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January 23, 2018

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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?

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!