Image Rejection
Understanding Image Rejection
Image rejection is a fundamental challenge in superheterodyne receiver design. Without adequate image rejection, interference at the image frequency would be converted to the same IF as the desired signal, causing interference that cannot be filtered afterward.
Image Rejection Methods
- Image-reject filter: A bandpass or bandstop filter before the mixer that attenuates the image frequency. Requires adequate frequency separation (2 x IF) between desired and image.
- High IF: Using a high IF increases the separation between desired and image, making the filter easier to design. Trade-off: harder to build a narrow channel filter at high IF.
- Image-reject mixer (Hartley/Weaver): Uses two mixers with 90-degree phase shift to cancel the image response. Achieves 20-40 dB image rejection without a filter.
Frequently Asked Questions
What is image rejection?
Image rejection is the suppression of the image frequency in a superheterodyne receiver. The image is at f_LO +/- 2*f_IF from the desired signal. Without rejection, image-frequency interference converts to the same IF as the desired signal.
How much image rejection is needed?
Communications receivers typically require 60-80 dB of image rejection. This is achieved by combining filter rejection (40-60 dB) with mixer architecture rejection (20-30 dB). Higher IF makes filtering easier but requires better IF processing.
Does zero-IF have an image problem?
Zero-IF receivers have f_IF = 0, so the image frequency equals the desired frequency. There is no traditional image problem, which is one advantage of zero-IF. However, I/Q mismatch creates a similar effect called self-image interference.