What is the image frequency and how is it rejected?
In a heterodyne receiver with LO frequency fLO and IF fIF, two RF frequencies produce the same IF: the wanted fRF and the image on the opposite side of the LO, fimage = fRF ± 2fIF (plus for high-side LO, minus for low-side). Image signals and noise must be rejected before or by the mixer.[1]
How is the image rejected with filters?
An RF pre-selector or image-reject filter before the mixer attenuates the image. A higher IF moves the image farther away and eases filtering, at the cost of harder channel selection at IF, which is why dual-conversion superheterodynes use a high first IF and a low second IF. See superheterodyne receiver.
How do Hartley and Weaver architectures work?
The Hartley architecture mixes with quadrature LOs, shifts one path by 90° and sums, cancelling the image. The Weaver architecture replaces the 90° shift with a second pair of quadrature mixers. Both stem from single-sideband techniques.[2][3][1]
How does mismatch limit image rejection?
With fractional gain error ε and phase error θ (radians), image rejection ratio is approximately IRR ≈ 4/(ε² + θ²). 1% gain error and 1° phase error give about 40 dB, so analog image-reject mixers usually reach 30–40 dB without calibration; digital calibration improves this.[1]
Frequently asked questions
Does a direct-conversion receiver have an image?
Its image is the mirror of the signal itself around DC, so I/Q mismatch causes self-image interference rather than an external image.
What is high-side injection?
An LO above the RF; the image lies above the LO.
Does image noise add to NF?
Yes; without rejection, image-band noise raises SSB noise figure by about 3 dB.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “RF Microelectronics, 2nd ed.”. B. Razavi, Prentice Hall. 2012.
- “Modulation System (U.S. Patent 1,666,206, single-sideband phasing method)”. R. V. L. Hartley. 1928.
- “A Third Method of Generation and Detection of Single-Sideband Signals”. D. K. Weaver, Proceedings of the IRE, vol. 44, no. 12, pp. 1703-1705. December 1956.