Hi Airy -

I generally agree with your description of IP3; but I would add
a few points.

The IP3 model was first published in a now-classic article
back in the 60s. (I could probably dig up the specific
reference, if someone really wants to know.)

The original author observed that many practical devices
(e.g., mixers) exhibit distortion levels that rise as the "power"
of the product in question. For example, third-order distortion
rises 3 times as fast (dB scale) as the desired (linear) signal.

If the subject distortion is plotted against input/output levels,
and approximated by a best-fit straight line, that line will
intersect a similar linear extrapolation of the desired signal
at a point dubbed the "Intercept Point."

The utility of all this is that you can use a single specification--
intercept point--to make quite good predictions of distortion
levels over a wide range of input conditions.

But it is important to remember that IP is only a MODEL,
and an empirical one at that. Real devices will never follow
the model exactly and completely--as you note in your
discussion of the saturation region.

73, Ed, W6LOL

"Airy R.Bean" wrote in message
...
From off the top of my head, without any revision.....

IP3, or "Third Order Intercept Point" is an indication
of how good a mixer is, but it is not a physical point!

If you were to plot the wanted output of a mixer stage against
the input signal (ignoring the local oscillator input), you would
get a graph that is a nearly-straight line from the origin
which then starts to flatten off.

At the point of the line where it starts to curve over to
flatness, and therefore starts to be non-linear, other
mixer products, mainly those based upon the third
harmonic of the input signals start to appear in the
output. if you plot these other products on your graph in
addition to the wanted output signal, they grow at a rate (the slope)
which is 3 times greater than was the initial straight line
of the wanted output.

If you take the original straight line of the wanted output, and
extrapolate it so that it meets the other line growing at 3 times
the slope, you get what is known as the "Third Order Intercept
Point". The reason that this is a theoretical point is because the
wanted output has long since flattened off!

The better a mixer is, the higher is IP3 for the outputs of the mixer.

IP3 will be given in terms of the power of the wanted output signal,
say, 50 dBm - other respondents have informed you that this is
50dB (or 10^5) times greater than 1mW, or 100W (Perhaps not
a good figure for an example - a mixer with an output of those
levels could be a PA stage!). In this case dBm gives us the power relative
to the mW.

If we now go back to the flattening off of the curve, at some point,
the curve will be 1dB less than what it would have been had the curve
not been a curve but had carried on as a straight line. This point is
known
as the "1dB Compression Point" - In this case we use dB and not dBm
because
we are talking relative to some other point on the line.

There is a mathematical derivation (which I don't know off-hand) which
shows that the 1dB Compression Point is 10.4dB below IP3.

So, I hope that I have gone some way to explaining (or increasing your
confusion) on the points that you raised!


"jason" wrote in message
ups.com...
May I know what actually the unit of dbm and db is different from one
another?
If they are different how can we minus the gain in unit of db from a
IP3 in unit of dbm?
Kindly enlighthen
Thank you all