The release of a license-free 7-GHz band around 60 GHz has prompted researchers around the globe to focus on millimeter-wave circuit and system design for this frequency band. Although such frequencies suggest the use of semiconductor processes like GaAs and InP, the constant scaling of CMOS technology has spawned receivers that are fast enough for millimeter-wave operation. Recently, a 90-nm, digital-CMOS, two-path, 52-GHz phased-array receiver based on local-oscillator (LO) phase shifting was demonstrated. The quadrature voltage-controlled oscillator (QVCO) flaunts an 8-GHz tuning range. The receiver achieves 30 dB of maximum gain and 7.1 dB of minimum noise figure per path around 52 GHz while consuming 65 mW. It occupies an area of just 0.1 mm2.
This achievement is credited to Karen Scheir, Stephane Bronckers, Jonathan Borremans, and Piet Wambacq from IMEC and Vrije Universiteit Brussel in Belgium together with Yves Rolain from Vrije Universiteit. To implement beamforming, the researchers' 52-GHz phased-array receiver relies on phase shifting in the LO path. The receiver comprises two antenna paths that each has a low-noise amplifier (LNA), mixer, and phase generator. A QVCO generates quadrature LO outputs, which are buffered and distributed to the phase generators. The QVCO is tunable between 48.2 and 51.7 GHz. It boasts phase noise of 87 dBc/Hz offset 1 MHz from the carrier and draws 19 mA from a +1.2-VDC supply.
A high-impedance cascading approach was adopted between stages. The researchers also proposed an algorithm to allow in-situ measurement of the LNA center frequency. Thanks to techniques for the implementation of variable gain and the expansion of the QVCO tuning range, a gain-selection range of 12.6 dB and a QVCO tuning range of 8 GHz were achieved. See "A 52 GHz Phased-Array Receiver Front-End in 90 nm Digital CMOS," IEEE Journal Of Solid-State Circuits, December 2008, p. 2651.