Image Frequency
Understanding Image Frequency
The image frequency problem is inherent to superheterodyne receivers. The mixer responds equally to signals at f_RF and f_image because both produce the same IF output. Without image rejection, noise and interference at the image frequency degrade receiver performance.
Image Frequency Example
f_LO = f_RF + f_IF
f_image = f_RF + 2*f_IF
Example: f_RF = 10 GHz, f_IF = 1 GHz
f_LO = 11 GHz
f_image = 12 GHz
Both 10 GHz and 12 GHz mix with 11 GHz LO
to produce 1 GHz IF!
Image Rejection Methods
- Pre-selection filter: BPF before the mixer that passes f_RF and rejects f_image. Requires 2*f_IF separation. Higher IF = easier filtering.
- Image-reject mixer (Hartley/Weaver): Uses two mixers with 90-degree phase shift to cancel the image. 20-40 dB rejection without filtering.
- Zero-IF (direct conversion): f_IF = 0. No image frequency. But introduces DC offset and 1/f noise challenges.
Frequently Asked Questions
What is the image frequency?
The image frequency is an unwanted input that produces the same IF as the desired signal. Located at f_RF +/- 2*f_IF (depending on LO injection side). Must be rejected by filtering or image-reject mixer architecture.
How do you reject the image?
Pre-selection BPF before the mixer (most common), image-reject mixer (Hartley/Weaver architecture), or direct conversion (zero-IF, no image). Higher IF makes pre-selection filtering easier because the image is farther from the desired signal.
Why choose high IF vs low IF?
High IF: easier image rejection (image is far from desired signal) but harder IF filtering for channel selection. Low IF: better channel selectivity but difficult image rejection. Dual conversion provides both: high first IF for image rejection, low second IF for selectivity.