IP3 is a little more complex than stated below. It is an important part of the overall dynamic range of a receiver. This is important if you are attempting to receive a weak signal when a strong local signal is in the receiver passband. An attenuator will not be helpful under such conditions.
If you look at some of the newest transceiver designs, many are emphasizing narrow band roofing filters in the first IF just to improve that IP3 number. The ARRL added close in IM measurements a while back for the same reason. Leif Asbrink wrote: > Hello Frank, > >> I am trying to understand the IIP3 concept a bit better. I get the >> general idea that it is a non-linearity measure of mixer that takes >> one tone, applies a non linearity and it will interfere with another >> one of different frequency. According to this graph >> http://en.wikipedia.org/wiki/Image:Interceptpoint.png >> am I supposed to look for a large positive dBm to get good performance? > > IIP3 on its own has no meaning. Put a 30 dB attenuator in front of > the radio and enjoy a 30 dB better IIP3! > > IIP3 is the upper limit of the permissible signal range (One of them. > Other limits may be lower.) It is meaningful only in relation to > the lower limit of the permissible signal range which is the noise floor. > The noise floor can be specified in many different ways. One convenient > way is to look at IIP3-NF when comparing systems. > > (Or you may adjust the gain/attenuation of an imaginary pre-amplifier > that you add in front of each radio to give it a standardized > sensitivity and then compare IIP3 with receivers having the same > noise floor.) > > 73 > > Leif / SM5BSZ > >> Looking at the below figures: >> AS8347: +11.5dBm >> softrock40: above 30dBm >> would this mean that the H mode FET switch mixers are far superior to >> analog mixers in the IIP3 spec? >> >
