Single-Chip Tri-Band WCDMA/HSDPA Transceiver without External SAW Filters and with Integrated TX Power Control

Abstract

With growing WCDMA adoption there is a strong demand to reduce the cost of WCDMA terminals. Contributing to the relatively high WCDMA terminal cost is the low integration of today's WCDMA radios. Since WCDMA is an FDD system the transmitter and receiver are on simultaneously. A duplexer is used to isolate the receiver from the transmit signal but the isolation of small low-cost duplexers is limited, with a guaranteed isolation at the TX frequency of not more than 52dB. For a Class-3 terminal with 2dB TX path loss due to duplexer and switch, the power at the PA output is +26dBm and the receiver will see a TX leakage of up to −26dBm. Today's state-ofthe-art WCDMA transceivers [1-3] deal with the TX blocking problem by using an external TX SAW filter to eliminate TX noise in the RX band and an external LNA and RX SAW filter to achieve sufficient RX linearity. A typical tri-band transceiver requires 6 SAW filters and 3 LNAs. The tri-band WCDMA transceiver presented here eliminates all external SAW filters and LNAs and achieves excellent performance with standard low-cost duplexers.In the presence of a strong modulated TX blocker at the receiver input the total noise at the output of the receiver consists of four contributions: the IC RX noise figure, the TX noise in the RX band [6], the reciprocal mixing of the TX blocker and the IM2 products arising from the modulated TX blocker. Using the method described in [4] and targeting only a 0.2dB noise-figure degradation at maximum TX power due to reciprocal mixing and IM2 products, then the LNA referred receiver requirement is for an IIP2>+50dBm and LO phase noise <−160dBc/Hz. The presence of a CW blocking signal together with the TX signal will produce an in-band spurious response when this blocking signal is either at 0.5× or 2.0× the duplex frequency. To achieve sufficient margin against the requirements [7] an IIP3 of −5dBm is targeted. Figure 10.2.1 shows a block diagram of the transceiver in a tri-band WCDMA application. The transmitter is discussed in detail in [6]. The receiver front-end is shown in detail in Fig. 10.2.2. A singleended LNA is used to interface to standard single-ended duplexers. The LNA has a high IP3 common-source input stage, cascode gain control and a notch filter at the source of the cascode. A low-noise Qenhancement circuit is used to improve notch selectivity. Together with the LC load this notch filter provides additional attenuation of the TX leakage. The LNA is followed by a high IP3 transconductor and a passive mixer optimized for high IP2. A low-pass filter is inserted between the passive mixer and the transimpedance amplifier to further improve out-of-band IP3. The transimpedance amplifier also implements the first real pole of a 5 th -order baseband filter. The remaining 4 poles are implemented as two biquads, and a 1 st -order equalizer is added to reduce group-delay variation as shown in Fig. 10.2.1. The overall baseband filter response is optimized for minimum ripple and group-delay variation to maintain low EVM for HSDPA applicat...

Managing linearity in radio front-ends

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Equalization of Third-Order Intermodulation Products in Wideband Direct Conversion Receivers

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