Modulation — Page 48, Lesson 47

Let’s pick up with phase modulation, because it’s the natural bridge from what we’ve already covered on amplitude and frequency modulation. In phase modulation, the phase of the carrier wave is modified by the input signal. Now, there are two cases here, and they depend on the nature of the input. The first case is an analogue signal — here, the phase of the carrier wave is modified by the amplitude of that signal. So the strength of the analogue input directly controls how much the carrier’s phase shifts. The second case is a digital signal, and this is known as phase shift keying. In that case, the phase change reflects a binary 0 or a 1. For example, a 0° phase shift indicates a zero, and a 180° phase shift represents a one. Now, note that this is the simplest case — because multiple data values can be represented by using many degrees of phase shift, not just two. So you can encode more than just one bit per phase change. Now, there are two cases used in navigation systems specifically: MLS and GPS. GPS uses binary phase shift keying, and MLS uses differential phase shift keying. So GPS is the simpler binary scheme — just the 0° and 180° shifts we talked about. MLS uses differential phase shift keying, which is a more advanced variant. I want you to hold onto those two names — GPS with binary, MLS with differential — because they’re the two navigation applications you’ll be tested on. Let’s move to pulse modulation. This is used extensively in radar systems and for data exchange in communications systems. The key idea here is that 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’re sending a series of brief bursts — that’s what makes it “pulse” modulation. The carrier is on for a short period, then off, then on again, in a repeating sequence. Now I want to introduce emission designators, because this is how we identify any electronic signal’s characteristics and the information it carries. A list of designators has been devised for this purpose. Each designator comprises three alphanumerics — that’s three characters, a mix of letters and digits. The first letter defines the nature of the modulation. The second digit defines the nature of the signal used for the modulation. And the third letter defines the type of information carried. So you read it left to right: modulation type, then the modulating signal type, then the information type. Let me walk you through the table, because you need to know the symbols. For the first symbol — the type of modulation of the main carrier — we have N for 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. Now the second symbol — the nature of signals modulating the main carrier. 0 means no modulating signal. 1 means a single channel containing quantized or digital information without the use of a modulating sub-carrier, excluding time division multiplex. 2 means a single channel containing quantized or digital information with the use of a modulating sub-carrier, excluding time division multiplex. 3 means a single channel containing analogue information. 7 means two or more channels containing quantized or digital information. 8 means two or more channels containing analogue information. 9 means a composite system — 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. Then the third symbol — the type of information transmitted. N means 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. And W is combinations of the above. Let me give you a concrete example to tie this together. VHF radio telephony communications have the designation A3E. Let’s break that down using the table. The first character, A, means amplitude modulation, double sideband. The second character, 3, means a single channel containing analogue information. And the third character, E, means telephony, including sound broadcasting. So A3E tells you: it’s amplitude-modulated, double sideband, carrying a single analogue channel, and the information is telephony — voice. That’s exactly what VHF radio telephony is. So the whole point of the designator system is that with just three characters, you can instantly identify the modulation type, the nature of the modulating signal, and the type of information being carried — without having to inspect the signal itself. That’s the professional shorthand you’ll use throughout your career.