about IIP3 and intermodulation

[super]

Dear all:
I don't understand some concept.
1. LNA and Mixer are AC couple to get rid of second-order
intermodulation products generate by LNA.

Why AC couple can improve second-order
intermodulation products from LNA ???


2. The result high impedance at signal frequency improve CMRR
while low impedance , both at low and high frequency , improve
IIP3

I don't understand this means .
Why can improve IIP3 ??

Somebody can help me ?

THANKS .

 

[flatulent]

AC coupling reduces the number of lower frequency signals getting to the amplifier. It is this that reduces the spurious output signals of the amplifier. The coupling does not change the basic amplifier distortion characteristics.

It is Always good practice to put a filter before an amplifier that only passes the frequency range that is used. For wide band systems you can switch in different filters.

It is common practice for HF receivers to have a bank of half or third octave filters between the antenna and the active electronics.

The famous R390 HF receiver had continuously tuned filters. Most of the size and weight was the mechanical cams that adjusted the LC values.