|
This is the base diagram for a simple radio wave, compare AM & FM to this diagram on the left. |
|
AM stands for Amplitude Modulation, and is a type of transmission, that uses a carrier wave to carry your voice. Modulation is speech, Morse code, picture information, etc. When you talk through a radio microphone you are modulating. Refer to Diagram on left. |
| |
SSB or single sideband is another type of transmission related to AM, only it uses one sideband and not a carrier wave like AM because AM is actually DSB, which is double sideband. You can either use USB, which is upper sideband or LSB, which is lower sideband. SSB sounds different on the radio compared to AM, especially over long distance communications. |
|
FM stands for frequency modulation, and is another type of transmission, using a carrier but is slightly different. The frequency is modulated and the amplitude of the wave remains constant. |
H = high, V = very, L = Low, U = ultra, F = frequency, S = super, E = extremely, and M = medium.
In the 'HF' radio band, being widely recognised to the general population
as Shortwave band, the wavelengths are between 10 meters to 100 meters
in length, so they are still highly frequent, hence the term High
Frequency. At frequencies where we communicate using terrestial
microwave (satellites, and ground dish-to-dish) extremely short wavelength
frequencies are used. This signifies Super High Frequency
rate of wavelengths.
ASK transmits 1's and 0's by representing each one as a different amplitude at the same frequency. Therefore, the amplitude of a 1 is larger than the amplitude of the 0.
This time the amplitude does not change, but instead the frequency is altered. A 1 has a lower frequency than a 0. These frequencies are defined by the CCITT standard. The transmitter transmits a 0 at 1180Hz and a 1 at 980Hz. The receiver transmits a 0 at 1850Hz and a 1 at 1650Hz. (and the transmitter listens at these freqeinces)
In this case, the frequency and amplitude are both kept constant. However, the phase of the sinewave is shifted to signify logic 0 and logic 1. Phase Coherent PSK uses two fixed signals, and a logic 0 is represented by a 180 degree phase shift and a logic 0 by a 0 degree phase shift. This is a complex method to demodulate for the receiver because the phase shifts are from an absolute value. Another method called Differential PSK exists (DPSK), which is a phase shift relative to the previous logic bit transmitted. Binary 0 is therefore a 90 degree phase change from the previous logic bit and binary 1 is a 270 degree phase change from the previous logic bit. In this case, the receiver only needs to detect the phase change which occured from the previous bit rather than off an absolute value that it has to know.
To acheive higher speed data communication a combination of PSK and AM can be used, which produces Quadrature Amplitude Modulation (QAM). This makes use of 0, 90, 180, 270 degree phase shifts together with ASK. These are used to produce bit rates of 2400, 4800 & 9600 bps.
 |