Chapter 5 AM, FM, and Digital Modulated Systems Amplitude Modulation (AM) Double Sideband Suppressed carrier (DSSC) Single sideband signals (SSB) Hüseyin Bilgekul & Erhan Ince Eeng360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University

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

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%.

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

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.

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

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

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

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

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

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

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

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.

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.

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.

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

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

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

Single Sideband Signal

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:

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

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)