Receiver Desensitisation Calculator

Desensitisation, or desense, is the loss of receiver sensitivity when unwanted power lands in the receiver passband and raises its noise floor. The receiver does not need to be tuned to the interferer, because broadband transmitter noise and reciprocal mixing both put noise across the band. The rise in noise floor, in dB, is the sensitivity lost.

The interference power and the receiver noise floor are added as linear powers in the same bandwidth, and the desense is the rise above the original floor: D = 10 log₁₀ (1 + 10^((I − N)/10)). Interference 6 dB below the floor gives about 1 dB of desense, interference equal to the floor gives 3 dB, and well above the floor the desense is about how far the interference sits above it.

It is set by the service's own protection criteria or by the system or site specification, not by one universal figure. For the fixed service, ITU-R F.758-8 Table 5 gives an aggregate interference to noise ratio of −6 dB between 30 MHz and 3 GHz and −10 dB above 3 GHz as the generally applicable values for long-term interference, which cost 0.97 dB and 0.41 dB of desense. Turned round, a 1 dB desense budget allows interference up to 5.87 dB below the noise floor, and a 0.5 dB budget up to 9.14 dB below it.

The usual causes are transmitter broadband noise, reciprocal mixing and front end overload. A nearby transmitter emits noise across a wide band that falls on the receive frequency, even though the carrier is on another channel. A strong off-channel carrier can also mix with the receiver's local oscillator phase noise to create noise in the passband. The calculator models these two, as well as any measured in-band interferer. It does not model the receiver's front end circuits, so overload by the same strong carrier is left out: a receiver's measured blocking figure covers both effects.

Enough to bring the transmitter noise at the receive frequency below the level that gives your allowable desense. The calculator works this out from the desense budget, the transmitter power and its noise density, and shows the margin against the isolation you already have. Isolation for transmitter noise comes from antenna separation or from filtering at the transmitter output, such as a cavity or duplexer: noise already on the receive frequency cannot be filtered out at the receiver.

Desense reduces sensitivity dB for dB, which shortens the usable range. With a power-law path loss, the range shrinks to 10^(−D/(10n)) of its original value, where D is the desense and n the path loss exponent. In free space (n = 2), 1 dB of desense costs about 11 per cent of range and 3 dB about 29 per cent. The ITU-R site-general models (P.1411, P.1238) give other exponents over the distances they were fitted to.