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!
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