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New for July 2011! This content was contributed by Aviv, from Israel. Thanks! Aviv won our prestigious pocketknife for his contribution. And what have you contributed today?
Blocking is defined as the degradation of receiver sensitivity, usually by 3 dB, in the presence of a much stronger (blocking) signal. Therefore we must first understand that Sensitivity is the measure of what the smallest possible detectable signal is.Sensitivity is measured through signal-to-noise ratio (SNR) at the output of the receiver. Different SNR values can be chosen according to the specific task. Blocking would therefore be the reduction in SNR caused by an interfering signal.
What does this mean?
Let's assume you have a radio receiver and you monitor a certain signal at an SNR of 6 dB. Now if a much stronger signal was present near your signal, the SNR of your desired signal would now be less than 6 dB.
But why does this happen?
Unfortunately for us, in all radio systems, the blocking signal does not have to be masking the desired signal, as is commonly used in electronic warfare. Blocking can also be caused by regular high power radio signals spectrally near your desired signal.The simplest method for this kind of blocking to occur would be if the interfering signal was strong enough causing the receiver LNA (the first amplifier in the receiver) to reach its compression point, thus causing a decrease in gain and potentially raising the amplifier's noise figure.Due to this, "blocking dynamic range" is often defined as "the difference in dB between the level of an incoming signal which will cause 1 dB of gain compression, and the level of the noise floor".Practically, a strong signal at the input port would mean that an attenuator of some sort would have to be put to work in order to avoid compression. Adding attenuation in the beginning of the RF chain causes your overall noise figure to increase by the value of attenuation (noise figure would increase by 2 dB for a 2 dB attenuator, 3 dB for a 3 dB attenuator and so on). This in turn means that your desired signal's SNR would degrade by the same value as that of the attenuation.Because of these two reasons, blocking is usually measured while maintaining receiver linearity and not altering the attenuation along the RF chain.Still an interfering signal can degrade the SNR of the desired signal due to intermodulation. There are two different possible causes for blocking by intermodulation. This is where life gets mathematical but I will try to simplify.The first phenomenon is caused by intermodulation of the second order usually referred to as IM2. Let's assume the desired signal is at 
1 and the interferer is at 2 while another signal which could be just noise is at 3:
The interferer mixes with the off-channel noise according to:
If the term is true then the IM2 product will appear around your desired signal. If this third signal is just noise then it can cause a raise in noise level around the desired signal, degrading SNR. The worst such case will occur if and in which 1/f noise will be mixed up to the desired frequency.The second phenomenon, and the more severe of the two, is caused by third order intermodulation. Let's assume:
If we look at the terms at frequency 1 for the Maclaurin series we have:
Further investigation into the Maclaurin series and with the restriction of the solution being physically possible we arrive at the conclusion that a3 will have an opposite sign to a1, meaning that the second term is in opposite phase to the first.This second term causes desensitization in the desired signal effectively lowering its amplitude according to a3 and V2.
Postscript:
Aviv had to delay his writeup for a while, as little Ofri appeared on the scene and demanded some attention. So we'll keep her picture right here along with her father's excellent work.The Unknown Editor is jealous of all of that hair!