slope of IIP3 graph - RF Cafe Forums
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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 ???
Please help me . . .
I have second doubt that if it is correct that slope will decrease then everyone must use cascading as
it increases IIP3 point !
please . .. . . .. . .. . .
Post subject: Posted: Sun Jan 22, 2006 1:38 pm
your probablycompressing the second amplifier.. Back off on the
Post subject: Posted: Sun Jan 22, 2006 1:52 pm
really could not
understand first reply
Post subject: Posted: Sun Jan 22, 2006 3:15 pm
Joined: Mon Jun 27, 2005 2:02 pm
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.
Posted: Mon Jan 23, 2006 1:47 am
really very very impressed by IR's answer.
Thanks a lot ,