Noise may be defined as any unwanted form of energy tending to interfere with the proper and easy reception and reproduction of signals. It may be subdivided according to type, source, effect, relation to the speaker, depending on circumstances.

1. External Noise. * Atmospheric Noise – consist of spurious radio components distributed a wide range of frequencies. Majority of these radio signals come from the natural sources of disturbance generally called static. * Extraterrestrial Noise * Solar Noise * Cosmic Noise * Industrial Noise – the intensity of noise made by humans easily outstrips that created by any other source, internal or external to the receiver.

2. Internal Noise * Thermal agitation noise – it is due to the rapid and random motion of molecules, atoms and electrons of which any such resistor is made up. Vn = 4kTδƒ R Example a. An amplifier operating the frequency rate from 18 to 20 MHz has a 10-kΩ input resistor. What is the rms noise voltage at the input to this amplifier if the ambient temperature is 27° C? Vn = 4kTδƒ R =4 x 1.38 x 3 x 2 x 10-11 = 1.82 x 10-5 = 18.2 μV * Shot noise – it is caused by random variations in the arrival of electrons at the output electrode of an amplifying device and thus as a randomly varying noise current superimposed on the output. 1.6 x 10-19 C in =2eip δf * Transmit Time Noise – if the time taken an electron to travel from the emitter to the collector of a transistor becomes comparable to the period of the signal being amplified at the frequencies in the upper VHF range and beyond the so called transmit time effect takes place. * Miscellaneous Noise Flicker – a poorly understood form of noise at low audio frequencies. Resistance – thermal noise(resistance noise) Noise in mixers – mixers are noisier than amplifiers.

* NOISE CALCULATIONS
*Addition of noise due to several sources Vn,tot = 4kt δf Rtot where Rtot = R1 + R2 + . . . Example b. Calculate the noise voltage at the input of a television RF amplifier, using a device that has a 200-Ω equivalent noise resistance and a 300-Ω input resistor. The bandwidth of the amplifier is 6 MHz and the temperature is 17°C.
Vn,tot = 4kt δf Rtot =4 x 1.38 x10-23 x17+273 x 6 x 106 x (300+200) =4 x 1.38 x 2.9 x 6 x 5 x 10-13 =48 x 10-12 = 6.93 x 10-6 = 6.93 μV

*Addition of noise due to several amplifiers in cascade

Req = R1 + R2A12 + R3A12A22

Example c. The first two stage of a two-stage amplifier has a voltage gain of 10, a 600-Ω input resistor, a 1600- Ω equivalent noise resistance and a 27-Ω output resistor. For the second stage, these values are 25, 81 kΩ, 10kΩ and 1 MΩ respectively. Calculate the equivalent noise resistance of this two stage amplifiers.
R1 = 600 + 1600 = 2200Ω
R2= 27 x 8127+ 81 + 10 = 20.2 +10 =30.2 kΩ
R3 = 1 MΩ
Req = 2200 + 30,200102 +1,000,000102x 252 = 2200 + 302 + 16 =2518 Ω

Noise in Reactive Circuits V = 4kT δf Rp
V is the noise voltage across a tuned circuit due to its internal resistance and Rp is the equivalent parallel impedance of the tuned circuit at resonance.

Signal to noise ratio It is the ratio of signal power to noise power at the same point. SN = XsXn = Vs2/RVn2/R = (VsVn) 2

Noise figure The ratio of the signal to noise power supplied to the input terminals of a receiver or amplifier to the signal to noise power supplied to the output or load resistor. F = input S/Noutput S/N

Noise figure from equivalent noise resistance R’eq = Req - Rt F = 1+R'eqRa
Example d. calculate the noise figure of the amplifier of ex. C if it is driven by a generator whose output whose output impedance is 50 Ω. R’eq = Req - Rt = 2518 – 600 = 1918Ω F = 1+R'eqRa = 1+ 38.4 = 39.4 (=15.48 dB)

Noise temperature Teq = T0 (F-1)

A receiver connected to an antenna whose resistance is 50Ω has an equivalent noise resistance of 30Ω. Calculate the receiver’s noise figure in decibels and its equivalent noise temperature.

F = 1+R'eqRa =1 + 3050 =1 + 0.6 =1.6 = 10 log 1.6 =10 x 0.204 = 2.04 dB

Teq = T0 (F-1) =290 (1.6 - 1) =290 x 0.6 =174K

AMPLITUDE MODULATION A system of modulation in which the amplitude of the carrier is made proportional to the instantaneous amplitude of the modulating voltage.

The tuned frequency of the oscillator in a simple AM transmitter employs a 50 µH coil and a 1nF capacitor. If the oscillator output is modulated by audio frequencies up to 10 kHz, what is the frequency range occupied by the sidebands? fc = 12πLC = 12π5 x 10-5 x 1 x 10-91/2 = 7.12 x 105 =712 kHz

Power relations A 400-watt carrier is modulated to a depth of 75 percent. Calculate the total power in the modulated wave.
Pt = Pc (1 + m22) =400(1 +0.7522) =400 x 1.281 =512.5 W A broadcast radio transmitter radiates 10 kW when the modulation percentage is 60. How much of this is carrier power?
Pc = Pt1+ m2/2 =101+ 0.62/2 = 101.18 =8.47 kW

FREQUENCY MODULATION A system in which the amplitude of the modulated carrier is kept constant, while its frequency is varied by the modulating signal