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I am reading the datasheet of this IC. The specification has listed in-band and out-band 3rd order intercept point (Page 11). I have read about intercept points and learned that intercept point of nth order gives the relative magnitude of gain and nth inter-modulation products. However, I am confused between in-band and out-band terms. What are they?

I can make a guess and my instinct says in-band 3rd order intercept point is related to 3rd-order inter-modulation product which is closet to input signal frequency and out-band refers to the relative magnitude of gain (first-order) and amplitude of inter-modulation products which are far away from input signal frequency.

Am I right?

If yes, then I should be concerned with in-band IIP3 only? I think I can filter out out-band frequencies components using a cheap filter. Am I correct?

abhiarora
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2 Answers2

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The datasheet is pretty good on this - they give you the exact frequencies used in the IP3 tests.

The in-band IP3 refers to two tones both in the band of interest, which would be for example weak TV harmonics, or an intentional jammer, interfering with, and distorting the signal.

The out of band IP3 refers to two tones well out of band, mixing into the band. They've chosen a pair of cellphone signals which could intermodulate to 1575 MHz and would interfere with the receiver. So while both tones are far away, the mixing product is in-band.

There must be some modest band-pass filtering in front of the active element to achieve this. They need to specify it as two different IP3 figures, as simply stating one inputIP3 and the gain would not deliver the same benefit in the RF chain linearity analysis.
If you're analysing linearity against in-band jammers, use the in-band figure, and if youre analysing it against cellphone signals, use the out-of-band figure.

You can filter out the out-of band frequencies, but you need to do it before the amplifier, and this will affect the noise figure. Up to you as the designer!

tomnexus
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  • Are you saying, they have divided the input signals in two categories. One is desired input signal and another is non-desired input signal (out-of-band)? – abhiarora May 11 '18 at 13:13
  • Yes, though if it's a GPS signal, it will be weaker than the frint-end noise... So rather just call them in-band and out of band. – tomnexus May 12 '18 at 05:06
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OUT OF BAND energy ----- aka BLOCKERS ----- may beat or mix to produce inband energy, or the BLOCKERS may simply demand the amplifiers energy be devoted to the BLOCKERS, to detriment of the desired RF signals; hence the name BLOCKERS.

Basically the BLOCKERS would cause clipping or railing or overload of the amplifier chain.

analogsystemsrf
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