Glossary

Front end – The very first amplification stage in a radio frequency receiver.  Usually this stage has limited filtering and can be sensitive to energy far removed from the desired receive frequency.  This stage is greatly responsible for the basic receiver sensitivity and can be overloaded with very strong signals fairly easily.  It is usually a non-linear amplifier by design.

Non-linear – When used with radio frequency amplifiers, the term refers to the inability of an RF amplifier to increase the strength or level of radio energy without distorting or changing the energy waveform in some way.  For example, if you turn up an audio amp too much the audio often begins to sound different or distort.  This is because the audio amplifier is non-linear at high power demands.  In an RF amplifier, this non-linearity causes harmonic and intermodulation distortion.  This is the creation of RF energy at frequencies not originally present in the non-amplified RF waveform.

Intermodulation Distortion --  The non-linear amplifier effect of creating sums and differences signals from multiple signals present in the amplifier.  For example, suppose a signals of 150 MHz  and 155 MHz are present in a non-linear amplifier.  Then these signals would mix and create new signals of (155-150)+155 or 160 MHz and/or 160+150 or 310 Mhz and so on.  If any of these non-linear signal products lands near the desired receive frequency, then interference occurs within the receiver.  Since most two-way communications type amplifiers are non-linear, communications systems are designed to avoid receiving intermodulation created signals.