Signals

Definition of Terms Modes – ways of conveying information. Modulation – the process of impressing or overlaying information to be conveyed onto the RF carrier wave. RF carrier – the radio signal that is sent from the transmitter to receiver. Bandwidth – the amount of spectrum required (occupied) to convey the information.

Voice Modulation AM – Amplitude Modulation SSB – Single Side Band FM – Frequency Modulation PM – Phase Modulation

AM Vary the amplitude of the RF carrier in step with the applied voice waveform.

SSB SSB – one sideband is filtered out, as well as the carrier (suppressed when no audio is present).

SSB Transmitter This allows for available power to be concentrated into the remaining sideband.

SSB Receiver The missing sideband is recreated in the receiver as a mirror image of the sideband sent. –(through a beat frequency oscillator (BFO) and another mixer)

FM/PM Frequency is varied in step with the voice waveform. Large amplitude – large frequency shift (deviation). Small Amplitude – small frequency deviation. Important point – voice volume depends on frequency deviation, not amplitude variation.

FM/PM The deviation setting between the TX and RX equipment must match for effective operation. TX narrow, RX wide – low volume. TX wide, RX narrow – splatter, distortion, clipping.

Data Transmission CW – Morse Code RTTY –Radioteletype AMTOR – Amateur Teleprinting Over Radio PACTOR PSK31 Packet

CW Data modulation concerns two states, on and off. Morse code is simplest data mode, you turn the RF carrier on and off to form dits and dahs. A BFO in the receiver adds some audio to the dits and dahs so that you hear a tone in the speaker.

Data Modulation Techniques FSK (frequency shift keying) AFSK (audio frequency shift keying) PSK (phase shift keying) All techniques depend on one state for on, the other state for off. Techniques used to “capture” the receiver circuitry to improve signal to noise ratio (and therefore accuracy). All ?SK modes are 100 percent duty cycle.

RTTY Based on a 5 bit code called BAUDOT. Used primarily on HF. Comes from the old mechanical teletype days though now mostly by computer. Uses FSK 170 Hz shift on HF. No error correction capabilities.

AMTOR Two modes of AMTOR –ARQ (Automatic Repeat reQuest) –FEC (Forward Error Correction)

AMTOR ARQ ARQ –TX and RX linked by handshaking. –Three letters sent at a time with error check. –Letters repeated until timeout or received correctly. –Almost 100%.

AMTOR FEC FEC –No handshaking. –Three letters sent, then the second 3 letters, then first 3 repeated and so-on. –RX compares 3 letter pairs and prints the best copy. –Not 100% but better than RTTY.

PACTOR Similar to AMTOR but more and variable group length to take advantage of changing band conditions. Code more computer like which allows for easier transmission of computer compatible data. Very good error correction.

PSK31 Similar to RTTY in that no handshake between RX/TX. Code is a 7 bit code, therefore more characters (upper and lower case etc.). Error correction allowed by transmitting operation (backspace and delete). Transmission technique requires very little bandwidth. Many, many contacts allowed simultaneously in the same space taken by one voice QSO.

Packet Very much like computer communication mode. HF and VHF. Full error correction. Forward and relay capabilities. Requires a smart modem (TNC) or specialized software that takes advantage of sound card technology.

Video SSTV (Slow Scan TV) – sending snap- shots. Generally an HF mode. Uses SSB for transmission. Gray (color) scale represented by amplitude of the wave, low – black, high – white. Picture built one line at a time.