Field Guide · term

Also known as: P1dB, 1-dB compression point, gain compression point

1 dB compression point (P1dB) is the power level at which an amplifier’s gain has fallen 1 dB below its ideal linear value.1 Below it the device is essentially linear — output tracks input dB for dB; at P1dB the amplifier is starting to saturate and can no longer keep up, so P1dB marks the practical top of a device’s linear operating range and the onset of overload. It complements the third-order intercept: where IP3 predicts intermodulation spurs, P1dB pins down single-signal gain compression and the blocking dynamic range ceiling.

P_out P_in ideal linear actual gain 1 dB down = P1dB IP1dB
The gain curve tracks the ideal line at low power, then bends; the 1 dB compression point is where actual output falls one decibel short of linear — the practical edge of clean operation.

How it works

Every amplifier has a finite supply voltage and current, so the output cannot swing past a hard limit. As input power climbs, the output approaches that ceiling and the incremental gain shrinks — gain compression. P1dB is a simple, agreed-upon threshold for “meaningfully compressed”: the input (or output) power at which the measured gain is exactly 1 dB less than the small-signal gain.

  • Input-referred (IP1dB) describes how strong a signal the device can accept.
  • Output-referred (OP1dB) describes how much clean power it can deliver; the two differ by the compressed gain (OP1dB ≈ IP1dB + G − 1 dB).

Beyond P1dB the device continues into hard saturation, where output barely rises with input and the signal is grossly distorted. For a receiver, driving the front end past compression is overload: gain collapses, harmonics and intermod explode, and a strong signal can desensitize the radio to everything else — see desensitization. P1dB, together with the noise floor, therefore brackets the usable dynamic range.

In practice

For a receive low-noise amplifier, a high P1dB means it stays linear even when a strong local transmitter appears — essential for scanning near paging or broadcast sites. For a transmit power amplifier, P1dB (and its output-referred form) roughly marks the top of clean, low-distortion output; digital modes with high crest factor must be backed off several dB below P1dB to keep their peaks out of compression and preserve EVM and spectral cleanliness. Engineers also use P1dB as a quick linearity sanity check, since it needs only a single-tone sweep, whereas IP3 requires a two-tone setup.

Relevance to SDR

For SDR reception, P1dB is really a statement about the front end ahead of the analog-to-digital converter — the LNA, mixer, and IF chain. Set gain too high and a strong nearby signal pushes some stage (often the ADC’s full-scale, an equivalent compression) into overload, wiping out the weak channels you actually wanted. This is the mechanism behind the common advice to reduce gain or add an attenuator or front-end filter when a scanner degrades in a strong-signal environment: you are keeping the whole chain comfortably below its compression point.

GopherTrunk sees only the digitized result. Once a stage has compressed and clipped, the distortion is in the samples and no decoder can remove it; a signal that decodes in isolation but fails when a strong carrier is present is compression/overload limited, fixed by gain staging and filtering upstream, not by software.

Sources

  1. Gain compression — Wikipedia, definition of the 1 dB compression point and its role in amplifier saturation. ↩

See also