Radio Signals Modulation Defined The purpose of radio

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 FSK SSB AM FM ATV - 0. 1 to 0. 3 k. Hz 0. 5 to 3 k. Hz 2 to 3 k. Hz 6 k. Hz 5 to 15 k. Hz 6000 k. Hz (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 1 s and 0 s that represent a character to be sent make up the code. • Numerous codes have been developed for specific applications: – Baudot – ASCII – PSK 31 – 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. • PSK 31 – backspace error correction, low power, minimum bandwidth.