A 71-86 GHz Bidirectional Image Selection Transceiver Architecture Najme Ebrahimi 1 , James F. Buckwalter 1,2 n1ebrahi@ucsd.edu, buckwalter@ece.ucsb.edu Abstract— A bidirectional image selection transceiver is presented that operates over 71-76 GHz and 81-86 GHz with only 3 GHz of LO tuning range. A sliding-IF architecture with bidirectional VGAs allows operation in transmit and receive modes. The sliding IF and narrow LO tuning range allow wideband image rejection using a single stage polyphase filter. The circuit is implemented in a 90-nm SiGe BiCMOS process. Measurements indicate conversion gain of -2.5 dB to 3 dB with less than ±0.75 dB variation over 10 GHz in TX mode and -4dB to 0 dB with less than ±1 dB variation over 10 GHz bandwidth in RX mode. With 16- and 64-QAM, the EVM is below 5% and 4% at data rates of 6 Gb/s and 9 Gb/s. The RF and LO circuitry consumes at most 150 mW and 250 mW. Index Terms— Image-selection transceiver, Image- rejection transceiver, E-band, SiGe, Bidirectional I. INTRODUCTION E-band (71-76/81-86 GHz) is particularly attractive for point-to-point backhaul applications because the large (10 GHz) bandwidth will enable high rate communication [1-6]. However, the 20% fractional bandwidth (FBW) required to cover this range poses significant problems for wideband circuit design. In a conventional heterodyne or homodyne radio, the local oscillator (LO) would require similar FBW requirements and result in gain and phase noise trade-offs. Here, a bidirectional sliding-IF Weaver architecture is proposed to select either the upper sideband (USB) or lower sideband (LSB) to significantly reduce the tuning range requirements of the LO while operating over a broad millimeter-wave band. Most importantly, polyphase filters (PPFs) introduce amplitude and phase imbalances over wideband (2 GHz) channels and degrade the image rejection ratio (IRR) over the bandwidth. Point-to-point applications often demand high-order QAM, e.g. 64- QAM, and impose EVM targets under 3%. In this case, an IRR better than 30 dB is required with phase and amplitude imbalance under 2˚ and below 0.5 dB. The proposed solution reduces the LO tuning range thereby relaxing the PPF design requirement and avoiding the gain and IRR variation. Low gain variation and high IRR is important to reach low EVM with wideband QAM [5]. In section II, a frequency plan for 71-76 and 81-86 is presented to demonstrate how to extend operation over a wide bandwidth. The proposed bidirectional image selection architecture is presented in section III and the circuit implementation is presented in section IV. The Fig. 1. Frequency planning for E-band, a) conventional scheme from [1-3], b) proposed image-selection scheme for reduced FBW tuning. measurement results are presented in section V. II. PROPOSED IMAGE SELECTION FREQUENCY-PLANNING Fig. 1a illustrates the conventional frequency planning for a 71-86 GHz transceiver and requires an LO tuning range on the order of 15 GHz (20% FBW) for frequency conversion [1-6]. Wide LO tuning range is very challenging at the mm-wave regime due to the lossy passive and parasitic elements and tends to result in conversion gain variation over a wide bandwidth; often more 10 dB variation of conversion gain across E band [1- 3]. In addition, the LO must tune across this wide range at the expense of phase noise [2-3] and I/Q mismatch [5]. To relax the LO tuning range, this paper proposes the image-selection planning for the E-band transceiver shown in Fig. 1b. In this scheme, the LO tuning should be placed in between the 71-76 GHz band (LSB) and 81-86 GHz band (USB). Consequently, this approaches requires the LO to tune across only 3 GHz tuning range with quadrature phases and phase inverting option to select the upper or lower images of LO with a 3 GHz IF bandwidth. The resulting 3 GHz IF band is centered at 5 GHz. Here, the proposition of this paper is that this frequency plan reduces the tuning range and results in more uniform circuit performance in terms of conversion gain and IRR. The IRR is also subject to a smaller tuning range and therefore is not prone to bandwidth induced amplitude and phase mismatch, which relax the PPF requirements. Fig. 1 shows the EVM dependence to the tuning range of LO. Under ideal conditions, the LO tuning across a ±20% FBW results in an EVM of around 5.5%; considering a 10% RC variation in the PPF, the EVM increases to 10%. For a ±4% FBW corresponding to the proposed approach, the theoretical EVM is under 1% and increases to 4% EVM with under 10% RC variations. -15 -10 -5 0 5 10 15 0 2 4 6 8 10 LO- Fractional BW(%) EVM (%) 1 University of California, San Diego, USA 2 University of California, Santa Barbara, USA PREPRESS PROOF FILE CAUSAL PRODUCTIONS 1