Updated July 9,
here to go to our main page on receivers
here to go yo our page on dynamic range
New for July 2011! This
content was contributed by Aviv, from Israel. Thanks! Aviv won our
for his contribution. And what have you contributed today?
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!
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
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
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
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.
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.