A Tunable Wideband Frequency Synthesizer Using LC-VCO and Mixer for Reconfigurable Radio Transceivers

Hindawi Publishing Corporation Journal of Electrical and Computer Engineering Volume 2011, Article ID 361910, 7 pages doi:10.1155/2011/361910 Research Article A Tunable Wideband Frequency Synthesizer Using LC-VCO and Mixer for Reconfigurable Radio Transceivers Yusaku Ito,1 Kenichi Okada,2 and Kazuya Masu1 1 ICE Cube Center, Tokyo Institute of Technology, Tokyo 226-8503, Japan 2 Department of Physical Electronics, Tokyo Institute of Technology, Tokyo 152-8552, Japan Correspondence should be addressed to Kenichi Okada, Received 2 May 2011; Accepted 6 June 2011 Academic Editor: Antonio Liscidini Copyright © 2011 Yusaku Ito et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper proposes a novel wideband LC-based voltage-controlled oscillator (VCO) for multistandard transceivers. The proposed VCO has a core LC-VCO and a tuning-range extension circuit, which consists of switches, a mixer, dividers, and variable gain combiners with a spurious rejection technique. The experimental results exhibit 0.98 to 6.6 GHz continuous frequency tuning with −206 dBc/Hz of FoMT , which is fabricated by using a 0.18 µm CMOS process. The frequency tuning range (FTR) is 149%, and the chip area is 800 µm × 540 µm. 1. Introduction Recently, dozens of wireless communication standards have been used for small mobile terminals, for example, GSM, UMTS, LTE, WiMAX, WLAN, Bluetooth, UWB, GPS, DTV, and RFID, and the standards use several frequency bands spreading in a quite wide range such as 800 MHz to 6 GHz. The mobile terminals have been obtaining multistandard operations, smaller size, and lower power operation [12]. However, the present multistandard RF front end consists of several LNAs, VCOs, mixers, and PAs for each frequency band (Figure 1). A multistandard RF front end implemented in a single chip is required for smaller size, lower power, and more flexible wireless communication terminals such as 800 MHz to 6 GHz. The software defined radio (SDR) has been studied [9, 13], and the multistandard RF front end is also needed to realize the SDR with feasible power consumption. Several multistandard RF front ends have been proposed. Digital-assist architectures are suitable for Si CMOS chips [14, 15]. As a common component for the multistandard RF front ends, this paper proposes a wideband frequency synthesizer covering 0.98 GHz to 6.6 GHz [20]. 2. Previous Work Ring-oscillator-based VCOs have unacceptably large phase noise for the wireless communication while it has very wide frequency tuning range. Thus, LC-based VCOs are required for the application due to the phase noise requirement. However, the tuning range of LC-based VCOs is usually very narrow such as 2 to 3 GHz even through the 800 MHz-to6 GHz tuning range is required for the multistandard RF front ends. The conventional LC-VCO cannot overcome the trade-off, so a new wideband LC-based VCO architecture has to be developed. A VCO using switched capacitors is a well-known topology to extend the tuning range [7, 21], and a switched inductor and a variable active inductor are also utilized [8, 16]. However, these circuits have a trade-off between the phase noise and the tuning range. The VCO using a variable MEMS inductor achieves wide-tuning range with superior phase noise characteristics [18]. However, it is difficult for these pure CMOS VCOs to obtain wide-tuning range with adequate phase noise. Recently, wideband VCOs for MB-OFDM UWB have been reported [1, 2, 4, 17, 22, 26], which use a tuning range extension technique using QVCO, dividers, and singlesideband mixer (SSBM). These VCOs achieve quite wide tuning range and high spurious rejection using SSBM with I/Q signals. However, the VCOs in [1, 2] use two oscillators and have large layout area and larger power consumption. Although the VCOs in [10, 22, 26] use only one QVCO, these VCOs also have larger phase noise and larger power consumption. 2 Journal of Electrical and Computer Engineering RF front end I SW LPF Q LO PA ADC PGA PLL LPF FD 1/N VCO LPF MIX Baseband LSI MIX LNA DAC Reconfigurable analog RF circuit Variable passive device DAC Variable bias voltage Switch Control Digital circuit Measure circuit Control circuit Memory If switch turns to (B), gain B is controlled to zero SV in 2 ASV in 1 0◦ Variable gain combiner SV in 1 1/2 fo 3/2 fo BSV in 2 180◦ + 1/2 fo SV out = 3/2 fo SV out = ASV in 1 + BSV in 2 fo or 1/2 fo Gain control VCO fo Divider 1/2 Div Switch 1/2 fo (A) 1/2 fo (A) 1/2 fo and 3/2 fo MIX DC and 2 f o Switch turns to (A) Figure 1: Concept of the reconfigurable RF circuit design. Reject spur Combiner Divider 3/2 fo (A) fo (B) 2 fo (B) fo (B) 1/2 Div 3/4 fo fo Bias, switch, and gain control Figure 2: The proposed wideband VCO architecture. 2 fo 3/2 fo fo 3/4 fo 1/2 fo 1 1.5 2 2.25 3 4 4.5 5 6 (GHz) 1 to 6 GHz Figure 3: Frequency plan from 1 GHz to 6 GHz. Wideband VCOs for multistandard transceivers are also reported [10, 13, 23]. The VCO in [10] use a QVCO and SSBMs, which also has larger phase noise and larger power consumption. The VCOs in [13, 23] use differential oscillators and 1/2 frequency dividers to avoid utilizing SSBM and the quadrature generation. The VCO in [13] uses two oscillators, and it requires, moreover, three oscillators for continuous frequency tuning. The VCO in [23] still requires two oscillators. The wideband VCO proposed in [19] uses divide-by2, divide-by-3, divide-by-4, divide-by-5, divide-by-6, divideby-8, and divide-by-10 frequency dividers for the tuning range extension. This architecture requires a wideband QVCO, and continuous tuning cannot be realized in the measurement [19] because ±20% tuning range is difficult for QVCOs. Various topologies for tuning range extension can be utilized depending on the required performances. In this paper, we propose a novel extension architecture to achieve wider tuning range with lower power, smaller layout area, and lower phase noise, which achieves ±71% of tuning range from a ±20%-range core VCO [20]. The proposed architecture utilizes a differential VCO to generate the Journal of Electrical and Computer Engineering Vdd 3 M3 Vbias L L C1 = 660 fF C2 = 1200 fF Cvar2 Cvar1 C1 Vctrl C1 M4 M1 C2 C2 VCO− VCO+ M5 M2 Figure 4: Schematics of core VCO using switched capacitors. Vdd 3. Wideband VCO Architecture Vdd OUTmix− OUTmix+ M7 M6 M5 M4 LO+ LO− M3 M2 IN Vmix bias IN − M1 (a) wideband mixer Vdd Vdd OUT− OUT+ OUTmix+ M4 M3 OUTmix− Vgain1 M1 M5 M6 LO− LO+ M2 Vgain2 (b) variable gain combiner Figure 5: Circuit schematics used in the proposed wideband VCO. fundamental frequency with smaller layout area, lower power consumption, and lower phase noise characteristics than quadrature VCOs. A variable gain combiner is employed to reje (...truncated)