Apparatus and method for feed-forward image rejection in a dual conversion receiver

A method of image channel suppression in a dual conversion receiver having a first mixer, a bandpass filter coupled to an output of the first mixer, and a second mixer coupled to the output of the bandpass filter, the method comprising:

• receiving an RF input signal having a plurality of channels;
• up-converting the RF input signal to generate a first If signal;
• filtering the first If signal in the bandpass filter to select a desired channel from the plurality of channels that falls in a passband of the bandpass filter, so as to produce a filtered first If signal having the desired channel and an image channel that is attenuated by a stopband attenuation of the bandpass filter;
• generating an image cancellation signal from the first If signal, the image cancellation signal having an amplitude at the frequencies of the desired channel and image channel substantially equal to the amplitude of the image channel in the first filtered If signal and a phase that is offset by approximately 180 degrees from the image channel in the first filtered If signal;
• and
• combining the filtered first IF signal with the image cancellation signal;
• down-converting the result of the combining step to a second IF signal having the desired channel.

A receiver portion for suppressing an image channel, comprising:

• a bandpass filter having a passband determined to pass a desired channel and attenuate an image channel of said desired channel;
• and
• an image cancellation circuit coupled across input and output terminals of said bandpass filter, including
• an attenuator having an attenuation at said desired channel and said image channel both determined by an attenuation of said image channel at an output of said bandpass filter, and
• a phase shifter coupled to said attenuator that provides a phase shift of approximately 180 degrees at the image channel frequency.

A dual conversion receiver, comprising:

• a receiver input configured to receive an RF input signal having a plurality of channels;
• a first mixer having a first input coupled to said receiver input and a second input coupled to a first local oscillator signal, said first mixer outputting a desired channel of said plurality of channels at a first IF frequency (IF1);
• a bandpass filter, having an input coupled to an IF output of said first mixer, and having a passband centered at IF1 that passes said desired channel and attenuates an image channel of said desired channel;
• and
• a second mixer having a first input coupled to an output of the bandpass filter and a second input coupled to a second local oscillator signal, said second mixer outputting said desired channel at a second IF frequency (IF2);
• and
• an image cancellation circuit, coupled in parallel with said bandpass filter, having a passband response at said image channel and said desired channel that is substantially equal in amplitude and opposite in phase of a response of said bandpass filter at said image channel.