1. Chapter 5 AM, FM, and Digital Modulated Systems
Amplitude Modulation (AM)
Double Sideband Suppressed carrier (DSSC)
Assymetric Sideband Signals
Single sideband signals (SSB)
Frequency Division Multiplexing (FDM)
Huseyin Bilgekul
Eeng360 Communication Systems I
Department of Electrical and Electronic Engineering
Eastern Mediterranean University

2. Bandpass Signaling Review
The modulated bandpass signal can be described by
Where
Modulation Mapping function: Convert m(t) →g(t) Ref : Table 4-1
The voltage spectrum of the bandpass signal is
The PSD of the bandpass signal is
Where

3. Amplitude Modulation
The Complex Envelope of an AM signal is given by
Ac indicates the power level of AM and m(t) is the Modulating Signal
Representation of an AM signal is given by
Ac[1+m(t)] In-phase component x(t)
If m(t) has a peak positive values of +1 and a peak negative value of -1
AM signal  100% modulated
Envelope detection can be used if % modulation is less than 100%.

4. Amplitude Modulation An Example of a message signal m(t)
Waveform for Amplitude modulation of the message signal m(t)

5. Carrier component together with the message
Amplitude Modulation B
An Example of message energy spectral density.
Carrier component together with the message 2B
Energy spectrum of the AM modulated message signal.

6. AM – Percentage Modulation
Definition: The percentage of positive modulation on an AM signal is
The percentage of negative modulation on an AM signal is
The percentage of overall modulation is
If m(t) has a peak positive values of +1 and a peak negative value of -1
AM signal  100% modulated

7. AM Signal Waveform % Positive modulation= 50%
% Negative modulation =50%
Overall Modulation = 50%
Amax = 1.5Ac
Amin = 0.5 Ac

8. AM – Percentage Modulation
Under modulated (<100%)
100% modulated
Envelope Detector
Can be used
Gives Distorted signal
Over Modulated (>100%)

9. Discrete Carrier Power
AM – Normalized Average Power
The normalized average power of the AM signal is
If the modulation contains no dc level, then
The normalized power of the AM signal is
Discrete Carrier Power
Sideband power

10. AM – Modulation Efficiency
Definition : The Modulation Efficiency is the percentage of the total power of the modulated signal that conveys information.
Only “Sideband Components” – Convey information
Modulation Efficiency:
Highest efficiency for a 100% AM signal : 50% - square wave modulation
Normalized Peak Envelope Power (PEP) of the AM signal:
Voltage Spectrum of the AM signal:
Unmodulated Carrier Spectral Component
Translated Message Signal

11. Example 5-1. Power of an AM signal
Suppose that a 5000-W AM transmitter is connected to a 50 ohm load;
Without
Modulation
Then the constant Ac is given by
If the transmitter is then 100% modulated by a 1000-Hz test tone , the total (carrier + sideband) average power will be
The peak voltage (100% modulation) is (2)(707) = 1414 V across the 50 ohm load.
The peak envelope power (PEP) is
The modulation efficiency would be 33% since < m2(t) >=1/2

12. Double Side Band Suppressed Carrier (DSBSC)
Carrier Power
Sideband power
Power in a AM signal is given by
DSBSC is obtained by eliminating carrier component
If m(t) is assumed to have a zero DC level, then
Spectrum 
Power 
Modulation Efficiency 
Disadvantages of DSBSC:
Less information about the carrier will be delivered to the receiver.
Needs a coherent carrier detector at receiver

13. No Extra Carrier component
DSBSC Modulation B
An Example of message energy spectral density.
No Extra Carrier component 2B
Energy spectrum of the DSBSC modulated message signal.

14. Carrier Recovery for DSBSC Demodulation
Coherent reference for product detection of DSBSC can not be obtained by the use of ordinary PLL because there are no spectral line components at fc.

15. Carrier Recovery for DSBSC Demodulation
A squaring loop can also be used to obtain coherent reference carrier for product detection of DSBSC. A frequency divider is needed to bring the double carrier frequency to fc.

16. Single Sideband (SSB) Modulation
An upper single sideband (USSB) signal has a zero-valued spectrum for
A lower single sideband (LSSB) signal has a zero-valued spectrum for
SSB-AM – popular method ~ BW is same as that of the modulating signal.
Note: Normally SSB refers to SSB-AM type of signal
USSB
LSSB

17. Single Sideband Signal
Theorem : A SSB signal has Complex Envelope and bandpass form as:
Upper sign (-)  USSB
Lower sign (+)  LSSB
– Hilbert transform of m(t) 
Where
and
Hilbert Transform corresponds to a -900 phase shift
H(f) f -j j

18. Single Sideband Signal
Proof: Fourier transform of the complex envelope
Upper sign  USSB
Lower sign  LSSB
Using
Recall from Chapter 4
Upper sign  USSB
If lower signs were used  LSSB signal would have been obtained

19. Single Sideband Signal

20. SSB - Power The normalized average power of the SSB signal
Hilbert transform does not change power.
SSB signal power is:
Power of the modulating signal
Power gain factor
The normalized peak envelope (PEP) power is:

21. Generation of SSB Advantages of SSB SSB signals have both AM and PM.
The complex envelope of SSB:
For the AM component,
For the PM component,
Advantages of SSB
Superior detected signal-to-noise ratio compared to that of AM
SSB has one-half the bandwidth of AM or DSB-SC signals

22. Generation of SSB SSB Can be generated using two techniques
Phasing method
Filter Method
This method is a special modulation type of IQ canonical form
of Generalized transmitters discussed in Chapter 4 ( Fig 4.28)

23. Generation of SSB Filter Method
The filtering method is a special case in which RF processing (with a
sideband filter) is used to form the equivalent g(t), instead of using
baseband processing to generate g(m) directly. The filter method is the
most popular method because excellent sideband suppression can be
obtained when a crystal oscillator is used for the sideband filter.
Crystal filters are relatively inexpensive when produced in quantity at
standard IF frequencies.

24. Weaver’s Method for Generating SSB.

25. Generation of VSB

26. Frequency Divison Multiplexing