Presentation on Noise from Analog Communication

Noise in Analog Communication

1. Course on ANALOG COMMUNICATIONS Presented by: G Sandeep V Padmakar Assistant Professor Dept. of ECE RCE DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING R16 Regulation II year II Semester

2. contents Introduction to noise Classification Internal noises External noises Receiver Receiver model Figure of merit Noise in DSB-SC receiver Noise in SSB-SC receiver

3. Introduction to noise Noise is an unwanted electrical or electromagnetic energy that interferes with the transmitted message and degrade the quantity of message signal.

4. Classification of noise NOISE INTERNAL EXTERNAL THERMAL NOISE or WHITE NOISE or JHONSON NOISE SHOT NOISE TRANSIT TIME NOISE MISCELLANEOUS NOISE FLICKER NOISE PARTITION NOISE ATMOSPHERIC NOISES EXTRATERRESTRIAL NOISES SOALR NOISE COSMIC NOISE MANMADE NOISES or INDUSTRIAL NOISE

5. Cont… Internal noises are generated within the receiver or communication system. External noises are generated from the external sources. If the noise gets added to the signal, then it is known as additive noise. x(t) + n(t) = additive noise If the noise gets multiplied to the signal, then it is known as fading. x(t) * n(t) = fading

6. Internal noises Thermal noise : This type of noise is generated by all resistances(e.g. Resistor, transistor, semiconductor, the resistance of resonant circuit etc.) Due to thermal agitation, the molecules in the electrical component gain energy, moves in random fashion and collide each other therefore produces heat and this heat produced is corresponds to the thermal noise. Thermal noise increases with temperature and resistance values. thermal noise power N = KTB watts where KT = No = power spectral density ⇒ N = NoB watts

7. Cont… N = KTB K = Boltzmann Constant = 1.38 x 10^-23 J/K = 8.65 x 10^-5 ev/k f B -B f B No No/2 S(f) S(f) one sided PSD two sided PSD

8. Cont… Shot noise : Shot noise is produced by the random movement of electrons or holes across a PN junction. Electrons or holes enter the junction region from one side, drift or are accumulated across the junction and are collected on the other side. The random movement give rise to a type of noise which is referred to as shot noise. Shot noise is also encountered as a result of emission of electrons from a heated surface. P N

9. Cont… Transit time noise: This noise occurs in transistors It is the time duration that is taken by current carrier such as electrons or holes to move from the input to the output. At low frequencies this time is negligible but when the frequency of operation is high then problem arises. The transit time shows up as a kind of random noise within the device and this is directly proportional to the frequency of operation.

10. Cont… Flicker noise : Flicker noise is also known as modulation noise or pink noise. Inversely proportional to frequency. Also known as 1/f noise occurs in almost all electronic devices and it has a variety of different causes although these are related to the flow of direct current. Partition noise : Partition noise occurs whenever current has to divide between two or more paths and results from the random fluctuations in the division. Due to this noise diode would be less noisy than a transistor.

11. External noises Atmospheric noise : Caused by lightning discharges in thunderstorms and other natural electrical disturbances occurring in the atmosphere. These electrical impulses are random in nature, hence the energy is spread over the complete frequency spectrum used for radio communication. Large atmospheric noise is generated in low and medium frequency bands while very little noise is generated in VHF and UHF bands. Therefore the atmospheric noise becomes less severe at frequencies above 30 MHz

12. Extraterrestrial noise: Solar noise : Electrical noise emerging from the sun Sun is a large body at a very high temperatures and radiates electrical energy in the form of noise over a very wide range of frequency spectrum used for radio communication. The intensity of noise produced by sun varies with time. Cosmic noise : Generated by distant stars having high temperatures . The noise receives from distant stars is cosmic noise and is distributed almost uniformly over entire sky.

13. Man – made noise or Industrial noise : Industrial noise is an electrical noise produced by the sources such as automobiles and aircraft ignition, electrical motors and switch gears, leakage from high voltage lines etc. Man – made noise is most intensive in industries & densely populated areas.

14. Receiver model Receiver : a receiver is a collection of electronic circuits designed to convert the signal back to the original information. It consists of amplifier, detector, mixer, oscillator, transducer etc. The model consists of modulated signal S(t) and noise signal n(t) The receiver input is the sum of S(t) & n(t). BPF is used for filtering action of tuned amplifier for the purpose of signal amplification prior to demodulation. Σ BPF demodulator noise w(t) Modulated signal S(t) x(t) Output signal

15. Cont… The bandwidth of a BPF is kept just wide enough to pass the modulated signal S(t) without distortion. We denote No/2 as the PSD of the noise w(t) for both +ve and –ve frequencies. f fc - fc No/2 Sn (f) Ideal characteristics of BPF noise Bt

16. Cont… No is the average noise power per unit bandwidth measured at the front end of the receiver. Bandwidth of BPF is equal to the transmission bandwidth of the modulated signal S(t) and it is denoted as ‘B t ’ or ‘w’. Midband frequency is equal to the corner frequency and it is denoted as ‘fc’. The carrier frequency fc >> B t and therefore we may consider the filtered noise n(t) as a narrowband noise and it is defined in the canonical form by n(t) = n I (t) Cos(2 π fct) – n Q (t) Sin(2 π fct) where n I (t) is in-phase noise component and n Q (t) is quadrature noise component

17. Cont… The filtered signal x(t) available for demodulation is given by x(t) = S(t) + n(t) The average noise power = (Avg noise power/unit BW) x BW = 2 x No/2 x B t = NoB t (or) Now Input signal to noise ratio Output signal to noise ratio (S/N) i = Avg power of the modulated sig S(t) Avg power of filtered noise n(t) (S/N)o = Avg power of filtered noise n(t) Avg power of the demodulated msg sig

18. Cont… Figure of merit is the ratio of signal to noise at output to the signal to noise ratio at input i.e. FOM = Noise figure : noise figure = Higher the value of the figure of merit, better the performance of the receiver. The value of the figure of merit also depends upon the type of modulation used. (S/N)o (S/N)i (S/N) i (S/N)o

19. Noise in dsb-sc receiver Σ BPF Low pass filter Product modulator Local oscillator noise w(t) x(t) y(t) v(t) DSB - SC s(t) Coherent detector Cos (2 π fct ) Block diagram of DSB-SC receiver model using coherent detection + +

20. Noise in ssb-sc receiver Σ BPF Low pass filter Product modulator Local oscillator noise w(t) x(t) y(t) v(t) SSB - SC s(t) Coherent detector Cos (2 π fct ) Block diagram of SSB-SC receiver model using coherent detection + +

21. Noise in AM receiver using envelope detector In the case of AM signal both sidebands and the carrier is transmitted. S(t) = Ac [1 + Ka m(t)] Cos(2 π fct) The average power of AM signal is calculated as follows S(t) = Ac Cos(2 π fct) + Ac Ka m(t) Cos(2 π fct) Σ BPF Envelope detector x(t) w(t) y(t) S(t) AM signal o/p signal noise signal Model of AM Receiver + +

22. Noise in FM receiver BPF limiter discriminator Baseband LPF Σ w(t) y(t) x(t) v(t) S(t) FM signal + +

23. SNR at output The output of the BPF is distorted FM signal. It is passed through a limiter which is a type of clipper circuit. It clips the undesired amplitude levels and produces a clipped FM wave. The output of a limiter is passed through a discriminator which performs two operations as a differentiator and then as a envelope detector. Finally the output of the discriminator is passed through a LPF to recover the msg signal. we have S(t) = Ac Cos(2 π fct + 2 π Kf ∫ m(t) dt )

24. Pre-emphasis & de-emphasis Capacitor, Resistor  HPF  differentiator Resistor, Capacitor  LPF  integrator Pre-emphasis and de-emphasis are used to improve fidelity of FM transmission of audio signals Fidelity : fidelity is defined as the ability of the receiver to reproduce all audio frequencies at the output. p re –emphasis HPF FM modulator FM demodulator De- emphasis LPF m(t) Tx R x o/p

25. PSD of audio signal PSD of noise f f f₁ f₁ s(f) S/N ↑ S/N ↓ up to freq f₁ : S/N >>> 1, so the low frequency component can be reproduced comfortably above f₁ : S/N <<< 1, so the high frequency component cannot be reproduced comfortably

26. Cont… In FM noise has a greater effect on the higher modulating frequencies. This effect can be reduced by increasing the value of modulation index for higher modulation frequencies. This can be done by increasing the deviation by increasing the amplitude of modulating signal at higher modulating frequencies. Thus if we boost the amplitude of higher frequency modulating signals artificially then it will be possible to improve the noise immunity at higher modulating frequencies. The artificial boosting of higher modulating frequencies is known as pre-emphasis.

27. pre – emphasis ckt f₁ f₁ f₁ H(f)

28. Pre-emphasis is done at the transmitter before frequency modulation.

29. de -emphasis It is the process of decreasing the strength of high frequency component of message signal to get back the original transmitted message signal. de – emphasis ckt f₁ f₁ H(f)

30. Cont… De – emphasis is performed at the receiver after demodulation.

31. CAPTURE EFFECT In the frequency modulation, the signal can be affected by another frequency modulated signal whose frequency content is close to the carrier frequency of the desired FM wave. The receiver may lock such an interference signal & suppress the desired FM signal when interference signal is stronger than the desired signal. When strength of the desired signal and the interference signal are nearly equal, the receiver fluctuates back and forth between them, i.e. receiver locks interference signal for sometime and desired signal for sometime and this goes on randomly. This phenomenon is known as capture effect.

32. THRESHOLD EFFECT IN FM Let us consider the concept of FOM in both DSB and SSB modulation techniques. In these both techniques FOM = 1  (s/n)o/(s/n) i = 1 ⇒ (s/n)o = (s/n)I Converting the above equation in to DB scale by applying log on both sides ⇒ 10 log (s/n)o = 10 log (s/n)I The above equation gives the linear relation between the SNR at output and input.

33. Questions to prepare from noise Define figure of merit and noise figure? Define pre-emphasis and de-emphasis. Explain threshold effect in FM. Classify various sources of noise. Write a short note on thermal noise, shot noise and solar noise. Explain the effect of noise in AM receiver using envelope detector. Explain noise performance in DSB-SC receiver. Explain the effect of noise in SSB receiver. Define capture effect. Explain the effect of noise in FM receiver.

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