A CDMA2000 zero IF receiver with low-leakage integrated front-end

Abstract

I. SYSTEM REQUIREMENTS INCREASED levels of radio integration with low external component count are essential for cellular handset manufacturers to achieve low product cost. These demands have led to the popularity of low and zero intermediate frequency ( As a full duplex system, one of the key challenges that CDMA2000 presents is dealing with signal leakage from transmit to receive paths; at the present state of the art, separate transmit and receive chips are the rule to avoid substrate-and package-related signal coupling. For the receiver alone, there is also the requirement to tolerate and indeed to work with legacy narrow-band FM analog systems (AMPS) in the same 880-MHz frequency spectrum [1] (Fig. 1), leading to several severe performance restrictions, in terms of input linearity, local oscillator (LO) leakage, and LO phase noise. In this design, we tackle the first two requirements, and meet the limits with a fully integrated signal path. Other designs employing zero IF (ZIF) for U.S. CDMA have opted to use a separate low-noise amplifier (LNA) to achieve this isolation [5]. A companion transmitter IC [6] also uses a direct conversion scheme to keep total system costs to a minimum. II. ZIF RECEIVER ARCHITECTUREThe overall system architecture is shown in Fig. 2, and the key performance requirements are given in Tables I and II. The worst case interference scenario is presented in Fig. 1, where transmit leakage cross-modulates with a strong adjacent FM carrier. This situation tends to dictate the linearity performance specifications. In practical designs, the attenuation afforded at the input by commercial duplexer blocks is typically around 55 dB, which is insufficient to keep the power in the transmit spectrum present at the receiver input low enough to eliminate this cross-modulation problems associated with FM carriers. To minimize the impact of cross-modulation without employing a sharper, more lossy (and costly) filter at this point, the LNA must have very high linearity (IIP3 of 9 dBm). Note that even with this demanding performance, it is necessary to employ an additional bandpass SAW filter in the receive path between the LNA and the mixer to provide additional attenuation of transmit band leakage (typically 40 dB) and thus ease the mixer linearity requirements.An external oscillator module running at double the required frequency provides the drive to the LO quadrature generator. The signal path is split into in-phase (I) and quadrature (Q) paths in the mixer switching core and passed to low-pass channelselection filters. III. LNA DESIGNThree gain settings are used to achieve the best overall gain and linearity; only high ( 15-dB) and low ( 5-dB) gain settings are shown for clarity. The target here is to achieve 20-dB reverse isolation in all gain modes. To meet the overall leakage requirements, the majority of the required 80-dBm isolation is then achieved in the mixer core and its associated interfaces, and maintained with careful layout and pad assignments.To meet the need for very high reverse ...

A Single-chip dual-band CDMA2000 receiver for cellular terminals in 0.13 µm CMOS

“…The presented design exhibits outstanding performance compared to published receivers [1], [2]. Moreover, this receiver integrates additional functionality, especially by the DFE performing high attenuating channel selection filtering, gain control, decimation and matched filtering.…”

“…Succeeding the demodulator combination is a Tow-Thomson biquad, which, together with the preceding 1stpole filter, forms a 3rd-order butterworth filter mainly for attenuation of the 1700 kHz blocker and far-off interferers. In contrast to previously published designs using higher-order filters [1], [2], this solution benefits from little sensitivity of the attenuation with process variations. Hence, a 5-bit alignment is sufficient to set the corner frequency to 750 kHz with an accuracy of 1 %.…”

“…Recently published work [1], [2], [3] includes receivers for CDMA, W-CDMA and AMPS in SiGe-BiCMOS and CMOS, with all of them comprising a traditional analog baseband interface and matched filtering on the BB-IC. Furthermore, Rx-DFEs have been discussed in [4], [5].…”

“…II A. Analog Receiver Architecture 1) General Problems in CDMA2000: Principal challenges in receiver design for CDMA2000 arise from the 900 kHz AMPS-blocker in three ways [1], [2]. Firstly, the TX leaking with a level of approximately -25 dBm through the duplexer cross-modulates the interferer and causes its product to fall into the band of interest (Fig.…”