Modulation — Page 48, Lesson 45

Let’s pick up where the power table left off, because that table is the whole reason single sideband exists. I want you to see the numbers first, because they explain everything. The table showed that the power in the audio frequency, the AF, is divided equally between the two sidebands. So if you have a carrier and two sidebands, the information in the AF is contained in both sidebands. And here’s the kicker: only one third of the total signal is actually carrying the information. Two thirds are wasted. Now, why is that? Because in a double sideband transmission, you’ve got redundancy. The information is in the upper sideband and the lower sideband — the same information, twice. And the original RF carrier wave? It served its purpose. Its only job was to get the audio information up into radio frequencies. Once it’s done that, it’s redundant too. So the idea behind single sideband, SSB, is simple: remove one of the sidebands and remove the carrier wave, because the remaining sideband contains all the information. That’s it. That’s SSB operation. Let me show you what that looks like in practice, because the figure gives you a concrete example. We’re working around 2182 kHz, which is the international distress frequency in the MF band. The audio frequency, the AF, is 3 kHz. So the upper sideband, USB, sits at 2182 kHz plus 3 kHz — that’s 2182.001 kHz, wait, let me re-read that. Actually the figure shows the upper sideband at 2182.001 kHz with 100 watts, and the lower sideband, LSB, at 2181.999 kHz with 25 watts. And the carrier at 2182 kHz itself has 150 watts. So you can see the power split: the carrier is the biggest chunk, and the two sidebands share the rest. Now, why does this matter for aviation? Because of sky wave propagation. When you use sky wave for long-range communication, the ionosphere refracts different frequencies by different amounts. If your bandwidth is too large, that differing refraction causes distortion. And the ionosphere is full of electrically charged particles that cause high levels of static interference on radio waves. SSB significantly reduces that interference. There’s also a spectrum problem. The MF and HF frequencies used for long-range communication are in great demand. Because SSB uses less bandwidth, it doubles the number of channels available compared to double sideband. And because you’re not transmitting the carrier and one sideband, you need less power. So the main advantages of SSB, and I want you to remember these three: first, double the number of channels available compared to double sideband. Second, a better signal-to-noise ratio — less interference. And third, less power required, which means lighter equipment. Now let’s move to a completely different way of modulating: Frequency Modulation, FM. In AM, we varied the amplitude of the carrier. In FM, the amplitude of the audio frequency modifies the frequency of the carrier wave. The carrier’s amplitude stays constant; its frequency is what changes. Here’s the key relationship. The change in the carrier frequency is dependent on the rise and fall of the amplitude of the modulating wave — the audio frequency. The greater the amplitude, the greater the frequency deviation. So amplitude of the audio controls how much the carrier frequency swings. And the frequency of the modulating wave determines the rate of change of frequency within the modulated carrier wave. So audio frequency controls how fast the carrier frequency swings back and forth. Now, FM has a bandwidth problem. When it’s used for sound broadcasting — music radio stations, for example — the bandwidth permitted by international agreements is 150 kHz. Compare that to 9 kHz allowed for AM. That’s a huge difference. In general, therefore, FM is unsuitable for use on frequencies below VHF. The bandwidth is just too wide for the lower bands. But for voice communications, you can reduce the bandwidth considerably while still maintaining the integrity of the information. That’s called Narrow Band FM, NBFM. Typically, NBFM systems have a bandwidth of 8 kHz. Now here’s the aviation problem: 8 kHz is greater than the 6 kHz permitted for Aeronautical Communications, and greater than the 3 kHz used in HF Communications. So NBFM communication systems are not yet used in aviation. The bandwidth is still too wide for our standards. So to tie it together: SSB saves power and spectrum by throwing away the redundant carrier and one sideband. FM trades bandwidth for noise immunity, but its bandwidth is too wide for aviation voice use — even in narrow band form. That’s the modulation picture.