Microwave Engineering Europe - March 2008 - (Page 11)

WIRELESS INFRASTRUCTURE — RECEIVER DESIGN 11 A direct conversion I/Q demodulator drives favorable basestation cost-performance metrics By James Wong, RF Product Marketing Manager, Min Zou, Design Manager, Doug Stuetzle, Senior Applications Engineer, and Sunny Hsiao, Applications Engineer, Linear Technology Corporation irect conversion (or low IF) receiver designs have been widely used in cell phones and wireless LAN radios because of their simplicity and lower cost. Direct conversion receiver ICs have only recently met the level of performance required for basestations. The drive to reduce cost of the next generation basestations, particularly acute for micro-cell and pico-cells, compel attention to the direct conversion architecture which offers signiﬁcant cost savings. The challenge is how to contain its susceptibility to interfering signals that are in-band to the receiver. This necessitates that the I/Q Demodulator used in the direct conversion receiver be able to handle demanding dynamic range requirements that can be as high as 80 dB. This paper highlights some of these design issues unique to direct conversion receivers for basestation applications, and describes a RF direct conversion IC that takes great strides in resolving these issues. A new generation demodulator such as the LT5575 from Linear Technology makes it possible to exploit the advantages of the direct conversion receiver architecture. The LT5575 is designed for applications where high receiver linearity is required to mitigate high blocker signals. They include wireless basestations (for GSM, CDMA, WCDMA, UMTS, etc.) and wireless infrastructure such as WiMAX and broadband microwave links, as well as RFID applications. The device is particularly suited for single-carrier Microand Pico-cell basestations where the costperformance metrics is demanding. Direct conversion receiver ICs eliminate the need for additional intermediate frequency (IF) stages, and relax the demands on high frequency ﬁlters, especially by eliminating the IF channelselect ﬁlter. With its +22.6 dBm input 3rdorder intercept (IIP3) and +60 dBm input 2ndorder intercept (IIP2) at 1900 MHz, this direct conversion receiver meets the strict dynamic range requirements of basestation receivers. The LT5575 integrates the functionality of an LO precision quadrature phase splitter and two high linearity down-converting mixers. The D Table 1: Summary of the typical RF performance of the LT5575. integrated on-chip broadband transformers provide simple 50 Ohm single-ended interfaces at both the RF and LO ports while still maintaining excellent RF-to-LO isolation and minimum LO-to-RF leakage across a wide RF operating frequency range. A single-ended RF signal is applied to the primary side of the on-chip RF transformer and is converted into a differential signal at the inputs of the integrated RF signal splitter. The chip directly down-converts the RF signal to baseband, while separating the in-phase (I) and quadraturephase (Q) signal components. The matched I and Q channels ensure precise gain and phase matching. The LT5575 operates over the input frequency range of 0.8 GHz to 2.7 GHz. The chip also integrates single-pole, low pass ﬁlters with 490 MHz bandwidth on each of the I and Q channels. The typical RF performance of the LT5575 is summarized in Table 1. Direct conversion receivers introduce a different set of design issues The block diagram of a direct conversion receiver, as it might be used in a W-CDMA basestation application, is shown in Figure 1. There are several considerations with this design that distinguish it from multi-stage, superheterodyne receiver designs. These can be summarized as: (1) DC offsets, (2) vulnerability to 2nd order distortion, (3) increased sensitivity to port-to-port coupling, (4) I – Q mismatch, and (5) requirement for proper high frequency termination of the baseband outputs. DC offset errors DC offset voltage at the I or Q outputs of the I/Q demodulator (i.e. non-zero DC output voltage when there is no RF signal) can be a problem for DC-coupled receiver designs due to its subsequent ampliﬁcation by the baseband ampliﬁers. If the DC offset voltage is too large, then it can limit the baseband ampliﬁer signal swing and thus degrade the dynamic range of the baseband A/D converters. DC offset can arise from mismatches in the I/Q demodulator itself. The LT5575 realizes a typical differential DC offset voltage of less than 10 mV at the I and Q outputs, which is low enough to allow Microwave Engineering ● March 2008 ● www.mwee.com 011_012_013_014_MWEE.indd 11 20/02/08 12:03:30 http://www.mwee.com

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Microwave Engineering Europe - March 2008 News Contents Comment Wireless Infrastructure: A Direct Conversion I/Q Demodulatordrives Favorable Basestation Cost-performance Metrics Wireless Infrastructure: Mobile World Set to Reshape the Internet RF Amplifiers: Latest Advances in RF Amplifiers Include a CMOS PA Operating at 77 GHz and Significant Advances in PAs for WiMAX and Broadband Applications Many Applications Still Require Unique Performance Benefits of BeO ACE Automated Circuit Extraction Returns to Real Design by Exploring Design Alternatives and Changes in Seconds Exceeding the Standard for Wireless Sensor Networks Products Calendar

Microwave Engineering Europe - March 2008

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