What is a double conversion superheterodyne receiver?

 We have seen that usual superheterodyne radio receivers have single conversion of received frequency to the intermediate frequency using a single local oscillator and mixer. In many of the modern communications receivers, multiple successive stages of frequency conversions with multiple intermediate frequencies are used. A receiver with two frequency conversions and two intermediate frequencies is known as a double or dual conversion superheterodyne receiver. One with three levels of conversion and three intermediate frequencies is known as a triple conversion superheterodyne receiver. Use of additional conversions improve adjacent channel selectivity and image frequency rejection. First IF stage has a wide separation between the intended signal and the image frequency, enabling RF amplifier to reject the image frequency. Second IF filter has a narrow bandwidth providing good adjacent channel selectivity.

Single IF stage in conventional superheterodyne receivers is a tradeoff between low image response and adjacent channel selectivity. We have seen earlier that the separation between the intended received frequency and the image frequency is twice the intermediate frequency. So with a higher initial intermediate frequency it is easier to design an RF filter to remove the image frequency from the input and achieve low levels of image frequency in the subsequent stages. But higher initial IF makes it difficult to remove adjacent channels with high selectivity. At HF and higher frequencies, high intermediate frequencies needed for low image response impacts performance by reducing adjacent channel selectivity. It is to overcome this that additional frequency conversions are designed. That is the basis for design of double conversion and triple conversion superheterodyne receivers.