I see the following figures quoted for preamps, for example:

Output IP3: >+32dbm or Output IP3: 39dbm

What does "Output IP3" refer to and how does it differ from the gain figures quoted for preamps? What does IP3 measure?

What would be the difference between two preamps that both have 20 db gain but the first one has an "Output IP3" of 32 and the second one of 39? Would the second one be better because of its higher IP3 and in what way?

Thanks

simple explanation

IP3 = Third-Order Intercept Point

IP3 is a figure-of-merit for linearity or distortion. Higher IP3 means better linearity and less distortion.

IP3 is commonly tested with two input tones. The inputs to the amplifier are two sinewaves (fundamental), At the output of the amplifier are the two desired signals. Because the amplifier is not perfectly linear, it also produces two third-order intermodulation (IM3) products. IM3 is often given in dBm. To reduce third order distortion products, the IP3 specification must be increased.

In mathematical terms, IP3 is a theoretical input power point at which the fundamental and third-order distortion output lines are intercepted .

You can build an amplifier with very high IP3 by adding another gain stage so that it has a very high output voltage swing.
Which has nothing to do with TV Preamps and everything to do with overloading the attached HDTV.

What is important is the sensitivity, which is determined by two factors; Noise Figure and SFDR (Spurious Free Dynamic Range).

The Noise Figure is the difference between the Preamp's
measured output noise power and the theoretical thermal noise power.

SFDR is more complicated, as was discussed here:
http://hdtv.forsandiego.com/messages/1/5666.html#POST28204
The trick is to MATCH the Preamp choice (and sometimes input attenuation) so that the Intermod products
caused by the strongest input signals are as close to the thermal noise floor as possible.

Yeah, I know, WTMI....

Which has nothing to do with TV Preamps and everything to do with overloading the attached HDTV.

What is important is the sensitivity, which is determined by two factors; Noise Figure and SFDR (Spurious Free Dynamic Range).

Not to nitpick here, but IP3 is an important specification for every amplifier. IP3 is directly related to the output power capability (1 dB compression point + 11 dB). More is generally better and higher values will ensure less chance of overloading and amplifier desensitizing from local sources. The downside is it may overload the tuner input as discussed in another thread but that can be mitigated with some attenuation between the pre-amp and tuner.

You have it backwards. Sensitivity is equal to noise figure times noise bandwidth only . SFDR is partially determined by sensitivity but not the reverse.