Analog Integrated Circuits and Signal Processing, 25, 261–269, 2000 A 1.9-GHz Direct Conversion Transmitter IC with Low Power On-Chip Frequency Doubler SHOJI OTAKA, RYUICHI FUJIMOTO AND HIROSHI TANIMOTO (MEMBERS) Research and Development Center, Toshiba Corporation, Kawasaki-shi, 212–8582 Japan Received June 25, 1998; Revised August 26, 1998 Abstract. A direct conversion transmitter IC including a proposed frequency doubler, a quadrature modulator, and a 3-bit variable attenuator was fabricated using BiCMOS technology with f T of 12 GHz. This architecture employing frequency doubler is intended for realizing wireless terminals that are low in cost and small in size. The architecture is effective for reducing serious interference between PA and VCO by making the VCO frequency different from that of PA. The proposed frequency doubler comprises a current-driven 90  phase- shifter and an ECL-EXOR circuit for both low power operation and wide input power range of local oscillator (LO). The proposed frequency doubler keeps high output power even when rectangular wave from LO is applied owing to use of the current-driven 90  phase-shifter instead of a voltage-driven 90  phase-shifter. An LO leakage of less than  25 dBc, an image rejection ratio in excess of 45 dBc, and a maximum attenuation of 21 dB were measured. The transmitter IC successfully operates at LO power above  15 dBm and consumes 68 mA from 2.7 V power supply voltage. An active die size is 1.5 mm  3 mm. Key Words: frequency doubler, quadrature modulator, variable attenuator, transmitter, PHS, 90  phase-shifter, ECL-EXOR 1. Introduction A direct conversion transmitter is a candidate for realizing low cost and small size terminals, because IF filters can be eliminated [1,2]. The direct conversion transmitter, however, suffers from interference between power amplifier (PA) and VCO (PA-VCO interference) [3], and local oscillator (LO) leakage due to direct coupling between LO input port and RF output port through substrate and package leads, because LO frequency and RF frequency are the same. One of the practical solutions to the problem is to use a frequency doubler (DBLR) in an LO signal path. This architecture is tolerant of distortion caused by the frequency doubler in addition to be subject to less PA-VCO interference and 2f LO leakage, because harmonics caused by the frequency doubler can be reduced by the following RF bandpass filter and PA. This paper presents new circuit techniques for a low power frequency doubler which comprises a current-driven 90  RC-CR phase-shifter and a mixer. The proposed frequency doubler can handle LO signal of rectangular waveform including odd order harmonics which cause a serious degradation of a phase-accuracy for the conventional voltage-driven RC-CR phase-shifter. It is also shown that the proposed frequency doubler has a wide LO input range using these techniques. Furthermore, this paper describes characteristics of a complete 1.9 GHz direct conversion transmitter IC (TX-IC) fabricated using BiCMOS technology  f T  12 GHz. The TX-IC integrates the frequency doubler in LO signal path, a quadrature modulator (QMOD) [4], and a variable attenuator (ATT) [5]. The measured results indicate that the TX-IC is applicable for low GHz band wireless systems. 2. Frequency Doubler A frequency doubler is a key component for realizing a direct conversion transmitter. In this section, two Copyright, 1999, IEICE, reprinted with permission from IEICE Transaction.