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Why do we have to care IMD2 for Direct conversion receiver ?

For a Direct-conversion(Zero-IF) receiver, RF signal down converts to BaseBand signal near 0 Hz directly.


Let’s assume there are 2 tones , f1 is interference, f2 is signal, and their frequency are close, i.e. f1 = f2.
If LNA IIP2 is not good enough, f1 and f2 will compose IMD2,
which is near 0 Hz (IMD2 = f1 – f2 =0Hz) and degrades SNR and sensitivity.


Because mixer input is LNA output,
that is to say, input power of mixer is larger than which of LNA.
It’s the reason why mixer has more effect on Rx chain linearity than LNA.
Thus, even if your LNA is extremely linear,
but there is still IMD2 issue with nonlinear mixer.


Besides, IMD2 caused by mixer is more difficult to eliminate than which caused by LNA.
Because you can eliminate IMD2 after LNA by means of DC block without attenuating desired signal.
if you eliminate IMD2 after mixer by means of DC block,
the desired signal (already becomes baseband signal near 0Hz) is attenuated as well.


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