Modulation — Page 48, Lesson 47

We’ve just covered phase and frequency modulation, so now I want to walk you through the last two modulation types in this section, and then the system that ties all of them together for identification purposes. First, phase modulation. In phase modulation, the phase of the carrier wave is modified by the input signal. There are two cases. The first is where the input is an analogue signal — here, the phase of the carrier wave is modified by the amplitude of that signal. The second case is with a digital signal, and this is known as phase shift keying. In phase shift keying, the phase change reflects a 0 or a 1. For example, a 0° phase shift indicates a zero, and a 180° phase shift represents a 1. Note that this is the simplest case, because multiple data can be represented by using many degrees of phase shift. There are two cases used in navigation systems: MLS and GPS. GPS uses binary phase shift keying, and MLS uses differential phase shift keying. So remember that distinction — GPS is binary, MLS is differential. Now, pulse modulation. Pulse modulation is used extensively in radar systems and for data exchange in communications systems. An intermittent carrier wave is formed by the generation and transmission of a sequence of short period pulses. So instead of a continuous wave, you get a series of brief pulses. Now, the big topic here: emission designators. In order to easily identify the characteristics and information provided by electronic signals, a list of designators has been devised. They comprise three alphanumerics. The first letter defines the nature of the modulation. The second digit defines the nature of the signal used for the modulation. The third letter defines the type of information carried. Let me walk you through the table. The first symbol — the type of modulation of the main carrier. N is emissions of an unmodulated carrier. A is amplitude modulation, double sideband. H is amplitude modulation, single sideband, full carrier. J is amplitude modulation, single sideband, suppressed carrier. F is frequency modulation. G is phase modulation. P is a sequence of unmodulated pulses. K is a sequence of pulses modulated in amplitude. And X is cases not otherwise covered. The second symbol — the nature of signals modulating the main carrier. 0 is no modulating signal. 1 is a single channel containing quantized or digital information without the use of a modulating sub-carrier, excluding time division multiplex. 2 is a single channel containing quantized or digital information with the use of a modulating sub-carrier, excluding time division multiplex. 3 is a single channel containing analogue information. 7 is two or more channels containing quantized or digital information. 8 is two or more channels containing analogue information. 9 is a composite system with one or more channels containing quantized or digital information, together with one or more channels containing analogue information. And X is cases not otherwise covered. The third symbol — the type of information transmitted. N is no information transmitted. A is telegraphy for aural reception. B is telegraphy for automatic reception. C is facsimile. D is data transmission, telemetry, telecommand. E is telephony, including sound broadcasting. F is television (video). W is combinations of the above. And X is cases not otherwise covered. Now let me give you a concrete example. VHF radio telephony communications have the designation A3E. Reference to the table gives the following breakdown. A — amplitude modulation, double sideband. 3 — single channel containing analogue information. And E — telephony, including sound broadcasting. So A3E tells you everything about that signal: how the carrier is modulated, what kind of signal is doing the modulating, and what information is being carried.