Receiver overload happens when a signal that is far stronger than the wanted one pushes the receiver's first stages out of their straight-line range. Nothing is wrong with the strong signal's purity: the receiver cannot handle it. The same cause explains interference to your neighbour's TV and interference to your own receiver from a nearby station.
Why strong signals do damage
Fix it at the receiver that is being overloaded: block the strong signal before it gets in.
Every amplifier is straight-line only over a limited range. Push harder and the output flattens at the peaks.
Idealised amplifier, illustrative numbers. The flattened peaks are new frequencies (harmonics and mixing products) and the lost gain is desensitisation.
Wanted signal gain-9.0 dB
A flattened peak is no longer a clean sine, so it carries new frequencies, and the amplifier has less gain left for anything weak. That gives four symptoms, often confused:
Effect
What happens
What you notice
Desensitisation
strong signal squeezes gain from the wanted signal
receiver "goes deaf"
Cross-modulation
the strong signal's modulation is copied onto the wanted signal
you hear the other station underneath
Intermodulation
two or more signals mix into new frequencies
phantom signals on clear channels
Audio rectification
a junction in an audio stage detects the RF
your voice or buzz in speakers or phones
Intermodulation arithmetic
Third-order products sit the same spacing beyond each signal: 2f₁ − f₂ and 2f₂ − f₁.
Receiver at 146.70interference
Two signals f₁ and f₂ create third-order products at 2f₁ − f₂ and 2f₂ − f₁. They fall right beside the originals, so once made they cannot be filtered out. Set the slider to 146.61: two strong stations at 146.52 and 146.61 MHz make a product at 2 × 146.52 − 146.61 = 146.43 MHz and another at 2 × 146.61 − 146.52 = 146.70 MHz.
Their level rises 3 dB for every 1 dB of input, while the originals rise 1 dB per dB. That is why a small increase in strength brings a big jump in interference, and why a higher intercept point means a better receiver. The full treatment is in Sensitivity, selectivity and dynamic range.
Telling them apart and curing them
Fundamental overload hits the victim on whatever it is tuned to, because the whole front end is swamped. It is cured by a filter at the victim's input, or by attenuation ahead of the first stage.
Intermodulation appears only when both interferers are on. Switch one off and it goes. Attenuate, or tune a preselector to reject at least one.
Audio rectification ignores the tuning knob, and it often ignores the volume control too. Fix it with chokes or bypass capacitors on the cables, see Interference to consumer electronics.