A Bandwidth-Enhanced Differential LC-Voltage Controlled Oscillator (LC-VCO) and Superharmonic Coupled Quadrature VCO for K-Band Applications

electronics Article A Bandwidth-Enhanced Differential LC-Voltage Controlled Oscillator (LC-VCO) and Superharmonic Coupled Quadrature VCO for K-Band Applications Farman Ullah 1,2,3 , Yu Liu 1,2, *, Zhiqiang Li 1,2 , Xiaosong Wang 1,2 , Muhammad Masood Sarfraz 1,2,3 and Haiying Zhang 1,2 1 2 3 * Beijing Key Laboratory of Radio Frequency IC Technology for Next Generation Communications, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China; (F.U.); (Z.L.); (X.W.); (M.M.S.); (H.Z.) Institute of Microelectronics, University of Chinese Academy of Sciences, 19A Yuquan Rd., Shijingshan District, Beijing 100049, China COMSATS University, Wah Campus, G. T. Road, Wah Cantt 47040, Pakistan Correspondence: ; Tel.: +86-108-299-5811 Received: 29 June 2018; Accepted: 20 July 2018; Published: 25 July 2018



 Abstract: A novel varactor circuit exhibiting a wider tuning range and a new technique for quadrature coupling of LC-Voltage Controlled Oscillator (LC-VCO) is presented and validated on a 25 GHz oscillator. The proposed varactor circuit employs distribute-biased parallel varactors with a series inductor connected at both ends of the varactor bank to extend the tuning range of the oscillator. Similarly, the quadrature coupling is accomplished by employing the 2nd harmonic, explicitly generated in the stand-alone free-running differential oscillator using frequency doubler. As an example, the Differential VCO (DVCO) is tunable between 20 GHz and 31 GHz and exhibits the best Phase Noise ( PN ) of −100 dBc/Hz at 1 MHz offset frequency. Similarly, the Quadrature VCO (QVCO) covers 42% tuning bandwidth around 25 GHz oscillation frequency, which is significantly wider than other state-of-the-art VCOs at comparable frequencies. In addition, all the oscillators are designed in class-C to further improve their performances both in term of low power and low phase noise. The presented oscillators are designed using high-performance SiGe HBTs of the GlobalFoundries (GFs) 130 nm SiGe BiCMOS 8HP process. The presented DVCO and QVCO draw currents of approximately 10 mA and 21 mA, respectively from a 1.2 V supply. Keywords: millimeter wave oscillator; wideband VCO; superharmonic coupling; QVCO; oscillation bandwidth; varactor 1. Introduction Modern wireless communication is transforming into future 5G and the demand for high data-rate communication with increased bandwidth requirements is increasing. Voltage Controlled Oscillator (VCO) as one of the key components is used in modern phase-locked loop (PLL) to provide local frequency signal. It is critical for VCOs that can robustly provide wider tuning range ( TR) at mm-wave frequency region with low phase noise. To date, many oscillators featuring low-power consumption ( PDC ), low phase noise ( PN ) and wide TR in K-band have been reported [1–8]. However, it is still challenging and uncommon to design VCO with optimized trade-off to simultaneously achieve a wide TR with low PN and low PDC . For example, in [1], two K-band SiGe bipolar VCOs using transformer-coupled varactors are presented. But they can only be tuned from 18.6 to 21.2 GHz and 20.4 GHz to 24.2 GHz, resulting only in 13% and 17% tuning ranges, respectively. Similarly, Electronics 2018, 7, 127; doi:10.3390/electronics7080127 www.mdpi.com/journal/electronics Electronics 2018, 7, 127 2 of 14 triple-couple inductor as part of the LC-Tank, to couple multiple varactors, is implemented to get more linear and wider TR, but only 15.8% TR is achieved at K-band [2]. In [3], the reported differential VCO utilizes current-reuse and transformer-feedback techniques fabricated in the standard bulk 90 nm CMOS process, can only achieve 4.8% tuning bandwidth at K-frequency band. Similarly, various Negative Capacitance ( NC ) circuits were integrated with traditional LC-VCOs to cancel out the parasitics (produced mainly by the LC-tank and cross-coupled pair) to expand the TR [9,10]. For example, in [9], the NC circuit is used to shift the oscillation frequency from 20.1 to 31.6 GHz, which, although it improves the TR% from 5.7 to 12.4%, it makes the power consumption too high (590 mW) and worsens the phase noise to −88 dBc/Hz. Similarly, a tunable differential NC circuit was designed in [10] to compensate the parasitic capacitances and is realized by connecting the NC circuit to the source of the cross-coupled transistors of the LC-VCO; however, it affected the start-up condition. Therefore, the NC of [10] only had a small effect on the resonance frequency ( f RES ) and TR. Similarly, the Quadrature Voltage Controlled Oscillators (QVCOs) play an important role in many fully-integrated, low cost, radio-frequency transceivers requiring I/Q modulation/demodulation. To provide Quadrature Local Oscillator ( LO) signals, various techniques are endorsed. Two approaches are very common for quadrature LO generation: (1) divide-by-two [11] and (2) polyphase filter techniques [12]. However, the earlier is more power hungry since the system oscillator needs to be operated at twice the desired frequency, while the later suffers from low quadrature accuracy as well as requiring an additional buffer to boost the output power. Later, the LC-based QVCOs are presented for the generation of quadrature LO signals without employing divide-by-two or polyphase filters, which resulted in huge reductions in power consumption as well as improved accuracy [13]. LC-based QVCOs are obtained by employing antiphase coupling between two identical differential oscillators. The antiphase connection is realized using a coupling network, either an active or passive coupling. The circuit techniques employing active coupling are parallel coupling (P-QVCO) [13], series coupling (S-QVCO) [3], top- and bottom-series coupling (TS-QVCO and BS-QVCO) [14], sub- and super-harmonic coupling [15,16], body-biased coupling [17], In-phase injection-coupling [18], complementary coupling [19] etc. Similarly, passive coupling techniques like inductor-based superharmonic coupling [20], transformer coupling [21], and coupling using transmission lines [22] are used for quadrature LO generation. Firstly, we emphasized to attain wider TR at K-frequency band. To realize this, we proposed a novel varactor circuit that consists of two similar branches of varactors, biased at different voltages. They are also cascaded with two inductors, each connected to the common nodes of the varactor bank at both sides. The proposed varactor scheme exhibits wider TR at mm-wave frequency. Besides, the TR of the VCO is enhanced by properly designing the VCO-core and aligning the consecutive frequency tuning characteristics with sufficient overlap margin to avoid blind zones between them. In addition, a novel technique for quadrature generation is proposed in this work. Subsequently, the two similar bandwidth-enhanced differential oscillators that we proposed in our first work are locked in quadrature by implementing th (...truncated)