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 TA0022 TA0022 RF9938: Low-Cost HBT Upconverters for CDMA Transmitter ICs       To support the increasing demand for Code Division Multiple Access (CDMA), the latest digital communication technology in cellular industry, RF Micro Devices (RFMD) developed CDMA chip sets for both receiver and transmit chain. The CDMA chip sets were designed and manufactured on an advanced GaAs Heterojunction Bipolar Transistor (HBT) process from TRW. The HBT process offers several advantages over other processes such as high Ft, low noise transistors, and process consistency. The CDMA program is two-fold. The first chip set, which was designed for cellular band, consists of a receive and a transmit IF AGC amps (RF9907 and RF9909), a receive LNA/ mixer (RF9906), and a transmit upconverter (RF9908). The second chip set, designed for PCS band, consists of a receive LNA/mixer (RF9936) and a transmit upconverter (RF9938). This article describes the two transmit upconverters for this CDMA program. Design architecture is the same for both cellular and PCS designs. The PCS upconverter, RF9938, is a modified RF9908 with performance optimized for the PCS band. Both ICs operate at 3.6VDC with 0dB conversion gain. With -10dBm input at the IC, typical Adjacent Channel Power Rejection (ACPR) for the RF9908 is -37dB for the 880kHz offset and -58dB for the 1.98MHz offset. The typical ACPR for the RF9938 is -43dB. The nominal power supply current for the RF99...