Amplitude Modulation Chapter 2. Modulation process

Size: px

Start display at page:

Download "Amplitude Modulation Chapter 2. Modulation process"

Jesse Patterson
5 years ago
Views:

Question 1 Modulation process Modulation is the process of translation the baseband message signal to bandpass (modulated carrier) signal at frequencies that are very high compared to the baseband

1 Question 1 Modulation process Modulation is the process of translation the baseband message signal to bandpass (modulated carrier) signal at frequencies that are very high compared to the baseband frequencies. Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to information in a message signal. Frequency of the carrier signal is much greater than the highest frequency of input message signal

AM modulation AM modulation is used as a generic term for amplitude modulation, but it includes a number of types, the types can be summarized as follows.

3 AM modulation AM modulation is used as a generic term for amplitude modulation, but it includes a number of types, the types can be summarized as follows. Double sideband modulation: DSB Single sideband modulation: SSB Double sideband suppressed carrier: DSB-SC Single sideband suppressed carrier: SSB-SC WC (With Carrier) means that the modulated wave includes a carrier wave component, and SC (Suppressed Carrier) means that there is no carrier wave component.

4 Question 2 The theory of DSB-SC/SSB modulation Standard AM contains a sinusoidal component at the carrier frequency which does not convey any message information. It is included to create a positive envelope which allows demodulation by a simple inexpensive envelope detector. From an information theory point of view, the power in the carrier component is wasted. Double sideband suppressed carrier modulation Double-sideband suppressed-carrier transmission (DSB-SC) is transmission in which frequencies produced by amplitude modulation (AM) are symmetrically spaced above and below the carrier frequency and the carrier level is reduced to the lowest practical level, ideally being completely suppressed. In the DSB-SC modulation, unlike in AM, the wave carrier is not transmitted; thus, much of the power is distributed between the sidebands, which imply an increase of the cover in DSB-SC, compared to AM, for the same power used. DSB-SC transmission is a special case of double-sideband reduced carrier transmission. It is used for radio data systems.

5 As the name suggests, there s no wave carrier in the modulated wave. With DSB-SC, the amplitude of carrier wave Ac is shifted proportionally to the modulating signal m(t). When the modulating signal m(t) is a single sine wave, it s as follows. If the initial phase ΦC of the carrier wave is 0, and K dsb-sc *Am is modulation factor m, the result is as follows. The The first term on the right-hand side is proportional to the desired message. The second term has spectral components cantered around 2fc and 2fc

6 For this reason, DSB-SC is considered to have good electrical efficiency. However, synchronous detection is necessary on the receiving end, which involves a system with advanced technology and the associated costs. Demodulation- DSB-SC Demodulation is done by multiplying the DSB-SC signal with the carrier signal just like the modulation process. This resultant signal passes through a low pass filter; the higher frequency component is removed, leaving just the original message. DSB-SC can be demodulated by a simple envelope detector,

7 Question 3 Single sideband modulation (SSB) Single sideband, SSB uses only one sideband for a given audio path to provide the final signal. Single sideband modulation, SSB, provides a considerably more efficient form of communication when compared to ordinary amplitude modulation. It is far more efficient in terms of the radio spectrum used, and also the power used to transmit the signal. In view of its advantages single sideband modulation has been widely used for many years, providing effective communications, as well as forms being used for some analogue television signals, and some other applications. Single sideband modulation basics Two modulating (audio) sidebands are located on either side of the carrier signal--one just above the other just below. As a result, the sideband located just above the carrier frequency is called the upper sideband and that which is located just below the carrier frequency is called the lower sideband. Single sideband modulation can be viewed as an amplitude modulation signal with elements removed or reduced. In order to see how single sideband is created, it is necessary to use an amplitude modulated signal as the starting point.

An amplitude modulated carrier showing sidebands either side of the carrier From this it can be seen that the signal has two sidebands, each the mirror of the other, and the carrier.

of the other. A single sideband signal therefore consists of a single sideband, and often no carrier, although the various variants of single sideband are detailed below.

8 An amplitude modulated carrier showing sidebands either side of the carrier From this it can be seen that the signal has two sidebands, each the mirror of the other, and the carrier. To improve the efficient of the signal, both in terms of the power and spectrum usage, it is possible to remove the carrier, or at least reduce it, and remove one sideband - one is the mirror image of the other. A single sideband signal therefore consists of a single sideband, and often no carrier, although the various variants of single sideband are detailed below. Single sideband modulation showing upper and lower sideband signals Single sideband transmissions can consist of either the lower sideband (LSB) or the upper sideband (USB). There is no advantage between using either the upper or lower sideband. The main criterion is to use the same sideband as used by other users for the given frequency band and application. The upper sideband is more commonly used for professional applications. SSB's High Efficiency

9 Suppose you have a typical 5-kW broadcast transmitter. You will only be able to impress 2.5 kw of audio power on that signal. This means that each of the two sidebands will have only 1.25 kw of power. But in highly effective communications using single sideband, a single sideband signal removes the carrier and one sideband and concentrates all of its energy in one sideband. Thus, a 1-kW SSB signal will "talk" as far as a 4-kW conventional AM or FM transmitter. It is one reason why long distances can be covered effectively with SSB. Single sideband's benefit is not only evident on transmission. The reverse happens on receive. When you work out the math, the efficiency with an SSB signal is 16 times greater than with a conventional AM signal. Question 4 What is the carrier frequency in an AM wave when its highest frequency component is 850Hz and the bandwidth of the signal is 50Hz? Explanation: Upper frequency = 850Hz Bandwidth = 50Hz Therefore lower Frequency = = 800 Hz Carrier Frequency = ( )/2 = 825 Hz

AM Limitations. Amplitude Modulation II. DSB-SC Modulation. AM Modifications

AM Limitations. Amplitude Modulation II. DSB-SC Modulation. AM Modifications Lecture 6: Amplitude Modulation II EE 3770: Communication Systems AM Limitations AM Limitations DSB-SC Modulation SSB Modulation VSB Modulation Lecture 6 Amplitude Modulation II Amplitude modulation is