Updated May 23,
Search for image
rejection mixers on EverythingRF.com
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New for May 2010! The
spreadsheet that created the graphic below is now available in our
download area, thanks to The Professor!
expressed as color contours
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.
But first, let us comment on the "sign"... rejection as
defined by most scholars and other rogues should be a positive number
in dB. The above formula yields a positive number. But often you
will see results expressed as a negative dB value. It is OK to roll
your eyes at such a mistake, but it is considered bad etiquette
to slow down a design review because of it! Thanks to Wayne for
pointing out our minus sign error in May 2010!
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!