29-08-2013, 04:57 PM
Detection of Signals in Noise
Signals in Noise[.pdf (Size: 1.57 MB / Downloads: 97)
Receiver Thermal Noise
Noise is the unwanted electromagnetic energy that interferes with
the ability of a receiver to detect the wanted signal
Noise may enter the receiver through the antenna
Noise may also be generated by the thermal motion of the
conduction electrons in the ohmic portions of the receiver input
stages. This is known as thermal or Johnson noise
The noise power is expressed in terms of the temperature, To of a
matched resistor at the input to the receiver
Receiver Noise Figure
The noise power in practical receivers is always greater
than that which can be accounted by thermal noise alone
The total noise, N, at the output of the receiver can be
considered to be equal to the noise power from an ideal
receiver scaled by a factor called the noise figure, NF
Detected Noise
Consider a typical radar front end that consists of an
antenna followed by a wide band amplifier, a mixer that
down converts the signal to an intermediate frequency (IF)
where it is further amplified and filtered (bandwidth βIF )
This is followed by an envelope detector and further
filtering (bandwidth βV = βIF/2).
The noise entering the IF filter is assumed to be Gaussian
(as it is thermal in nature) with a probability density
function (PDF) .
The Matched Filter
This SNR can achieve its maximum value when the IF
filter is matched to the signal.
This should not be confused with matching in circuit
theory which maximises power transfer not SNR
The peak signal to (average) noise power ratio of the
output response of the matched filter is equal to twice
the received signal energy E divided by the single-sided
noise power per Hz.
Detection of Fluctuating Signals
All moving targets (with the exception of the sphere) will produce echoes
whose RCS changes with time
To account for these fluctuations, both the PDF and the correlation
properties of the target must be known
Ideally these should be measured for each target, however this is often not
feasible
A practical alternative is to postulate a reasonable model for target
fluctuations and the analyse the effects mathematically
Four fluctuation models have been proposed by Swerling for this purpose