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Unit – 5Communication Systems
Lecture_5.3
Types of modulation
Amplitude Modulation (AM)
• Amplitude of high frequency carrier signal is varied in accordance
with instantaneous value of modulating signal.
• Only amplitude of carrier wave changes.
• Frequency remains constant.
Derivation of expression of Amplitude modulated signal.
Amplitude Modulation (AM)
Angle Modulation
• Instantaneous angle of the carrier signal is varied in accordance with
the instantaneous value of modulating signal.
• Amplitude remains constant.
• Frequency and Phase angle changes.
Frequency Modulation (FM)
• Instantaneous frequency of the carrier signal is varied linearly with
variations of message signal.
• Amplitude remains constant.
Derivation of expression of Frequency modulated signal.
Frequency Modulation (FM)
Phase Modulation (PM)
• Phase of the carrier signal is varied with instantaneous value of
message signal.
• Amplitude remains constant.
• Phase angle affects frequency. So, phase and frequency modulation
are interrelated.
Derivation of expression of Phase modulated signal.
Types of Analog modulation
Advantages of FM over AM
• Amplitude of FM wave is independent of depth of modulation (In AM, it is
dependent on depth of modulation).
• In FM, Transmitted power remains same. Bandwidth increases for increase in
modulation index (In AM, transmitted power increases with increase in
modulation index).
• FM is immune to noise than AM (SNR is high).
• Noise is lesser for increased frequency deviation.
• Adjacent channel interference is less in FM than AM (as there are guard bands
between FM stations).
• FM operates at upper VHF and UHF.
Disadvantages
• Wider channel 7 to 15 times larger than AM.
• FM transmitter and receiver equipment are complex and expensive.
• Line of sight area of reception is smaller than AM.
Comparison of FM and PM
• 𝑚𝑝 ∝ 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑜𝑛𝑙𝑦 𝑃𝑀
• 𝑚𝑓 ∝ 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑜𝑛𝑙𝑦; ∝1
𝑚𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑛𝑔 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦𝐹𝑀
• PM and FM are indistinguishable for single modulating frequency.
• PM is more immune than FM to noise.
Pulse Modulation
• More number of signals can be transmitted through single channel simultaneously.
• Better SNR at transmitter end and high noise immunity.
• Carrier train of rectangular pulses.
• Message sampled at regular intervals. Information at sampling time is transmitted.
• For proper recovery of signal, sampling rate should be greater.
Pulse Modulation
𝑫𝒖𝒕𝒚 𝑪𝒚𝒍𝒄𝒆 =𝑻𝑶𝑵
𝑻𝑶𝑵 + 𝑻𝑶𝑭𝑭=𝑻𝑶𝑵𝑻
=𝒕
𝑻
Sampling Theorem
• “For reception and reconstruction of signals with minimal distortion, the
frequency of sampling should be greater than twice the maximum frequency
component in the signal”.
𝑓𝑠 > 2𝑓𝑚𝑎𝑥
• Ex. For audio frequency telephone channel of range 300 Hz to 3400 Hz , sampling
rate of 8000 samples/second is used world wide.
Pulse Analog Modulation
PAM
• Amplitude of each pulse of unmodulated pulse train is varied with sample values of message signal.
PTM
• Amplitude of carrier pulse train is constant.
• Time parameter is varied and is proportional to sampled signal amplitude.• PWM
• PPM
Pulse Analog Modulation
PWM
• Amplitude and starting time are fixed.
• Width of the pulse is proportional to amplitude of message sample.• PDM – Pulse duration.• PLM – Pulse length.
• Location of leading edge is fixed.
• Position of trailing edge is modulated.
PPM
• Amplitude and width of the pulse are constant.
• Position of the pulse is varied proportional to amplitude of message sample.
Pulse Modulation
Pulse digital modulation - PCM
• Analog message signal is transmitted after converting it into digital form.
• Operations involved are sampling, quantizing and encoding.
Low Pass Filter
Sampler Quantizer Encoder
Continuous time message signal
PCM waves
PCM Transmission
Pulse digital modulation - PCM
• SamplingSampled with train of rectangular pulses. 𝑓𝑠 > 2𝑓𝑚𝑎𝑥.
• QuantizingInstantaneous sample value is rounded off to nearest standard (or quantum) level. Ex. 4.7 V to 5 V.
• EncodingQuantized value to binary word Coding format.
Ex. 7 0111 using 8421 code.
Pulse digital modulation - PCM
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