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