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Ip3 is one of the most important parameter in the receiver.
Ip3 is one of the indication for the nonlinearity of the receiver. One can look at the 1dBc or other defenitions. For comunication applications the LNA that define the input NF must not saturated. For some applications there are several channels and the link of information between channels should be avoided. For other applications the input information includes AM modulation and so on.
If the precede stage's OIP3 is equal the next stage IIP3, the overall IIP3 will degrade 3dB.
For the RF chaine, the IP3 of the stages of the front especialy which have high gain will have strong influence to the total chain IP3.
Any received large signal (blocker) next to your band of interest might create nonlinearities in your band through nonlinearities in the LNA.
Or if your received signal in band is very strong you don't want it to distort. That's why you need high IP3.
Likewise to detect a weak signal you need small Noise Figure.
So your LNA and Mixer should have high dynamic range (IP3-NF)
IP3 of LNA is needed when a large dinamic range is expected from the input signal. Then LNA has to handle large input signals (situations that don't really need LNA at the input but due to very small signals expeccted, it has to be there). In that circumstances LNA must not generate nonlinear distorsion of the signal, signals on unwanted frequencies or oscillate, as was described in previous posts.
Perhaps your question is "Don't understand why check IP3 of LNA SINCE IT IS NOT A PA?
Immagine the following and not rare case.
A receiving system composed by a Large Antenna, a Broadband LNA (1/2 octave wide), a mixer, a narrowband IF filter, IF Ampli and so...
Of course the OIP3 of the overall chain is generally dominated by the IP3 of the last IF Ampli.
But...your bandwidth is determinated by the IF filter, every eventual strong RFI outside your BW, involve only the LNA and the mixer.
If the LNA and Mixer are wideband (i.e. 1/2 octave) the "integral" of all RFI may be stong enough to saturate the LNA and/or the Mixer :-(
From here the importance to know IP3 of both LNA and the mixer.
Think that a broadcasting FM TX (88-108 MHz) generally send a 200 KWeirp 8O
You could have two strong base station signals at your cellphone receiver input. The IM3 product of these two signals can land on your weak desired signal. The IP3 must be high enough to prevent this.
In the cellphone area, you can see it very well. (W-)CDMA, which are full-duplex standards, requires considerable more linear LNA than EDGE/GSM phones. Reason is because your own transmit signal acts as a strong blocker at your LNA.
IP3 point and 1dB compression point are two different specification. The term "blocking" means that some strong unwanted signal desensitize the wanted signal gain. Intermodulation means that two interferers' 3rd order product accidently located in our channel.
The following is an analogy example, many people are talking in their native language, someone speeking English, someone speeking Spanish and someone speeking Chinese. If everyone is talking in the same time, the ambient is very noisy, but you can still distinguish your native language if somebody is speeking with that. If I speek Chinese and two other unknown language is so loud that it "sounds like" Chinese, then I might be wonder if there is anybody trying to talk with me or not. This resembles intermodulation(indicated by IIP3). If someone is speeking an unknown language with extra loud volume around me, then it is impossible for me to listen to anyone speeking Chinese because it almost makes me deaf. This resembles gain compression(indicated by 1dB compression point), it is also known as blocking.
The language example is often used as a analogy to explain CDMA.
IP3 is not really that interesting if presented alone!! You can have arbitrary hight IP3 numbers if you sacify noise factor! The key is of course Dynamic Range! This number describes how good or bad you amplifier does handle strong and weak signals. A LNA (low noise amplifier) should have, yes, LOW noise factor per se, and thus will have low IP3 numbers compared to other types of amplifiers. BUT it should have good Dynamic Range.
Try a Google for IP3 and dynamic range and you will find numerous pages, for example **broken link removed** which discusses your question too!
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