I saw this on another forum a few days ago. Any
comments about what it says? Seems to be a lto of
different pinions out there. Sorry for the long
"I think you will
find that you do not directly measure IP3. IP3 is
an extrapolated point which is probably not achievable
in practice due to limiting and/or destruction of
the device. The use of IP3 is to calculate distortion
at low signal levels, not high signal levels.
It also matters, for 2 tone IP3 testing, what
the combined power in both tones is. You get a different
OIP3 prediction at different test powers.
OIP3 is a measure of the amount of unwanted
spurious signals generated by a device that is operating
near its compression point. Communications networks
are sensitive to these unwanted spurious signals,
as they can jam an adjacent communications channel,
or distort a modulation scheme.
OIP3 is measured
by having a device output two sine waves, and reading
the unwanted spurious tones generated. The two sine
waves superimpose. At some instances in time, the
two superimpose as a maximum envelope--at which
time they are being the most distorted (clipped
envelope) by the devices limited P1db capacity.
So, the lower the P1db, the more the evelope of
superimposed tones is distorted, and the worse the
OIP3 will be.
The IP3 (referred to input
or output) is just a figure of merit for the linearity
of the amplifier. Amplifiers need to be linear so
they can pass the undesired large adjacent signals
(the jammers) while still amplifying the small desired
signal (the channel of interest). The higher the
IP3 then the less gain compression the amplifier
will suffer... and less distortion from mixing of
the jammer with the desired signal will fall next
to the desired signal at baseband (and this 3rd
order distortion can't be filtered). Receiver designers
are more concerned with input IP3 because this gives
them an idea how large a jammer (at the antenna)
the amplifier can handle. If you're designing a
Power Amplifier then you're more concerned with
In the real world (in practice) when
you design receivers the spec's are typically more
specific than IIP3 or P1dB. If you have a good system's
engineer then he will taylor the block specifications
to more specific specifications like "the 3rd order
distortion rejection with out of band jammer at
X dBm = IMR3 spec or IP3 with specific tone power".
The IP3 considering in band jammer is also spec'ed
by determining the maximum acceptable signal swing
hitting the baseband ADC (ie 250mV) and then back
calculating the required input power hitting the
input of the receiver to generate 250mV at the baseband
output (considering mixer/filter/LNA gains)ie: Z
dBm. Then you apply two test tones to the input
of your receiver each with (Z-6) dBm and you should
get 250mV max peak swing out of the baseband. Now,
with the applied Z-6 dBm tones the 3rd order distortion
products should not be higher than the noise floor
of the receiver to avoid 3rd order distortion...
thus giving you your in band IP3 specificaiton .The
P1dB spec is not used to the extent which it is
taught in school. I have only seen it used in one
place. That is, making sure the amplifier gain does
not compress by more than 1dB when the worst case
(maximum power) jammer enters the receiver with
the desired signal. The gain compression suffered
by the desired low power signal is measured and
when the gain of the desired signal is reduced 1dB
then this is the gain compression breaking point.
The answer to your interview question (“what
is the relationship between P1dB and IP3”) was 9.6dB
as stated above. If any of you disagree then it's
because you don't know the answer. It's standard
textbook theory that you will learn in any graduate
RFIC circuit design course. I don't know of a web
page that shows this derivation but 9.6dB to 10dB
difference between P1dB and IP3 is the widely accepted
figure of merit. Any RF designer knows (or should
know) this. I'm not sure about the data sheets you're
looking at or how it was measured. The relation
between IP3 and P1dB being 9.6dB is assuming you
are measuring the IP3 with low power tones, meaning
that if you increase the tone power by 1dB then
the 3rd order products (IM3) will rise by 3dB. As
you turn up your test tone power and they approach
the 1dB compression point the device is no longer
small signal non-linear and (IM3:desired) is no
long 3:1 and this will cause the 9.6dB rule of thumb
to not apply because your test tones are too hot
and they're driving the device large signal non-linear.
To test the Input IP3 of your device you
drive it with 2 tones w1 and w2. The output will
have 3rd order distortion products 2w1+w2 and 2w2-w1
which is your IM3. Take either one of these tones
and find the IMR3 which is your output tone power
minus the adjacent IM3 power. Now, your IIP3 is
the power of one of your input test tones (at the
signal generator) plus 0.5*IMR3. If you're performing
this test in the lab with the signal generator and
spectrum analyzer you have to:
1. make sure there
is no distortion generated by the signal generator.
Typically you will have 2 signal generators (1 test
tone per sig. gen.) and both tones are combined
in a 2:1 power combiner that has only 3dB power
isolation. Therefore, the test tones from opposite
machines can enter the other machine and create
3rd order distortion in the signal generator itself.
Make sure you use attenuators on the signal generator's
SMA outputs (like 10dB pads) to ensure an extra
10dB isolation on each machine.
2. When you choose
the test tone power level you don't want it too
hot... because if it is then your distortion won't
be 3:1 for IM3-vs-test tone slope. If it is, then
you're not measuring small signal IP3. Make sure
that if you increase the test tone 1dB then the
3rd order distortion increases by 3dB.
can follow up your IP3 measurement by doing a P1dB
measurement. Apply 1 tone at a XdBm and then at
(X+10)dBm. See if the gain dropped by 1dB. If it
didn't then increase X by 1dB and re-check the gain
(at X+i+10)dBm. When your gain is down by 1dB then
the X+i is your input P1dB point. Check to see if
it is 10dB below your measured IP3!
caevat is when you have high frequency distortion.
The 9.6dB rule of thumb generally applies for frequency
independent circuits (ie: no caps/inductors or frequency
response). In a system with frequency response the
a1 + a2^2 + a3^3 + .... analysis typically gets
more complicated because you have to account for
phase and frequency effects... requiring you to
use volterra analysis. However, the 3:1 relationship
between 3rd order distortion and test tone still
applies unless your test tones are widely spaced
to place your IM3 distortion far away to suffer
different frequency response than the test tone.
AM to PM conversion effects on IP3 are typically
only considered in power amplifier designs where
you are basically more concerned with Adjacent Channel
Power Ratio (ACPR), which can be related to IP3.
If you measure the IP3 of your PA with small test
tones then AM-to-PM conversion isn't as much an
issue. However, small signal IP3 in a PA isn't very
useful information because you care about large
signal 3rd order and adjacent channel distortion...
not small signal IP3. THerefore, the 9.6dB rule
of thumb isn't really applicable because in a PA
your spec and required linearity is derived from
large signal distortion, not small signal distortion
which is 3:1."