analog data to analog signal - data communication

Data Communication Subject module-3

1. UNIT 3: Signal Encoding Techniques. Topic for the class: Analog Data to Analog Signal Conversion Techniques –AM,FM,PM. 1 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications 10 June 2024

2. Analog to Analog Modulation Analog-to-analog conversion or analog modulation is the representation of analog information by an analog signal. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications

3. Why to modulate an analog signal, if it is already analog ? Modulation is needed if the medium is band pass in nature or if only a band pass channel is available. An example is radio. The government assigns a narrow bandwidth to each radio station. The analog signal produced by each station is a low-pass signal, all in the same range . To be able to listen to different stations, the low-pass signals need to be shifted, each to a different range. Higher frequency can give more efficient transmission. Permits frequency division multiplexing. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications

4. Analog to Analog Modulation Electronic devices produce messages like analog baseband signals in the form of audio, video or even messages can be in the form of digital bits from computer. To send these messages, some communication channels like wires, co-axial cable, even wireless radio waves, microwaves or infrared are used. Voice, Video, bit streams from computer are having lower frequency band and can travel few distance with wires but cannot be sent through wireless media . Voice signal has lower Bandwidth therefore it will not propagate through space and will be attenuated . To transmit voice signal a large size antenna is required as antenna length is proportional to half of wavelength. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications

5. Why Carrier wave? A carrier, is a high frequency radio wave, which can travel long distance without attenuation and as the frequency is high, smaller antenna is required. Selecting different carrier frequency for different transmitting stations can eliminate overlapping of frequency band. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications

6. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Types of analog-to-analog modulation Analog-to-analog conversion can be accomplished in three ways: AM, FM and PM.

7. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Amplitude Modulation In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. The frequency and phase of the carrier remain the same. Only the amplitude changes to follow variations in the information. Thus, if m(t) is the message signal and c(t)=Acosw c t is the carrier signal then mathematically AM signal F(t) is written as where cos 2 π f c t is the carrier and x(t) is the input signal (carrying data), n a known as the modulation index, the ratio of the amplitude of the input signal to the carrier. By trigonometric identity, this may be expanded to The resulting signal has a component at the original carrier frequency plus a pair of components each spaced fm hertz from the carrier. As long as n a <1 , the envelope is an exact reproduction of the original signal. If n a >1 , the envelope will cross the time axis and information is lost .

8. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Amplitude Modulation The total bandwidth required for AM can be determined from the bandwidth of the audio signal: B AM = 2B.

9. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Spectrum of an AM Signal The spectrum consists of the original carrier plus the spectrum of the input signal translated to fc . The portion of the spectrum for is the upper sideband , and the portion of the spectrum for is lower sideband . Both the upper and lower sidebands are replicas of the original spectrum M(f) , with the lower sideband being frequency reversed. As an example, consider a voice signal with a bandwidth that extends from 300 to 3000 Hz being modulated on a 60-kHz carrier. The resulting signal contains an upper sideband of 60.3 to 63 kHz , a lower sideband of 57 to 59.7 kHz , and the 60-kHz carrier.

10. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Power of an AM Signal The total transmitted power in P t is given by where Pc is the transmitted power in the carrier. n a has to be as large as possible so that most of the signal power is used to carry information. However, n a must remain below 1.

11. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications The AM signal s(t) contains unnecessary components , because each of the sidebands contains the complete spectrum of m(t). A popular variant of AM, known as single sideband ( SSB ), takes advantage of this fact by sending only one of the sidebands , eliminating the other sideband and the carrier. The principal advantages of this approach are as follows: • Only half the bandwidth is required , that is, where B T = B , where B is the bandwidth of the original signal. For DSBTC, B T = 2B Less power is required because no power is used to transmit the carrier for the other sideband .

12. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Another variant is double sideband suppressed carrier (DSBSC), which filters out the carrier frequency and sends both sidebands . This saves some power but uses as much bandwidth as DSBTC . The disadvantage of suppressing the carrier is that the carrier can be used for synchronization purposes. For example, suppose that the original analog signal is an ASK waveform encoding digital data. The receiver needs to know the starting point of each bit time to interpret the data correctly. A constant carrier provides a clocking mechanism by which to time the arrival of bits. A compromise approach is vestigial sideband (VSB), which uses one sideband and a reduced-power carrier .

13. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Advantages and disadvantages of AM AM Advantages: AM is the simplest type of modulation. Hardware design of both transmitter and receiver is very simple and less cost effective. AM Disadvantages: AM is very susceptible to noise . Applications: AM radio broadcasting.

14. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications The bandwidth of an audio signal (speech and music) is usually 5 kHz. Therefore, an AM radio station needs a bandwidth of 10kHz . In fact, the Federal Communications Commission (FCC) allows 10 kHz for each AM station. AM stations are allowed carrier frequencies anywhere between 530 and 1700 kHz (1.7 MHz). However, each station's carrier frequency must be separated from those on either side of it by at least 10 kHz (one AM bandwidth) to avoid interference. If one station uses a carrier frequency of 1100 kHz, the next station's carrier frequency cannot be lower than 1110 kHz. Standard Bandwidth Allocation for AM

15. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Angle Modulation Frequency modulation (FM) and phase modulation (PM) are special cases of angle modulation. The modulated signal is expressed as Angle modulation For phase modulation, the phase is proportional to the modulating signal: where n p is the phase modulation index . For frequency modulation, the derivative of the phase is proportional to the modulating signal: where n f is the frequency modulation index and φ ’(t) is the derivative of φ (t). The phase of s(t) at any instant is just 2 π f c t + φ (t). The instantaneous phase deviation from the carrier signal is φ (t). In PM , this instantaneous phase deviation is proportional to m(t) .

16. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Angle Modulation Because frequency can be defined as the rate of change of phase of a signal, the instantaneous frequency of s(t) is and the instantaneous frequency deviation from the carrier frequency is which in FM is φ ’(t), which in PM is proportional to m(t) . PM signal FM signal

17. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Frequency Modulation In FM transmission, the frequency of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. The peak amplitude and phase of the carrier signal remain constant , but as the amplitude of the information signal changes, the frequency of the carrier changes correspondingly.  

18. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Frequency Modulation The total bandwidth required for FM can be determined from the bandwidth of the audio signal: B FM = 2(1 + β)B . Where  is usually 4 . FM is normally implemented by using a voltage-controlled oscillator as with FSK . The frequency of the oscillator changes according to the input voltage which is the amplitude of the modulating signal .

19. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications FM Bandwidth FM Bandwidth The actual bandwidth is difficult to determine exactly, but it can be shown empirically that it is several times that of the analog signal or 2(1 + β)B where β is a factor depends on modulation technique with a common value of 4. Standard Bandwidth allocation for FM Radio:   The bandwidth of an audio signal (speech and music) broadcast in stereo is almost 15 kHz. The FCC allows 200 kHz (0.2 MHz) for each station. This mean β = 4 with some extra guard band. FM stations are allowed carrier frequencies anywhere between 88 and 108 MHz. Stations must be separated by at least 200 kHz to keep their bandwidths from overlapping. To create even more privacy , the FCC requires that in a given area, only alternate bandwidth allocations may be used . The others remain unused to prevent any possibility of two stations interfering with each other. Given 88 to 108 MHz as a range, there are 100 potential PM bandwidths in an area, of which 50 can operate at any one time.

20. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications FM Advantage Modulation and demodulation does not catch any channel noise . FM Disadvantage: Circuit needed for FM modulation and demodulation is bit complicated than AM Application: FM radio broad cast is an example Frequency Modulation

21. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications Phase modulation In PM transmission, the phase of the carrier signal is modulated to follow the changing voltage level (amplitude) of the modulating signal. The peak amplitude and frequency of the carrier signal remain constant , but as the amplitude of the information signal changes, the phase of the carrier changes correspondingly. It is proved mathematically that PM is the same as FM with one difference.

22. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications In FM, the instantaneous change in the carrier frequency is proportional to the amplitude of the modulating signal; In PM the instantaneous change in the carrier frequency is proportional to the derivative of the amplitude of the modulating signal. Thus if m(t) is the message signal and c(t)=Acosw c t then PM signal will be F(t)= Acos ( w c t+k p m (t)) Phase modulation

23. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications PM is normally implemented by using a voltage-controlled oscillator along with a derivative . The frequency of the oscillator changes according to the derivative of the input voltage which is the amplitude of the modulating signal . Phase modulation

24. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications The actual bandwidth is difficult to determine exactly, but it can be shown empirically that it is several times that of the analog signal. Although, the formula shows the same bandwidth for FM and PM , the value of β is lower in the case of PM (around 1 for narrowband and 3 for wideband). The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal: B PM = 2(1 + β)B. Where  = 2 most often. Phase Modulation Bandwidth

25. 10 June 2024 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications PM Advantage: Modulation and demodulation does not catch any channel noise . PM Disadvantage: Circuit needed for PM modulation and demodulation is bit complicated than AM and FM. Application: Satellite communication. Phase modulation

26. 26 Department of CSE, GIT Course Code and Course Title: 19ECS201 & Data Communications 10 June 2024 THANK YOU