Also known as: desense, receiver desensitization, blocking
Desensitization (desense) is the reduction of a receiver’s sensitivity caused by a strong signal — usually out-of-band — that drives the front end toward compression or injects oscillator noise, raising the effective noise floor so weak wanted signals disappear.1 The interfering signal need not be on the tuned frequency at all; it simply has to be strong enough to disturb the receiver’s linear operating point or its local oscillator.
How it works
Two mechanisms dominate. First, gain compression: every amplifier and mixer is linear only over a limited range. A strong signal pushes the low-noise amplifier or first mixer toward its 1 dB compression point, where stage gain falls. A weak wanted signal sharing that stage sees the same gain drop, so its level relative to the internally generated noise worsens — sensitivity is lost even though nothing changed at the wanted frequency.
Second, reciprocal mixing: no local oscillator is spectrally pure; it carries phase noise skirts. A strong off-channel signal mixes with those noise skirts and lands broadband energy in the IF passband, directly raising the noise floor around the wanted channel. This is why a receiver can desense even when the front end is nowhere near compression: the culprit is oscillator purity, not amplifier headroom.
Desensitization is quantified as the interfering-signal level needed to degrade sensitivity by a fixed amount (commonly 3 dB or 6 dB), measured at a stated frequency offset. The span between the noise floor and that blocking level is the blocking dynamic range.
In practice
- A pager transmitter, broadcast-FM station, or nearby land-mobile repeater a few MHz away can desense a wideband scanner even though it is far outside the tuned channel.
- Software-defined radios with wide, unfiltered front ends — RTL-SDR dongles especially — are prone to desense because the whole spectrum reaches the analog-to-digital converter before any channel selection.
- Cures are physical: a bandpass or notch RF filter ahead of the receiver, an attenuator to pull the strong signal out of compression, better antenna placement, or reducing front-end gain via the automatic gain control settings.
- Adding gain with a preamplifier often makes desense worse, not better, because it amplifies the blocker toward compression too.
Relevance to SDR
Desensitization is one of the most common real-world reasons a trunking signal that “should” be receivable is not. In dense RF environments — mountaintop sites, apartment blocks near paging or cellular transmitters — a strong neighbor can lift the noise floor across the band a wide-front-end SDR is watching. Because GopherTrunk samples a wide swath of spectrum to follow a trunked control channel and its voice grants, a single strong out-of-band emitter can degrade decoding of an otherwise healthy channel. The fix lives in the RF plumbing rather than the software: front-end filtering, attenuation, and correct gain staging restore the receiver’s sensitivity. GT can report symptoms — a rising noise floor, falling demod SNR — but it cannot undo desense that has already corrupted the samples reaching the ADC.
Sources
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Desensitization (telecommunications) — Wikipedia, definition and mechanisms of receiver desensitization. ↩