extending the antenna feedline leading to a symmetrical structure. The mixer can be operated in active and passive mode depending on the DC bias at the shunt stubs of the RF port. The simulated conversion gain of mixer in the active mode with 3-dBm LO power is -10dB. The mixer is tuned for higher conversion gain at higher frequencies to compensate for the lower gain of the IF amplifier at high frequencies leading to a flatter frequency response. The simulated impedance matching is also shown in Fig. 4(b). The dipole antenna [3], transformers and differential transmission lines are constructed using the ~3–μm thick top copper layer. * This work is supported by SRC through TxACE at the UT, Dallas (Task ID: 1836.119) and Samsung GRO. Fig. 1. Block diagram of transmitter with power and frequency plans. Fig. 3. (a) Amplifier characteristics, (b) EIRP of amplifier-mixer chain and complete TX. Mixer LO Amplifier × 4 90 to 150 GHz -5 to 0 dBm 90 to 150 GHz -20 to -10 dBm 200 to 280 GHz EIRP > -10 dBm 110 to 140 GHz 0 to 5 dBm