1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite
Assignment # 2
AMPLITUDE MODULATION
Types of Amplitude Modulation
Power in Amplitude Modulation
Modulation Index)
Tagasa, Jerald A. June 29,2011
Communications 1 / BSECE 41A1 Score:
Eng'r. Grace Ramones
Instructor

2. AMPLITUDE MODULATION
Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied in accordance
with the information bearing signal.
The envelope, or boundary, of the amplitude modulated signal embeds the information bearing signal.
The total power of the transmitted signal varies with the modulating signal, whereas the carrier power remains
constant.
A nonlinear device is used to combine the carrier and the
modulating signal to generate an amplitude modulated
signal. The output of the nonlinear device consists of
discrete upper and lower sidebands.
The output of a nonlinear device does not vary in direct
proportion with the input.
Amplitude Modulation is abbreviated AM.

3. TYPES OF AMPLITUDE MODULATION
Double sideband (DSB), single side band (SSB), vestigial side band, is common modes used on all amateur
and commercial bands. All of these are correctly referred to as amplitude modulation. The reference to double
side band as AM and single side band as SSB on some amateur transceivers gives credence to the misconception
that SSB is not an amplitude modulated wave when in fact it is.
Double Sideband Full Carrier (DSB- LC). This type of Amplitude modulation is also known as 'Full AM' or
'Standard AM'. Here the frequency spectrum of the AM will have the carrier frequency, Upper sideband and the
Lower Sideband.
Double Sideband- Suppressed Carrier (DSB-SC).In this type of amplitude modulation, both the sidebands
namely Lower sideband and Upper sideband are present in the frequency spectrum but the carrier component is
suppressed, hence the name Double Sideband suppressed Carrier. The Carrier does not contain any information,
so it is suppressed during modulation to obtain a better Power Efficiency.
Single sideband- Suppressed Carrier (SSB-SC) . In this type of amplitude modulation, the carrier is
suppressed and it is either the Upper sideband (USB) or the Lower Sideband ( LSB) that gets transmitted. In DSC-SC
the basic information is transmitted twice, once in each sideband. This is not required and so SSB-SC has an upper
hand.
Single sideband Full Carrier. This could be used as compatible AM broadcasting system with DSB-FC
receivers.
Single Sideband - Reduced Carrier: Here an attenuated carrier is reinserted into the SSB signal, to
facilitate receiver tuning and demodulation. This method is steadily replaced by SSB-SC.
Independent Sideband Emission: Two independent sidebands, with a carrier that is most commonly
suppressed or attenuated is used here.
Vestigial Sideband: Here a vestige or trace of the unwanted sideband is transmitted, usually with the full
carrier.
Lincompex: This is an acronym that stands for 'linked compressor and expander'.
A3E double-sideband full-carrier - the basic AM modulation scheme
R3E single-sideband reduced-carrier
H3E single-sideband full-carrier
J3E single-sideband suppressed-carrier
B8E independent-sideband emission

4. POWER IN AMPLITUDE MODULATION
The utilisation of power by an amplitude modulated signal is very poor. When the carrier is modulated
sidebands appear at either side of the carrier in its frequency spectrum. Each sideband contains the information
about the audio modulation. To look at how the signal is made up and the relative powers take the simplified case
where the 1 kHz tone is modulating the carrier. In this case two signals will be found 1 kHz either side of the main
carrier.
When the carrier is fully modulated i.e. 100% the amplitude of the modulation is equal to half that of the
main carrier, i.e. the sum of the powers of the sidebands is equal to half that of the carrier. This means that each
sideband is just a quarter of the total power. In other words for a transmitter with a 100 watt carrier, the total
sideband power would be 50 watts and each individual sideband would be 25 watts.
The total power of the transmitted signal varies with the modulating signal, whereas the carrier power
remains constant.

5. MODULATION INDEX
Amplitude modulation requires a high frequency constant carrier and a low frequency modulation signal.
A sine wave carrier is of the form
A sine wave modulation signal is of the form
The high frequency carrier takes on the shape of the lower frequency modulation signal, forming what is called a
modulation envelope.
The modulation index is defined as the ratio of the modulation signal amplitude to the carrier amplitude.
where
The overall signal can be described by:
More commonly, the carrier amplitude is normalized to one and the am equation is written as:
In most literature this expression is simply written as:
If the modulation index is zero (mam = 0) the signal is simply a constant amplitude carrier.
If the modulation index is 1 (mam = 1), the resultant waveform has maximum or 100% amplitude
modulation.
Sidebands
Expanding the normalized AM equation:
we obtain:
where: sinωct represents the carrier
represents the lower sideband
represents the upper sideband
The sidebands are centered on the carrier frequency. They are the sum and difference frequencies of the carrier
and modulation signals. In the above example, they are just single frequencies, but normally the baseband
modulation signal is a range of frequencies and hence two bands are formed.
As a side point, note that multiplication in the time domain causes addition and subtraction in the frequency
domain.