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CMOS Receiver Eliminates SAW Filter

Full-duplex communication systems like code division multiple access (CDMA) and wideband CDMA (WCDMA) have both triple-beat (TB) and input second-order-intercept (IIP2) performance requirements at the transmit offset frequency. As a result, an external surfaceacoustic- wave (SAW) filter is typically employed after the low-noise-amplifier (LNA) stage. Yet a SAW-less receiver system would eliminate the costly filter as well as the external matching components between the LNA output and mixer input. Recently, an embedded filtering passive (EFP) mixer was used to overcome transmitter power leakage in a receiver without the use of a SAW filter. This work was conducted by Namsoo Kim and Lawrence E. Larson from the University of California at San Diego together with QUALCOMM's Vladimir Aparin.

This integrated-circuit (IC) receiver comprises a differential LNA that employs an active-predistortion, in-phase/quadrature (I/Q) embedded-filtering passive mixer and two transimpedance amplifiers (TIAs) for I/Q outputs. It exhibits more than +60 dBm of receiver IIP2 with a 2.4-dB receive noise figure. The receiver boasts 77-dB TB performance with 45 MHz offset transmit leakage at a 900-MHz receiver frequency while consuming just 18 mA from a 2.1-V supply. The TB specification can be measured by applying two tones in the transmit band and one jammer tone in the receive band. The resulting XMD tone appears at the difference frequency of the two transmit tones from the receive-band jammer offset. The TB specification is defined as the difference in power between the jammer and XMD tones.

Compared to a conventional receiver, the receiver IC offers an additional 15 dB transmit rejection because of the embedded filtering passive mixer. This additional transmit rejection improves the IIP2 by 10 dB and the triple beat by 30 dB. To avoid triple-beat and transmit second-order intermodulation (IM2) issues in a SAW-less receiver, the researchers recommend that the leaked transmit signal be rejected before the mixer. The fabricated receiver IC measures 2.25 mm2 including bonding pads, electrostaticdischarge (ESD) devices, local-oscillator (LO) input buffer, frequency divider, and mixer drivers. See "A Highly Linear SAW-Less CMOS Receiver Using a Mixer With Embedded Tx Filtering for CDMA," IEEE Journal Of Solid- State Circuits, August 2009.

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