Frank You have probably gathered that this is not a simple topic.
1st, I would say that IIP3 is not restricted to mixers, all components that utilise semiconductors have non-linearities, and so they could all have an IIP3. For example, low noise amplifiers have an IIP3, but you can't easily compare an amplifier to a mixer. Also, filters, especially crystal filters, are non-linear, as are inductors that use ferrite such as torroids. IIP3 means the third order intercept point, referred to the input, due to intermodulation distortion (IMD). IMD can ONLY occur if there are two (or more) signals present. Imagine only one signal at the input to a system (it doesn't matter what the system is), then no matter how strong that signal is, there cannot be any intermodulation. Now imagine two signals at the input to our system, and that system is a radio receiver. Let's assume their amplitudes are equal - this is unlikely in practice, but it makes the maths a lot easier and this is how IMD is best understood. Lets assume that the signals are quite strong, and are at 27 and 28MHz, i.e. 1MHz spacing. If we put a spectrum analyser on the input of the receiver we would see just the 2 signals. Now, if we put the spectrum analyser elsewhere in the receiver, for example after the first gain stage, we would see more than two signals, each separated by 1MHz. These extra signals are the Intermodulation products, caused by non-linearities - in this case in the Rx amplifier. The closest IMD products will be at 26 and 29MHz, and will be at equal amplitude. These are the third order IMD products, because they are procuced by the 2nd harmonic of each of the two input signals mixing with the fundamental of the other. We will ignore the other IMD products for this discussion. The level of the IMD products will be less than the two input signals. Now, it gets more tricky - if the level of BOTH of the input signals is increased by 1dB, then the levels of the 3rd order IMD products will increase by 3DB. So, the difference between the levels of the two, real, input signals (27 and 28MHz) and the IMD products (26, 29MHz) has decreased. The IIP3, (3rd order input intercept point) is the theorectical level at which the level of the two input signals and the IMD products is exactly equal. The IIP3 is always theoretical, because in practise the system (amplifier, mixer or whatever) will have gone way into compression before this point is ever reached. Therefore, IIP3 can never be measured directly - it has to be determined from measurements made at lower input levels. I had to make a few assumptions in the above statements, in order to explain the principle. I hope that helps. regards Grant G8UBN
