Image response

The image rejection ratio, or image frequency rejection ratio, is the ratio of the intermediate-frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency. The image rejection ratio is usually expressed in dB. When the image rejection ratio is measured, the input signal levels of the desired and image frequencies must be equal for the measurement to be meaningful. IMRR is measured in dB, giving the ratio of the wanted to the unwanted signal to yield the same output from the receiver. In a good design, ratios of >60 dB are achievable. Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity); the signal yields a perfectly valid IF frequency. Rather, it is the measure of the bandpass characteristics of the stages preceding the IF amplifier, which will consist of RF bandpass filters and usually an RF amplifier stage or two. == Image rejection formulas ==

The Image Frequency Rejection Ratio (IRR) is characterized by its RF filter which can be determined on the basis of its relative response of a parallel tuned circuit. IRR = \sqrt{1+\rho^2Q^2} where, \rho=\frac{f_{IMAGE}}{f_{RF}}-\frac{f_{RF}}{f_{IMAGE}}and Q is the quality factor. The Image Rejection Ratio for a given value of gain imbalance \gamma, (\epsilon=\gamma-1) and phase imbalance \phi is determined by, IMRR = \frac{\gamma^2+1-2\gamma cos(\phi)}{\gamma^2+1+2\gamma cos(\phi)} \approx \frac {\epsilon^2+\phi^2}{4} ==See also==