ECE 241

Analog Communication
Ch4: Analog Modulations
and Demodulations
By
Ahmad AbdAllah Hassan, PhD.
Modern Digital and
Analog Communication
Systems
/in/aahassan1/ 5th edition
B.P. Lathi, Zhi Ding
Oxford University Press, 2019
Ch4: 4-1
Outline
▪ Introduction
▪ Baseband Versus Carrier
Communications
▪ DSB Amplitude Modulation

Ch4: 4-2
Course
Introdu
ction:
1-3

Introduction
• Modulation: Shifts message signals to a frequency band defined by the
physical channel.
• Advantages:
o Ease of RF transmission
o Frequency Division Multiplexing (FDM)
• Types:
o Analog Modulation: AM, FM, PM
o Digital Modulation: Used in modern systems (e.g., cellular, HDTV,
Bluetooth).
• Why Study Analog Modulations?
o Foundation of all communication systems.

Ch4: 4-4
Course
Introdu
ction:
1-5

Baseband Versus Carrier

Communications
❑ Baseband Communications:
o Direct transmission of message signals without modification (give
some typical examples).
o Examples: Audio (0-3.5 kHz), Video (0-4.3 MHz).
o Limitations:
• Low-frequency content makes wireless transmission difficult.
• Requires dedicated channels (e.g., twisted pairs, coaxial cables).
❑ Carrier Communications:
o Use modulation to shift the signal spectrum to higher frequencies.
o Enables:
• Frequency Division Multiplexing (FDM)
• Efficient power radiation via antennas.

Ch4: 4-6
Carrier Modulation
•Analog Modulation:
• Modulates one of the three parameters of a sinusoidal carrier:
• Amplitude: AM
• Frequency: FM
• Phase: PM
•AM: Linear modulation
•FM & PM: Nonlinear, collectively known as angle modulation.
•Note: PAM, PWM, PPM, PCM, and DM signals are baseband signals.
The analog message signal is modulating parameters of a digital pulse
train. These pulse modulated signals can still modulate a carrier in
order to shift their spectra
Ch4: 4-7
Amplitude Modulation
• Basic Concept:
• Modulate the amplitude of a carrier signal with the source
message signal.
• Equation:

• Frequency Domain:

• Double-Sideband Suppressed-Carrier (DSB-SC):

• No discrete carrier component in the modulated signal.
Ch4: 4-8
Course
Introdu
ction:
1-9

DSB Amplitude
Modulation
 DSB-SC Modulation
•Modulation Process:
• Multiply message signal m(t) by carrier cos(ωct).
• Spectrum shifts to ±fc
• Bandwidth: 2B Hz (where B is the bandwidth of m(t)).
•Upper Sideband (USB) & Lower Sideband (LSB):
• USB: Frequencies above fc
• LSB: Frequencies below fc
• double-sideband, suppressed-carrier (DSB-SC) modulation.
• The modulated signal does not contain a discrete component of the
carrier frequency fc
Ch4: 4-10
DSB-SC Modulation

Ch4: 4-11
 DSB-SC Demodulation
Demodulation Process:
•Multiply the modulated signal by a local carrier cos(ωct).
•Lowpass filter to recover the original message signal.
•Equation:

•Lowpass filter removes the 2ωc component, leaving 1/2m(t).

Ch4: 4-12
DSB-SC Demodulation

Ch4: 4-13
Example 4.1 For a baseband signal m(t) = cos ωmt = cos 2πfmt find the DSB-SC
signal and sketch its spectrum. Identify the upper and lower sidebands (USB
and LSB). Verify that the DSB-SC modulated signal can be demodulated by the
demodulator in Fig. 4.1e

Notes:
• use the scaling property of delta function to find M(ω). Try to proof it!
• You can analyze the demodulation process in the time domain directly.
Check ref. page 193.

Ch4: 4-14
Ch4: 4-15
 Amplitude Modulators
•Multiplier Modulators:
• Use analog multipliers to directly multiply m(t) and cos(ωct).
•Nonlinear Modulators:
• Use nonlinear devices (e.g., diodes, transistors) to achieve
modulation.
• Example: Balanced modulators.
•Switching Modulators:
• Replace multiplication with switching operations.
• Example: Diode bridge modulators.
Ch4: 4-16
Nonlinear Modulators

Notes:
•This circuit is called single
balanced w.r.t. carrier as the
o/p does contain the carrier
•If the circuit is balanced w.r.t.
both inputs, it is called double
balanced
Ch4: 4-17
 Switching Modulators
•Diode Bridge Modulator:
• Uses diodes to switch the message signal on and off at the carrier
frequency.
• Output:

• Bandpass filter extracts the desired m(t)cos(ωct) component.

•Ring Modulator:
• Double balanced modulator that eliminates both the carrier and
message signal from the output.
• Output:
Ch4: 4-18
Ch4: 4-19
Diode Bridge Modulators

Ch4: 4-20
Ring
Modulators

Ch4: 4-21
 Switching Demodulators
•Demodulation Process:
• Similar to modulation but uses a lowpass filter instead of a
bandpass filter.
• Input: DSB-SC signal m(t)cos(ωct).
• Output: Recovered message signal m(t).
•Synchronous Detection:
• Requires a local carrier synchronized with the incoming carrier.

• Note: Review example 4.2 page 198

Ch4: 4-22