What are image frequencies and how do I reject them in a superheterodyne receiver?
Image Rejection
The image frequency problem is fundamental to all superheterodyne receivers. The mixer performs downconversion: fIF = |fRF - fLO|. But this same equation has two solutions for fRF: fRF = fLO + fIF (desired) and fRF = fLO - fIF (image), or vice versa depending on LO injection side. Both frequencies produce output at the same IF, and the mixer cannot distinguish between them.
The image frequency is always 2×fIF away from the desired signal. Higher IF means larger image separation, making image filtering easier. This is the primary reason for choosing a high first IF in dual-conversion receivers. For example, with a 10.7 MHz IF, the image is only 21.4 MHz away from the desired signal, requiring a very sharp preselector. With a 70 MHz IF, the image is 140 MHz away, and a moderate bandpass filter provides adequate rejection.
Image-reject (Hartley) and image-reject (Weaver) mixer architectures use two mixers with quadrature LO and 90° phase shift in the IF path to cancel the image response. The cancellation depends on the amplitude and phase balance between the two paths. Practical image rejection: 20-30 dB with analog components, 40-60 dB with digital I/Q correction.
Frequently Asked Questions
How much image rejection do I need?
The image rejection must be sufficient to keep the strongest possible image-band signal below the receiver's desired signal sensitivity. For commercial receivers: 40-60 dB is typical. For simple systems with no strong interferers at the image frequency: 20-30 dB may suffice.
What IF frequency minimizes the image problem?
Higher IF = easier image filtering but more expensive and higher-loss IF components. Common trade-offs: first IF of 70 MHz (VHF receivers), 140-170 MHz (UHF/SHF), or 1-2 GHz (microwave/mmWave receivers). Some modern receivers use zero-IF (direct conversion) to eliminate the image problem entirely.
What is the Hartley image-reject mixer?
Two mixers driven by the same RF signal but with LO phases shifted by 90°. The IF outputs are recombined with a 90° hybrid. Image signals cancel at the output; desired signals add. The cancellation is limited by the amplitude and phase balance: 1 dB amplitude and 5° phase imbalance limits rejection to about 20 dB.