IP2 and IP3 Design Considerations for a Direct Conversion I/Q Receiver

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  • Author: Doug Stuetzle
This article analyzes the levels of 2nd and 3rd order distortion products and their effects on a direct conversion I/Q receiver in a WCDMA applicationture offers several advantages over the traditional superheterodyne. It eases the requirements for RF front-end bandpass filtering, as it is not susceptible to signals at the image frequency. The bandpass filters need only attenuate strong out-of-band signals to prevent them from overloading the front end. Also, direct conversion eliminates the need for IF amplifiers and bandpass filters. Instead, the RF input signal is directly converted to baseband, where amplification and filtering are less difficult. The overall complexity and parts count of the receiver are reduced as well. Direct conversion does, however, come with its own set of implementation issues. For example, since the receive LO signal is at the same frequency as the RF signal, it can easily radiate from the receive antenna and violate regulatory standards.