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Understanding Radio Signals: Modulation, Demodulation, and Digital Communication

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emelda
19 diapositives · 328 Views · Oct 13, 2014

This overview explores the fundamental concepts of radio communications, including modulation and demodulation processes. It explains how information is embedded into carrier waves for transmission. Various modulation methods, such as AM, FM, SSB, and digital modes like PSK and FSK, are discussed along with their bandwidth requirements. The process of sending and receiving information across radio frequencies is detailed, emphasizing the importance of bandwidth related to the amount of information conveyed. Learn how digitization has transformed communication within radio technology.

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Understanding Radio Signals: Modulation, Demodulation, and Digital Communication
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  1. Radio Signals

  2. The purpose of radio communications is to transfer information from one point to another. The information to be sent is combined with a radio wave (the carrier wave). This process is called modulation. The carrier wave (with the embedded intelligence) is then transmitted into space by the transmitting equipment. Modulation Defined

  3. Carrier and Intelligence

  4. 0% Modulation (CW)

  5. 25% Modulation

  6. 50% Modulation

  7. 100% Modulation

  8. Over Modulation Causes Distortion

  9. Once the carrier wave is received, the carrier has done its job. The carrier and intelligence are then separated (demodulation) and the carrier is discarded. The intelligence is then processed and provided to the listener as audio, video or text. Demodulation Defined

  10. Modulated Carrier (blue) and Original Intelligence (red)

  11. Carrier Removed (blue) Compared to Original Intelligence (red) – Demodulated Signal

  12. Recovered Intelligence (blue) Filtering required to remove as much carrier as possible

  13. AM – amplitude modulation SSB – single sideband FM – frequency modulation CW – turn carrier on and off (Morse code) FSK – frequency shift keying PSK – phase shift keying Major Modulation Modes

  14. Sending intelligence via a radio carrier wave takes spectrum space – called bandwidth. As a general rule, the more intelligence to be sent, the more bandwidth is required. Morse code (CW) – minimum information and narrow bandwidth. Television (ATV) – large amount of information and wide bandwidth. Bandwidth

  15. CW - 0.1 to 0.3 kHz FSK - 0.5 to 3 kHz SSB - 2 to 3 kHz AM - 6 kHz FM - 5 to 15 kHz ATV - 6000 kHz (6 MHz) Approximate Bandwidths

  16. Sending text via computer (primarily). Morse code is a digital mode – usually sent and received manually but can be computer assisted. Requires a modem to convert text into bits and modulate the carrier in step with the bits. Bits have two states (binary) Either a 1 (high) or 0 (low) Digital Modes

  17. The sequence of 1s and 0s that represent a character to be sent make up the code. Numerous codes have been developed for specific applications: Baudot ASCII PSK31 And many others Binary Codes

  18. Generally the codes have one thing in common: the need to uniquely identify the two states of binary – on/off, 1 or 0. Accomplished by: Shifting the carrier frequency (FSK). Shifting the frequency of a modulating tone (AFSK). Shifting the phase of the carrier or audio (PSK). Two Unique States

  19. Radioteletype (RTTY). Uses Baudot code and FSK with 170 Hz shift between the two tone frequencies. TORs (Teletype over radio) – some error correction: PACTOR AMTOR PACKET – error correction and reliable transport. PSK31 – backspace error correction, low power, minimum bandwidth. Common Digital Modes