Receiver Blocking or Desensitisation

- notes and details about radio receiver blocking or desensitisation performance and the factors that affect it.

Radio receiver blocking, or as it is also called receiver desensitisation is an important parameter for any receiver.

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 sued for everything from Wi-Fi to cellular communications and Bluetooth as well as many more traditional applications, there are many more instances where two radios operate very close to each other and the receiver blocking performance will be very important.

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

Radio receiver blocking
Radio receiver blocking

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 blocking specification is now more important than it was many years ago. With the increase in radio communications systems in use, it is quite likely that a radio transmitter will be operating in the close vicinity to a receiver. If the radio receiver is blocked by the neighbouring transmitter then it can seriously degrade the performance of the overall radio communications system.

By Ian Poole

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