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Highly linear SAW-less receiver design techniques for CDMA


The proliferation of wireless communication systems has resulted in ever demanding low-cost handsets. The wide adoption of the Homodyne receiver system is a consequence of industrial requirements. Yet, the inter-stage high-Q filter, rejecting the leaked transmit (Tx) signal between the LNA and mixer, is prevalent in frequency division duplexed (FDD)receivers, so that the amplified Tx signal does not corrupt overall receiver performance. This research is focused on the method of eliminating the inter -stage high-Q filter by adopting new circuit topologies. The main consideration is given to mixer design, since the mixer needs to be either highly linear or to reject the Tx signal by itself. First, a highly linear passive mixer with simple degeneration, in order to improve noise figure (NF) and IIP₂ performance, is researched. A receiver using a proposed passive mixer with degeneration is fabricated in a 0.18[Mu]m Si CMOS process. The operating frequency is from 1.55 to 2.3 GHz. The measured performance shows less than 9.5dB double-sideband (DSB) NF, more than 22dB voltage gain, better than +50dBm uncalibrated IIP2, and higher than +7dBm of IIP₃, while consuming only 10mW from a 2 V supply. In the second approach, an embedded filtering passive (EFP) mixer is researched. Unlike a normal passive mixer, the EFP mixer performs narrow band downconversion, which provides filtering for the Tx signal. A complete receiver, consisting of LNA, EFP mixer, transimpedance amplifier (TIA), and local oscillator (LO) path, is fabricated in a 0.18[Mu]m Si CMOS process. The receiver IC exhibits more than +60dBm of Rx IIP₂, 2.4dB Rx noise figure, and +77dB of triple beat (TB) with 45 MHz offset transmit leakage at 900 MHz Rx frequency while consuming only 18mA from a 2.1 V supply. The proposed receiver IC shows an additional 15dB Tx rejection compared to a conventional receiver. The additional Tx rejection improved the IIP₂ by 10dB and the TB by 30dB

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