Hi Grant, All, Wow, good explaining!
I was struggling with this same subject a while ago and found a few explanations on the internet that cleared this up a bit for me. But I think this one is the best one I've seen so far, for someone new to the concept of IIP3. And it sure helped me! Regards, Henry. > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of Grant Hodgson > Sent: maandag 4 juni 2007 22:50 > To: [email protected] > Subject: [soft_radio] Re: IIP3 concept > > > 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 > > > > Yahoo! Groups Links > > > >
