Talk:Third-order intercept point

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I put this page in the category of amplifiers. It probably also belongs in a mixer category if there were one, but I couldn't find one and I can't justify creating the category for one article (two including the main Frequency mixer page). --Christaj 02:01, 6 September 2006 (UTC)

At the end of the text a small example is given:

" For example, assume a device with an input-referred 3rd order intercept point of 10 dBm is driven with a test signal of -5 dBm. This power is 15 dB below the intercept point, therefore nonlinear products will appear at approximately another 15 dB below the test signal power at the device output (in other words, 2*15 dB below the output-referred 3rd order intercept point). "

I think it's wrong: When the assumed device is driven with a signal of -5 dBm, the test signal at the output will appear 15 dB below the output-referred 3rd order intercept point (OIP3) and nonlinear (3rd order) products will appear at another 30 dB below the test signal (and 45 dB below the OIP3). Unless 2nd order nonlinear products are meant in the example, which would appear outside the frequency band of most narrow-band systems and would have no effect.

I have a question about this subject: can the 3rd order intercept point be used to specify the capability of the system to deal witch strong out of band signals?

[edit] IP3 comment

I also feel that the example given for tones 15dB below the intercept point is wrong for 3rd order products. In this case, the 3rd order distortion products will be 30dB down on the signal and 45dB below the intercept point.

[edit] Series expansion

According to the article, the nonlinearity is modeled using polynomial series expansion (Taylor series). I don't think this is a good model for a non-linearity. It works well when the signal gets smaller, but for growing input values, a polynomial grows without bounds. A real nonlinearity doesn't, but is rather bounded by the supply voltage or available current. There must be a better series expansion than a polynomial, that still fits with the IP3 concept.

A simple model for the nonlinearity of a bipolar differential stage is the tanh function. A Taylor series approximates tanh well only in a small range close to the origin, for a practical polynomial order. --HelgeStenstrom (talk) 10:52, 21 April 2008 (UTC)

The polynomial model allows one to easily work out the power of intermodulation products, and hence IP3. Oli Filth^(talk) 11:10, 21 April 2008 (UTC)

Yes, it's easy to work out the IM products of the polynomial, but it's only a model, and perhaps not a very good one, in particular at high amplitude levels, such as close to compression. Real amplifiers and mixers don't have a measured IP3 that is independent on the power level, like a polynomial does. --HelgeStenstrom (talk) 08:18, 30 April 2008 (UTC)