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A Level     Modulation     AM     >Digital<     FM     PAM     PCM     PPM     PWM    

Modulation Digital


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  A Level    Modulation  0 of 22    Question 1125    Bandwidth AM Transmissions 
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Deadline not set3 of 22    Question 1128    Bandwidth AM Transmissions 
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Deadline not set6 of 22    Question 1131    Bandwidth AM Transmissions 
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Deadline not set7 of 22    Question 114    FM Deviation Calculation 
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Deadline not set8 of 22    Question 53    Frequency Bands and Modulation Types 
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Deadline not set9 of 22    Question 109    Modulation - AM Bandwidth Calculation 
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Deadline not set10 of 22    Question 113    Modulation - AM bandwidth Calculation 
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Deadline not set11 of 22    Question 105    Modulation - Analogue Types 
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Deadline not set12 of 22    Question 108    Modulation - Analogue Types 
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Deadline not set13 of 22    Question 106    Modulation - Terminology 
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Deadline not set14 of 22    Question 107    Modulation - Terminology 
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Deadline not set15 of 22    Question 110    Modulation Depth 
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Deadline not set16 of 22    Question 112    Modulation Depth 
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Deadline not set17 of 22    Question 111    Modulation Depth Calculation 
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Deadline not set18 of 22    Question 1259    Radio Systems 
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Deadline not set19 of 22    Question 1260    Radio Systems 
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Deadline not set20 of 22    Question 307    Sub-systems 
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Deadline not set21 of 22    Question 50    Transmitter Basics 
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Deadline not set22 of 22    Question 51    Transmitter Terminology 
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Definitions

Sampling Rate

The sampling frequency must be at least double the highest frequency being sampled. (Nyquist).

Resolution

The size (usually in Volts) of the smallest step in the reconstructed analogue signal.
You also need to know how many steps there are. This is a 2n problem. A 5 bit converter has 25 = 32 steps.

Pulse Code Modulation

Resolution depends on the number of different values that can be encoded. With 4 a bit sample any number between 0 and 15 can be stored. If voltages between 0 and 3.2V are to be encoded into 16 levels, each level is 0.2 Volts. This is the resolution measured in Volts. By adding one more bit to each sample, the resolution is doubled to 0.1 volts. Above the image shows a reconstructed three bit signal and the steps are obvious. The lack of quality in the reconstruction is quite serious. Eight bit systems are much better. Sixteen bit systems are used for CD audio and these are superb.

Bit Rate

Analogue Modulation

The carrier wave is transmitted even when there is no music, speech or data. It sounds like silence or silence with background noise (hiss and other interference) on a weaker transmission.

Digital Modulation

Digital data is transmitted across radio links, over cables and through fibre optics. AM and FM are not ideal for digital systems so other modulation methods are used.

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