Field Guide · term

Also known as: BDR, blocking, blocking level

Blocking dynamic range (BDR) is the range in decibels between a receiver’s noise floor and the level of a strong off-channel signal that desensitizes it by a defined amount — usually 1 dB or 3 dB of sensitivity loss.1 It measures how loud an unwanted neighbor a receiver can tolerate before a weak wanted signal starts to fade, and it is one of the core dimensions of a receiver’s overall dynamic range.

noise floor blocking level BDR (dB)
Blocking dynamic range is the vertical span from the noise floor up to the off-channel level that causes a defined loss of sensitivity.

How it works

The lower bound of BDR is the receiver’s own noise floor, which fixes the weakest signal it can hear — its sensitivity. The upper bound is the blocking level: the power of an interfering carrier, placed at a stated offset (say 20 kHz or 100 kHz away), that reduces the wanted signal’s output by the reference amount. Subtract the two, in dB, and you have BDR.

A high blocking level, and therefore a wide BDR, requires two things. The front end must stay linear — high 1 dB compression point and third-order intercept — so the strong signal does not compress stage gain. And the local oscillator must be spectrally clean, because at large offsets the blocking level is set by reciprocal mixing: the strong signal beats against the LO’s phase noise and dumps broadband energy into the passband. Below a certain offset, LO phase noise usually dominates the limit; farther out, front-end compression does.

BDR is offset-dependent, so a meaningful figure always names the spacing. It is a sibling of spurious-free dynamic range, which instead measures the level at which internally generated intermodulation products emerge from the noise. A receiver can have excellent BDR yet poor SFDR, or vice versa; a good design needs both.

In practice

  • Narrow front-end selectivity — a cavity, helical, or SAW filter — raises the blocking level for out-of-band interferers by attenuating them before the first active stage.
  • A cleaner LO, such as one referenced to a TCXO or OCXO, pushes back the reciprocal-mixing limit and widens BDR at close offsets.
  • Wideband direct-sampling SDRs typically show lower BDR than a well-filtered superheterodyne set, because their entire input reaches the ADC unfiltered.

Relevance to SDR

Blocking dynamic range predicts how a scanner behaves at busy multi-transmitter sites, where a nearby high-power emitter sits only tens of kilohertz from the channel you want. P25, DMR, and TETRA trunked systems are frequently deployed alongside strong paging, broadcast, or cellular transmitters, so BDR often decides whether an SDR can follow the control channel at all. For a receiver-only decoder like GopherTrunk, BDR is a property of the RF hardware and its filtering, not of the decode software — but it directly bounds what GT can work with. When a strong neighbor blocks the front end, the wanted signal never reaches the DSP intact, and no amount of downstream processing recovers it.

Sources

  1. Dynamic range — Radio — Wikipedia, definitions of blocking and dynamic range in receivers. ↩

See also