Sensitivity, selectivity and dynamic range

What it is

Three numbers describe how good a receiver is. Sensitivity is how weak a signal it can hear. Selectivity is how well it separates a signal from close neighbours. Dynamic range is how strong the other signals can be before they spoil reception. A radio that is strong in one can be poor in another, which is why no single number describes a receiver.

Sensitivity: a question of noise

Every receiver adds noise of its own, and even a perfect one sees thermal noise. A signal is heard only if it stands above the noise floor:

Bandwidth	Floor with a 10 dB noise figure (typical example)
15 kHz (FM)	−122 dBm
2.4 kHz (SSB)	−130 dBm
500 Hz (CW)	−137 dBm

Two levers move the floor: a narrower bandwidth (about 7 dB lower from SSB to CW) and a lower noise figure. The minimum discernible signal (MDS) is roughly the noise floor.

On the lower HF bands, atmospheric and man-made noise arriving at the antenna is usually stronger than the receiver's own, so a better noise figure or a preamp gains little; on VHF and UHF, where outside noise is low, it can matter a lot. See Noise and signal-to-noise ratio.

Selectivity: filter width and skirts

Selectivity comes from filters (see Filters in radios). Two properties matter: the bandwidth (about 2.4 kHz for SSB, a few hundred Hz for CW) and how steeply the skirts fall away on each side. A filter that is narrow at its top but slopes gently still lets a strong neighbour through.

Dynamic range: strong signals

A strong signal does harm in two ways:

• Blocking (gain compression). A strong signal overloads an early stage, which turns its gain down. Weak signals on any frequency then fade or vanish.
• Intermodulation. Two strong signals mix inside the receiver, producing false signals at other frequencies, called third-order products (see Receiver overload and intermodulation).

Dynamic range is the span in dB from the noise floor up to the level at which one of these appears.

Good HF receivers reach more than 100 dB (typical for a modern design); simple ones are far less. Specifications state the signal spacing, since a close pair is much harder than a widely spaced one.

In practice

What you notice	Likely limit
a weak station is hidden by hiss	sensitivity (or just outside noise)
two stations heard together on one frequency	selectivity
false signals, or everything fades when a neighbour transmits	dynamic range

If strong signals are the trouble, reducing the receiver's gain (RF gain, attenuator, preamp off) usually helps, and costs little on the lower HF bands where noise dominates anyway.

Related

• Receiver basicsFrom antenna to speaker.
• Filters in radiosRoofing, crystal, mechanical and DSP filters.
• AGC, noise blankers and noise reductionTaming level changes and noise.
• Receiver overload and intermodulationWhen strong signals cause trouble.
• Noise and signal-to-noise ratioWhere noise comes from and why SNR decides what you can copy.
• DecibelsThe logarithmic scale for gain, loss and signal strength.
• Attenuators and padsReducing signal level without upsetting the impedance.