slope of IIP3 graph - RF Cafe Forums
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Post subject: slope of IIP3 graph Posted: Sun Jan 22, 2006
hi to all,
i am getting a strange resu;t, let me desicribe
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 ???
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
your probablycompressing the second amplifier.. Back off
on the input power.
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
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
Thanks a lot ,