Level of image rejection levels: Typical levels of image rejection will vary according to the product.There are a number of considerations to be taken into account when using an image reject mixer. Image reject mixer considerations and performance Using commercially available units levels of 35 to 40 dB should be obtained. However it is possible to achieve reasonable results with one manufactured from discrete components - levels of image rejection of around 20 dB should be obtainable using this approach. Often integrated or modular image reject mixers are employed as these offer the best performance. It is not possible to achieve perfect cancellation in a real image reject mixer - the mixers must be identical and as must the amplitude balance and phase shift of all the quadrature and in-phase power splitters. The LO drive to each mixer is in-phase and the IF output is combined in quadrature. The two balanced mixers within the image reject mixer are driven in quadrature by the RF signal.
To achieve this an image reject mixer utilises two balanced mixers and the quadrature (90°) hybrids as shown below. In order to be able to operate, image reject mixers, image rejection mixers utilise the phasing techniques to cancel out the unwanted mix products. There may also be other instances where the image signal may cause a problem. However in some applications this is not the case and the unwanted or image product can be close to the wanted signal and can require complicated filtering to remove it sufficiently. Often the unwanted one will fall well outside the required bandwidths and can often be removed very easily. Of the two products from a mixer, normally only one is required. This can be seen diagrammatically: Mixing two RF signals Output signals = ( f 1 + f 2 ) ( f 1 - f 2 )