Image rejection mixers

Updated April 19, 2005

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New for May 2005! An image rejection mixer is often used to eliminate one sideband from converting to the IF frequency. The typical reason for this is noise figure: image noise from the unwanted sideband can easily pollute the noise figure of your receiver by 3 dB. Here is a block diagram of a generic image rejection mixer:

Image rejection can be calculated by the vector subtraction of the mixer's two outputs at the image frequency; achievable errors in amplitude and phase reduce the image rejection. The formula is simple and can be entered into an Excel spreadsheet. The amplitude imbalance gamma is not in dB.

The plot below shows the effect.

A good goal for an image rejection mixer is 20 dB rejection. Why? because this will keep the noise figure contribution of the image frequency down below 0.1 dB. Referring to the figure, you can trade perfect phase balance for 1.7 dB amplitude imbalance and get 20 dB rejection, or perfect amplitude balance for about 11 degrees phase error and still get 20 dB rejection.

More to come!