# Slope of IIP3 graph - RF Cafe Forums

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Guest
Post subject: slope of IIP3 graph Posted: Sun Jan 22, 2006 9:59 am
hi to all,
i am getting a strange resu;t, let me desicribe that first.
When I am plotting I and III harmonics of a Common Source Amplifier (for IIP3 ) , I am getting slope of '3' of III harmonic curve , but when I cascade it with another Common Source Amplifeir , I get slope of less then '3' !!!

Isn't it strange ???

I have second doubt that if it is correct that slope will decrease then everyone must use cascading as it increases IIP3 point !

please . .. . . .. . .. . .

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Guest
Post subject: Posted: Sun Jan 22, 2006 1:38 pm
your probablycompressing the second amplifier.. Back off on the input power.

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Guest
Post subject: Posted: Sun Jan 22, 2006 1:52 pm
really could not understand first reply
sorry

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IR
Post subject: Posted: Sun Jan 22, 2006 3:15 pm

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Guest,

IP3 is measured under 'small signal' conditions.

To be more specific according to the IEEE: the IMR3 (3rd order Intermodulation Ratio), which is the difference between the 1st order signal to the 3rd order (in dB) should be greater than 20dBc.

For each 1dB increment in the fundamental signal (1st order) the IM3 product is increased in 3dB.

When you cascade 2 amplifiers and you want to measure the cascaded IP3, you should check that both amplifiers are working in small signal region and far enough from P1dB. If the slope is no more 3:1, that means that one of the amplifiers is working in non-linear region i.e. close to compression. The meaning of backing off is to reduce the input power to the amplifier and by that to decrease the output power and backing it off from compression. Try to reduce the input power to the first amplifier and measure the IP3 again.

Good luck!

_________________
Best regards,

- IR

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Guest
Post subject: Posted: Mon Jan 23, 2006 1:47 am
really very very impressed by IR's answer.
Thanks a lot ,
--amit

Posted  11/12/2012

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