Radio Blocking & Desensitisation

Blocking & desensitisation are key aspects of the performance of a radio receiver.

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Radio Receiver Overload / Strong Signal Handling Includes:
Strong signal performance basics Blocking & desensitisation Intermodulation distortion Cross modulation Receiver intercept point

Radio receiver blocking can also be referred to as receiver desensitisation and it occurs when strong signals are present and results in the level of lower level signals being reduced, i.e. the receiver is desensitised.

Good radio receiver blocking performance or receiver desensitisation performance is particularly important in the scenarios where a number of radios of various forms are used in close proximity to each other.

With wireless communications being used for everything from Wi-Fi to cellular communications and Bluetooth as well as many more traditional radio communications applications, there are many more instances where two radios operate in very close proximity to each other creating very large signals at the receiver input. In situations such as these, the receiver blocking performance is very important.

As an example, radio receiver blocking constitutes one of the out-of-channel receiver tests used for type testing cellular phones.

Receiver blocking basics

When a very strong off channel signal appears at the input to a receiver it is often found that the sensitivity is reduced. The effect arises because the front end amplifiers run into compression as a result of the off channel signal.

Overload characteristic of a generic amplifier — Characteristic curve for an amplifier showing overload area

This often arises when a receiver and transmitter are run from the same site and the transmitter signal is exceedingly strong.

When this occurs it has the effect of suppressing all the other signals trying to pass through the amplifier, giving the effect of a reduction in gain.

Effect of radio receiver blocking by adjacent strong signal — Effect of radio receiver blocking by strong signal

Radio blocking specifications

Blocking is generally specified as the level of the unwanted signal at a given offset - often 20 kHz - which will give a 3 dB reduction in gain, although receivers for particular applications such as cellular or Wi-Fi applications for example may quote the figures in a way that is more applicable to hat particular application.

Dependent upon the type of receiver, the values for blocking will vary considerably. As a reference point, a good communications style receiver may be able to withstand signals of about 10 dBm before this happens.

Receiver blocking and desensitisation cause

Receiver blocking or receiver desensitisation is caused by the odd order intermodulation products within a receiver amplifier / mixer chain. These affect the signal in such a way that the wanted signal strength is reduced.

When a signal is being received in the presence of a strong interfering signal, then non-linearities within the receiver mixer and amplifier chain result in intermodulation products. One of the resulting components of an even power of the sinusoid is a constant, so the desired signal is multiplied by that constant and an even power of the interferer's signal strength. If the interferer is sufficiently strong, the resulting product will subtract from the desired signal product from the first power term, reducing the effective gain of the device.

The radio blocking and desensitisation specifications are very important. With the phenomenal growth in mobile and other forms of wireless communication, most cell phones and other wireless enabled devices need to be able to operate successfully in the presence of other strong local signals. Even a single cell phone will have several different wireless transmitters and receivers.

For more traditional communications on the short wave bands or even at VHF and UHF, the radio blocking and desensitisation specifications and parameters are still very important as it is necessary to operate these receivers in the presence of local string signals and maintain performance at all times.