This link from Mini-Circuits explains it very clearly, with spectrum
analyzer plots, etc:
http://www.minicircuits.com/pages/pdfs/mxr1-18.pdf
I am not sure about this calculation that is mention here but as an example,
let's consider 0dBm for each tone into the RF port of the mixer under test.
If our IMR (delta) is 60dB, our IP3 will be +30dBm,
since IP3 = [(IMR/2) + Pin]

If we had -4dBm for each tone, using the same mixer as the first example,
the IMR would be 68dB.

If we had -10dBm for each tone, using the same mixer as the first example,
the IMR would be 80dB.

Once again, IP3 is calculated in this manner: IP3 = [(IMR/2) + Pin]

This is the method that Mini-Circuits, Synergy Microwave, Watkins-Johnson,
and other vendors in the industry use when making this calculation. The only
difference is that the term "Delta" us used for the IMR spec. I hadn't heard
of the "IMR" term until I did that stint at Motorola last year.
Take a look at the PDF link, and it will become very clear.
One characteristic of the IP3 terms is that as you increase the level of the
two tones at the input port of the mixer, the 3rd order distortion products
will increase by a 3:1 ratio over the desired tones.
As an example, if you increase your two RF tones by 1dB, your 3rd order
products will increase by 3dB.
3dB increase for each tone will cause a 9dB increase in the 3rd order
products.
This example is only valid if you are operating within the linear range of
the mixer. The linear range is defined as the range where conversion loss
(or conversion gain) is constant as you increase the signal level at the RF
port.
Consider that your typical Level 7 mixer has a conversion loss of 6.5dB.
There will be a point where you will increase your input tones and the
conversion loss will be 7.5dB. This is your 1dB compression point.
Now, the IP2 calculations can get confusing, since there are different
methods of measuring it. The [RFin + (I.F./2)] method is commonly used. With
LO power applied to the mixer under test, a desired frequency is applied to
the RF port.
A measurement of outpur power at the I.F. port is then recorded.
Next, a frequency of [RFin + (I.F./2)] is applied to the RF port. The power
at the I.F. port will now be between 50dB to 80dB below the initial recorded
value. This is your IMR (or Delta).
IP2 is calculated as (IMR + Pin). If your IMR is 70dB and your Pin is 0dBm,
your IP2 is 70dB.
If your IMR is 70dB and
your Pin is -10dBm, your IP3 is 60dB, etc,etc,etc.
I hope this provides further clarification!

Pete

Input tone power (both) -4.0
Output tone power (both) -3.8
(2*F1) - F2 power was -59.7
then the IP3 would be +24

--
Telamon
Ventura, California