radio signals modulation defined the purpose of radio communications is to transfer information from...
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Radio Signals
Modulation Defined
• 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.
Carrier and Intelligence
0% Modulation (CW)
25% Modulation
50% Modulation
100% Modulation
Over Modulation Causes Distortion
Demodulation Defined
• 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.
Modulated Carrier (blue) and Original Intelligence (red)
Carrier Removed (blue) Compared to Original Intelligence (red) – Demodulated Signal
Recovered Intelligence (blue) Filtering required to remove as much carrier as possible
Major Modulation Modes
• 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
Bandwidth
• 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.
Approximate Bandwidths
• 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)
Digital Modes
• 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)
Binary Codes
• 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
Two Unique States
• 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).
Common Digital Modes
• 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.
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