Receiver Sensitivity vs. Selectivity: Understanding the Difference

This page describes the difference between sensitivity and selectivity with respect to a receiver. Sensitivity and selectivity are very useful parameters for a wireless receiver.

Receiver Sensitivity

Receiver sensitivity is the smallest possible signal power level at the input of a receiver that assures proper functioning; that is, it will decode data correctly. This wireless receiver can be part of a mobile phone, base station, or any wireless standard compliant device such as WLAN, Zigbee, Bluetooth, etc. Up to this power level, the system delivers a Bit Error Rate (BER) within the desired range. Below this level, the system starts producing more bit errors and it’s not advisable to operate the system below this power level.

For example, the IEEE standard for Zigbee 802.15.4 specifies the receiver sensitivity of a 2.4 GHz device to be about -85 dBm. If one increases receiver sensitivity to, say, -90 dBm from -85 dBm, it will extend the coverage or distance (in meters) for that particular radio/wireless device.

Higher receiver sensitivity reduces the need for a power-hungry power amplifier at the transmitter end, which reduces power consumption and increases battery life. This is the main reason behind people aiming for higher receiver sensitivity when selecting a radio device.

Fig.1 Receiver sensitivity measurement test setup

As shown in the figure, start with a $P_{in}$ (input power) of 5 dB more than the receiver sensitivity and check the BER in the test system. Keep reducing $P_{in}$ until the BER is within an acceptable range. Stop when the BER is out of the range of acceptance and record the $P_{in}$. This $P_{in}$ is the receiver sensitivity of the Device Under Test (DUT).

Receiver Selectivity

Receiver selectivity refers to a wireless receiver’s capability to detect and decode the desired signal in the presence of other unwanted interfering signals. In other words, the selectivity of the receiver means how well a receiver performs in the presence of other unwanted co-channel and adjacent channel interfering signals.

Usually, these unwanted signals can never be eliminated, but if the levels of these signals are sufficiently lower than the wanted carrier frequency, then the radio receiver or wireless system will function without any errors. If the levels of these interfering signals are comparable to the desired signal, then the system starts producing errors.

Filters are employed in the wireless/RF receiver chain to improve the selectivity of the device.

Fig.2 Receiver selectivity measurement test setup

The setup is slightly changed from the sensitivity setup. Here, one more RF signal generator is used to generate an interfering signal either within the band or near the band of the desired signal. Both the desired and interfering signals are combined using a power combiner and fed to the DUT. The level of the interfering signal is increased slowly in steps from a specified limit, and the system will be checked for errors. This level until errors are within range is the selectivity level of the DUT.