imagereject

Imagereject refers to techniques and designs used to suppress an undesired image frequency in heterodyne receivers, which can produce the same intermediate frequency (IF) as the desired signal after mixing with the local oscillator (LO). If not attenuated, the image can mask or distort weak signals, particularly in crowded or weak-signal environments. The concept is central to radio receiver design and is quantified by the image rejection ratio (IRR), typically expressed in decibels.

In a single-conversion receiver, the LO mixes with the incoming RF frequency f_RF to produce an IF of f_IF = |f_LO − f_RF|. There exists an image frequency f_image that also satisfies |f_LO − f_image| = f_IF, meaning a second RF signal at f_image would fold to the same IF. For many architectures, f_image can be approximated by f_image = f_LO + f_IF (or f_LO − f_IF, depending on the LO arrangement). Without adequate filtering or cancellation, the image can appear at the IF and interfere with the desired signal.

Methods to achieve imagereject include: preselect RF filters with sufficient bandwidth and sharp skirts to reject the image before mixing; image-reject mixers that use quadrature (I/Q) demodulation to cancel the image term; dual-conversion or RF front-end architectures designed to suppress the image; and digital signal processing techniques in software-defined radios to remove residual image interference after sampling.

Practically, IRR values vary with application and technology, ranging from modest tens of decibels in simple receivers to hundreds of decibels in high-performance systems. Designing imagereject stages involves trade-offs among selectivity, noise, complexity, and cost.