(i)Noise figure:
Sometimes the noise factor is expressed in decibels. When noise factor is expressed in decibels it is called noise figure.
Noise figure = $F_{dB}$=10 $log_{10}$F………..(1)
Substituting the expression for the noise factor we get
Noise figure =10 $log_{10}$[(S/Nat the input)/(S/Nat the output) ]
=10$log_{10}$ $(S/N)_i$ - 10 $log_{10}{(S/N)_o}$
Noise figure = $F_{dB}$= $(S/N)_i$ dB – $(S/N)_o$dB….(2)
The ideal value of noise figure is 0dB.
(ii)Noise temperature
The concept of noise factor or noise figure is not always the most convenient way of measuring noise.
Another way to represent the noise is by means of the equivalent noise temperature.
The equivalent noise temperature is used in dealing with the UHF and microwave low noise antennas, receivers or devices.
Definition:
The equivalent noise temperature of a system is defined as the temperature at which the noisy resistor has to be maintained so that by connecting this resistor to the input of a noiseless version of the system, it will produce the same amount of noise power at the system output as that produced by the actual system.
(iii)Noise bandwidth
Assume that a white noise is present at the input of a receiver (filter).Let the filter have a transfer function H(f) as shown in figure.
This filter is being used to reduce the noise power actually passed on to the receiver. Now contemplate an ideal (rectangular) filter as shown by the dotted plot in figure .The center frequency of this ideal filter also is $f_0$.
Noise bandwidth of a filter.
Let the bandwidth “$B_N$ " of the ideal filter be adjusted in such a way that the noise output power of the ideal filter is exactly equal to the noise output power of a real R-C filter.
Then B_N is called as the noise bandwidth of the real filter.
Thus the noise bandwidth "$B_N$ " is defined as the bandwidth of an ideal (rectangular) filter which passes the same noise power as does the real filter.
(iv)Noise voltage
Thermal noise is distinct from shot noise, which consists of additional current fluctuations that occur when a voltage is applied and a macroscopic current starts to flow.
For the general case, the above definition applies to charge carriers in any type of conducting medium (e.g. ions in an electrolyte), not just resistors. It can be modeled by a voltage source representing the noise of the non-ideal resistor in series with an ideal noise free resistor.
(v)Modulation
Modulation is the addition of information to an electronic or optical carrier signal. A carrier signal is one with a steady waveform -- constant height (amplitude) and frequency.
Information can be added to the carrier by varying its amplitude, frequency, phase, polarization (for optical signals), and even quantum-level phenomena like spin.
Common modulation methods include the following:
Amplitude modulation (AM), in which the height (i.e., the strength or intensity) of the signal carrier is varied to represent the data being added to the signal.
Frequency modulation (FM), in which the frequency of the carrier waveform is varied to reflect the frequency of the data.
Phase modulation (PM), in which the frequency of the carrier waveform is varied to reflect changes in the frequency of the data (similar but not the same as FM).
Polarization modulation, in which the angle of rotation of an optical carrier signal is varied to reflect transmitted data.
Pulse-code modulation, in which an analog signal is sampled to derive a data stream that is used to modulate a digital carrier signal.
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