Noise, bandwidth and sensitivity

Noise starts before the receiver

In the classical model, a matched thermal source provides available noise power N = kBTB. kB is Boltzmann’s constant, T is absolute temperature in kelvin and B is equivalent noise bandwidthBandwidth×Bandwidth is the width of a frequency range. A receiver filter limits the analysed range; the signal’s occupied bandwidth is a separate quantity.Example & full explanation → in hertz. At 290 K, a 1 Hz bandwidth gives about −174 dBmdBm×dBm expresses an absolute power level relative to 1 milliwatt, combining the logarithmic dB scale with a fixed power reference.Example & full explanation →: a density reference, not a universal receiver sensitivity.

Doubling bandwidth doubles this noise power, an increase of about 3 dB. A 2400 Hz bandwidth adds about 33.8 dB relative to 1 Hz. A real antenna can collect considerably more noise from the atmosphere and local electrical equipment than an ideal room-temperature source would suggest.

What noise figure adds

Noise figure describes a receiver’s degradation of signal-to-noise ratio under defined reference conditions. The calculator converts it at 290 K into an equivalent added noise temperature. This is added to source temperature, avoiding the incorrect approach of simply multiplying a different source temperature by the noise factor.

Sensitivity also needs a criterion: what signal-to-noise ratio or error rate must be achieved? Mode, demodulator and required quality therefore belong with any meaningful specification. Two data sheets using different bandwidths or measurement criteria cannot be compared directly.

Narrow the filter, preserve information

A narrower filter helps only while enough of the wanted signal remains intact. Cutting speech or data can reduce usability despite lower noise. Use equivalent noise bandwidth in the calculator; the nominal filter width shown on screen may only approximate it, depending on filter shape.