What I Understand Already:
- Intermodulation products manifest in receiving equipment as a result of several near-by transmitters, each operating at different carrier frequencies, mixing at the receiving antenna.
- The signal sensed at the receiving antenna produces an alternating current that contains a mix of each carrier frequency (plus harmonics thereof, for each carrier).
- Presuming that frequency selective components before the RF amplifier don't attenuate the power at each carrier too much, non-linearities in the RF amplifier will produce intermodulation products at frequencies which are linear combinations of each input carrier.
- Some of these intermodulation products fall near the receiver's tuned bandwidth, creating undesired power in the side-band (which is why it's worth modeling intermodulation effects).
However, I've learned that transmitter intermodulation products can be important to model as well. How do these intermodulation products manifest themselves in a victim transmitter?
Let's consider a two-carrier mix at the input of the RF amplifier (block diagram shown below).
The first carrier is easy to explain: it is passed through the IF-stage into the RF-stage, to the input of the RF amplifier. However, there are only two possible routes for the second carrier to be passed to this victim transmitter's RF amplifier:
- The second carrier penetrates the victim transmitter's casing (seems nearly impossible, saying that a Faraday cage sole purpose is to attenuate external radio signals)
- The second carrier impinges on the victim transmitter's antenna, producing an alternating current that is passed through the output of the amplifier into its input (seems more likely, since there wouldn't be any protective casing over an antenna)
producing an alternating current that is passed through the output of the amplifier into its input
So far, I've accepted this as an explanation for how these IMPs manifest themselves in a victim transmitter. But, I realize that it's very hand-wavy. Can some provide a more technical explanation of this, if my understanding is even correct?