What is P1db(continued)
The same power restriction applies to a preamp or receiver front end. Although they are only Milliwatt amplifiers, they still have a limit of how much signal they can handle !
What is IP3? (Intermodulation Products 3rd Order)
IP3 is not directly measured on test equipment. It is calculated, based on measuring the performance of the amplifier, and determining where a line plotted on a table will intercept
a specific output power level. This point is where the preamp is past it's usefull power level.
To help understand IP3 we can again use the home stereo example. For compression point
we noticed that at some point as you increase the volume, the output just would not go any
higher. Simulating IP3, we continue to increase the volume, and notice that the sound becomes distorted, and frequencies that are not in the music are produced! The amplifier is now operating in a non-linear mode, and who knows what you will get out of it (if it does not fail).
You may want to investigate and learn more about the causes of IMD (Intermodulation Distortion) (Normally 2 signals applied, and the base data obtained with testing equipment)
The same effects can be found with a receiver front end that is pushed to the intercept point.
You may hear out of band signals, clicks, clunks, audio from adjacent stations, or in some
cases, nothing at all!
RF congestion, or RF "pollution" example.
Again, letís compare your stereo amplifier to your receiver or preamp front end. Think about
this experiment, it demonstrates the effects of unwanted signals robbing preamp power. If you connected an audio signal generator to your home stereo amplifier, and set it to 30Khz (sine wave) and adjusted the generator so your 100 watt amplifier produced 50 watts of output, you would not hear anything (your dog might).
Now, turn the generator output up to produce 100 watts at 30 Khz. You still don't hear a sound, since 30Khz is a frequency that is too high for you to hear. Now, turn on the music, and see how much sound the amplifier will produce. How much? It is all ready at full power, how can it amplify the music? It canít, when it is already at full power!
The same principle applies to your receiver RF amplifier, and your preamp. When we talk about a 10db improvement in compression point over older low bias preamps, it may not sound impressive. But, if we add 10db more capability to the 100 watt home stereo in the above examples, we now have a 1000 watt amplifier! When we add 10db of "overhead" even with the 30Khz signal that we canít hear wasting 100 watts of power, you still have power capability remaining (900watts) and you can now hear the music! (and so can the neighbors)