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.