COMMUNICATION SYSTEM EECB353 Chapter 4 NOISE ANALYSIS

1. COMMUNICATION SYSTEM EECB353Chapter 4NOISE ANALYSIS

2. Noise Analysis • Noise is any undesired signal that ultimately appears in the output of a communications system. • Electrical noise is defined as any undesirable electrical energy that falls within the passband of the signal. • Electrical noise may be said to be the introduction of any unwanted energy, which tend to interfere with the proper reception and reproduction of transmitted signals. • 2 types of noise: • Correlated – exist when a signal is present. • Uncorrelated – exist regardless of whether there is a signal presentor not.

3. Uncorrelated Noise • Subdivided into 2 categories: • External Noise • Present in a received radio signal that has been introduced in the transmitting medium. • Source - atmospheric, extraterrestrial and man-made • Internal Noise • Introduced by the receiver itself. • Electrical interference generated within a device i.e create from the communication equipment. • Type– shot, transit time and thermal.

4. External Noise 1. Atmospheric Noise • Caused by naturally occurring disturbances in the earth’s atmosphere, with lighting discharges being the most prominent contributors. • It is often in the form of impulse that spread energy throughout a wide range of freq. 2. Extraterrestrial Noise (Space Noise) • Originates from outside earth’s atmosphere (outer space), also call deep-space noise. • Sub-divided into 2 categories: • Solar noise – generated from the sun’s heat. The sun radiates a broad spectrum of freq, including those which are used for broadcasting. • Cosmic noise – originating from stars other than the sun. 3. Human-made Noise • Produce by mankind. Generated by equipment that produces sparks. • Eg. Automobile engine, switching equipment, fluorescent light.

5. External Noise • Impulse Noise (spikes) – characterized by high amplitude peaks of short duration in the total noise spectrum. • Consists of sudden burst of irregularly shaped pulses that generally last between a few milliseconds. • Some of the sources of impulse noise are voltage changes in adjacent lines, lightning flashes during thunderstorms and fluorescent lights. • Interference – form of external noise. • Electrical interference occurs when info signals from one source produce freqs outside their allocated BW and interfere with other info signal. • Most interference occurs when harmonics or cross-product freq from one source fall into the passband of a neighboring channel.

6. Internal Noise 1. Shot Noise • Produced in active devices such as transistors. • Caused by a random arrival of carriers (holes & electrons) in the pn junctions of semiconductor. • The carrier is not moving in continuous and steady flow i.e it moves in a random path of motion. 2. Transit-time Noise • Noise produced in semiconductors when the transit time of the carriers crossing a junction is close to the signal's period and some of the carriers diffuse back to the source or emitter of the semiconductor. • i.e Due to any modification to a stream of carriers as they pass form input to the output of a device (from emitter to collector). • Time taken for the carrier to propagate through a device produces irregular and random variation of noise.

7. Internal Noise 3. Thermal Noise (White Noise or Johnson Noise) • Generated by the agitation and interaction of electrons in a conductor due to heat. • Thermal Noise Power, N = KTB where N = noise power (W) K = Boltzmann’s contant (1.38 x 10-23 Joules/Kelvin) T = absolute temperature (Kelvin), and T= C + 273 B = bandwidth (Hz) • Thermal Noise Power in dBm, • Thermal Noise – dependent on temperature. • White Noise – another name for thermal noise because its frequency content is uniform across spectrum. • Johnson Noise – another name for thermal noise, first studied by J.B. Johnson.

8. Internal Noise • For worst case and max noise power transfer, RI = R. Thus, VR = VN/2 = VL • The rms noise voltage, VN that appears across a resistor at temperature T is:

9. Example 1 For an electronic device operating at a temperature of 27C over a 1 MHz frequency range, determine • thermal noise power in watts and dBm • rms noise voltage for a 100  resistance

10. Example 2 The noise produced by a resistor is to be amplified by a noiseless amplifier having a voltage gain of 75 and a bandwidth of 100 kHz. A sensitive meter at the output reads 240 µV rms. Assuming operation at 37°C, calculate the resistor’s resistance. If the bandwidth were cut to 25 kHz, determine the expected output meter reading. Solution

11. Correlated Noise • It is a form of internal noise that is correlated to the signal and cannot be present in a circuit unless there is a signal i.e – NO SIGNAL, NO NOISE! • Produced by nonlinear amplification and includes harmonic and intermodulation distortion, both of which are forms of nonlinear distortion. • Nonlinear distortion creates unwanted frequencies that interfere with the signal and degrade performance

12. Correlated Noise • Harmonic distortion (Amplitude Distortion) occurs when unwanted harmonics of a signal are produced through nonlinear amplification (nonlinear mixing). • Harmonics are integer multiples of the original signal. • The original signal = first harmonic = fundamental frequency. • 2 x the original signal freq is called the second harmonic, .. n x original signal freq = nth harmonic. Figure: Correlated Noise (Harmonic Distortion) Note – from Figure, the output spectrum contains the original input freq plus several harmonics (2f1, 3f1, 4f1) that were not part of the original signal.

13. Correlated Noise • Total Harmonic Distortion,THD is the ratio of the quadratic sum of the rms values of all the higher harmonics to the rms value of the fundamental. %THD =

14. Example 3 – Determine • 2nd, 3rd and 12th harmonics for a 1 kHz repetitive wave. • Percent second-order, third-order and total harmonic distortion for a fundamental frequency with an amplitude of 8Vrms, a second harmonic amplitude of 0.2Vrms, and a third harmonic amplitude of 0.1Vrms.

15. Correlated Noise • Intermodulation distortion is the generation of unwanted sum and difference frequencies produced when two or more signals mix in a nonlinear device. • The sum and difference freq are called cross product i.e mathematically Cross product = where f1, f2= fundamental frequencies, f1> f2 m,n = positive integers • Unwanted cross-product freq can interfere with the info signals in a cct or with the info signal in other cct. Figure: Correlated Noise (Intermodulation Distortion)

16. Example 4 - For a non linear amplifier with two input frequencies, 3 kHz and 8 kHz, determine • First three harmonics present in the output for each input frequency. • Cross-product frequencies produced for values of m and n of 1 and 2.

17. INTERFERENCE • Form of external noise • “to disturb or to detract from” • Electrical interference : When information signal from one source produce frequencies that fall outside their allocated bandwidth and interfere with information signals from other source. • Most interference occurs when harmonics or cross product frequencies from 1 source fall into the passband of a neighboring channel

18. Noise Summary