Chapter 1 : Part 3 Noise

Noise, interference and distortion  Noise  unwanted signals that coincide with the desired signals.  Two type of noise: internal and external noise.  Internal noise  Caused by internal devices/components in the circuits.  External noise  noise that is generated outside the circuit.  E.g. atmospheric noise, solar noise, cosmic noise, man made noise. EKT343 –Principle of Communication Engineering 2

Noise, interference and distortion (Cont’d) Interference  Contamination by extraneous signals from human sources.  E.g. from other transmitters, power lines and machineries.  Occurs most often in radio systems whose receiving antennas usually intercept several signals at the same time  One type of noise. Distortion  Signals or waves perturbation caused by imperfect response of the system to the desired signal itself.  May be corrected or reduced with the help of equalizers. EKT343 –Principle of Communication Engineering 3

Noise Temperature & Equivalent noise Temperature Thermal noise directly proportional to temperature ~ can be expressed in degrees, watts or volts. EKT343 –Principle of Communication Engineering 4 Where P N = noise power ( Watt ) k = Boltzman constant (1.38 x J / K ) T = environmental temperature ( K ) [Add 273 to C] B = Bandwidth of system ( Hz ) P n = k T B

Cont’d… Equivalent noise temperature, (T e ) T e = T(F-1) Where T = environmental temperature (kelvin) F = Noise factor T e often used in low noise, sophisticated radio receivers rather than noise figure. EKT343 –Principle of Communication Engineering 5

Example 1 A domestic television receives antenna delivers a sky noise power of -105 dBm to a matched coaxial feeder in a radio frequency bandwidth of 8 MHz. Find the antenna noise temperature. EKT343 –Principle of Communication Engineering 6

Insertion loss IL is a parameter associated with the frequencies that fall within the passband of a filter. The ratio of the power transferred to a load with a filter in the circuit to the power transferred to a load without the filter. IL (dB) = 10 log (P out /P in ) EKT343 –Principle of Communication Engineering 7

Signal to Noise Ratio EKT343 –Principle of Communication Engineering 8 SNR is ratio of signal power, S to noise power, N. Noise Factor, F Noise Figure, NF

For a nonideal amplifier with the following parameters: Table 1 Determine the following: 1)Input Signal-to-Noise ratio (dB). 2)Output Signal-to-Noise ratio (dB). 3)Noise factor and noise figure. 4)Equivalent noise temperature. Exercise EKT343 –Principle of Communication Engineering 9