Analog RC Polyphase Filter and Mixer Design for Large Image Rejection
Achieving adequate image rejection poses the major obstacle to full integration of a superheterodyne wireless receiver (RX). Zero-IF RX’s are attractive for full integration because the image of one half of a channel is the other half of the same channel. Thus, it is sufficient to reject the image by, say, 15 dB more than the final SNRmin required. Also, the receiver power dissipation is potentially lower because the subsequent amplification and filtering is at baseband. However, zero-IF suffers from several drawbacks such as DC offset and flicker noise, which cannot be easily filtered if the modulated spectrum at zero IF contains sensitive information at DC. The low-IF architecture solves this problem by downconverting the desired channel to frequencies beyond the flicker noise corner. The IF amplifiers and filters operate at frequencies substantially the same as in a zero-IF receiver, but now the image consists of some other unrelated channel two times IF away in frequency, which may be substantially larger than the desired channel. All of a sudden, an image rejection of up to 60 dB becomes necessary.
KeywordsInput Impedance Error Sequence Switch Resistance Voltage Gain Pole Frequency
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