Modes and Modulation


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Modulation is the process by which voice, music, and other "intelligence" is added to the radio waves produced by a transmitter. The different methods of modulating a radio signal are called modes. An unmodulated radio signal is known as a carrier. When you hear "dead air" between songs or announcements on a radio station, you're "hearing" the carrier. While a carrier contains no intelligence, you can tell it is being transmitted because of the way it quiets the background noise on your radio.

The different modes of modulation have their advantages and disadvantages. Here is a summary:

Continuous Wave (CW)

CW is the simplest form of modulation. The output of the transmitter is switched on and off, typically to form the characters of the Morse code.

CW transmitters are simple and inexpensive, and the transmitted CW signal doesn't occupy much frequency space (usually less than 500 Hz). However, the CW signals will be difficult to hear on a normal receiver; you'll just hear the faint quieting of the background noise as the CW signals are transmitted. To overcome this problem, shortwave and ham radio receivers include a beat frequency oscillator (BFO) circuit. The BFO circuit produces an internally-generated second carrier that "beats" against the received CW signal, producing a tone that turns on and off in step with the received CW signal. This is how Morse code signals are received on shortwave.

Amplitude Modulation (AM)

In amplitude modulation, the strength (amplitude) of the carrier from a transmitter is varied according to how a modulating signal varies.

When you speak into the microphone of an AM transmitter, the microphone converts your voice into a varying voltage. This voltage is amplified and then used to vary the strength of the transmitter's output. Amplitude modulation adds power to the carrier, with the amount added depending on the strength of the modulating voltage. Amplitude modulation results in three separate frequencies being transmitted: the original carrier frequency, a lower sideband (LSB) below the carrier frequency, and an upper sideband (USB) above the carrier frequency. The sidebands are "mirror images" of each other and contain the same intelligence. When an AM signal is received, these frequencies are combined to produce the sounds you hear.

Each sideband occupies as much frequency space as the highest audio frequency being transmitted. If the highest audio frequency being transmitted is 5 kHz, then the total frequency space occupied by an AM signal will be 10 kHz (the carrier occupies negligible frequency space).

AM has the advantages of being easy to produce in a transmitter and AM receivers are simple in design. Its main disadvantage is its inefficiency. About two-thirds of an AM signal's power is concentrated in the carrier, which contains no intelligence. One-third of the power is in the sidebands, which contain the signal's intelligence. Since the sidebands contain the same intelligence, however, one is essentially "wasted." Of the total power output of an AM transmitter, only about one-sixth is actually productive, useful output!

Other disadvantages of AM include the relatively wide amount of frequency space an AM signal occupies and its susceptibility to static and other forms of electrical noise. Despite this, AM is simple to tune on ordinary receivers, and that is why it is used for almost all shortwave broadcasting.

Single Sideband (SSB)

Since so much power is wasted in AM, radio engineers devised a method to transmit just one sideband and put all of the transmitter's power into sending useful intelligence. This method is known as single sideband (SSB). In SSB transmitters, the carrier and one sideband are removed before the signal is amplified. Either the upper sideband (USB) or lower sideband (LSB) of the original AM signal can be transmitted.

SSB is a much more efficient mode than AM since all of the transmitter's power goes into transmitting useful intelligence. A SSB signal also occupies only about half the frequency space of a comparable AM signal. However, SSB transmitters and receivers are far more complicated than those for AM. In fact, a SSB signal cannot be received intelligibly on an AM receiver; the SSB signal will have a badly distorted "Donald Duck" sound. This is because the carrier of an AM signal does play a major role in demodulating (that is, recovering the transmitted audio) the sidebands of an AM signal. To successfully demodulate a SSB signal, you need a "substitute carrier."

A substitute carrier can be supplied by the beat frequency oscillator (BFO) circuit used when receiving CW signals. However, this means that a SSB signal must be carefully tuned to precise "beat" it against the replacement carrier from the BFO. For best performance, a SSB receiver needs more precise tuning and stability than an AM receiver, and it must be tuned more carefully than an AM receiver. Even when precisely tuned, the audio quality of a SSB signal is less than that of an AM signal.

SSB is used mainly by ham radio operators, military services, maritime and aeronautical radio services, and other situations where skilled operators and quality receiving equipment are common. There have been a few experiments in using SSB for shortwave broadcasting, but AM remains the preferred mode for broadcasting because of its simplicity.

Frequency Modulation (FM)

In CW, AM, and SSB, the carrier of the signal will not change in a normally operating transmitter. However, it is possible to modulate a signal by changing its frequency in accordance with a modulating signal. This is the idea behind frequency modulation (FM).

The unmodulated frequency of a FM signal is called its center frequency. When a modulating signal is applied, the FM transmitter's frequency will swing above and below the center frequency according to the modulating signal. The amount of "swing" in the transmitter's frequency in any direction above or below the center frequency is called its deviation. The total frequency space occupied by a FM signal is twice its deviation.

As you might suspect, FM signals occupy a great deal of frequency space. The deviation of a FM broadcast station is 75 kHz, for a total frequency space of 150 kHz. Most other users of FM (police and fire departments, business radio services, etc.) use a deviation of 5 kHz, for a total frequency space occupied of 10 kHz. For these reasons, FM is mainly used on frequency above 30 MHz, where adequate frequency space is available. This is why most scanner radios can only receive FM signals, since most signals found above 30 MHz are FM.

The big advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and a FM receiver will not respond to AM signals. FM receivers also exhibit a characteristic known as the capture effect. If two or more FM signals are on the same frequency, the FM receiver will respond to the strongest of the signals and ignore the rest. The audio quality of a FM signal increases as its deviation increases, which is why FM broadcast stations use such large deviation. The main disadvantage of FM is the amount of frequency space a signal requires.

Frequency-Shift Keying (FSK)

Like FM, frequency-shift keying (FSK) shifts the carrier frequency of the transmitter. Unlike FM, however, FSK shifts the frequency between just two separate, fixed points. The higher frequency is called the mark frequency while the lower of the two frequencies is called the space frequency. (By contrast, an FM signal can swing to any frequency within its deviation range.)

FSK was originally developed to send text via radioteleprinter devices, like those used by the TeleType Corporation. This mode, often referred to as radioteletype or RTTY, involves shifting of the carrier between the mark and space to generate characters in the Baudot code, which can be thought of as a more elaborate version of the Morse code. At the receiver, the Baudot signals were used to produce printed text on printers and, later, video screens.

So What Modes Are Being Used Now?

To answer this question, we must divide the usage of these modes in two parts those used by amateur radio operators, and those used by virtually everyone else. It should be noted at the outset that there are places where these 2 groups overlap but basically use the same mode, but in different applications.

The amateur radio (ham) community is an easy place to start. The modes used by hams can never, ever be encrypted in any way. This is both the law and common sense. After all if you are the only one who can transmit a digital signal, and have the key, then to whom are you going to talk?

However the data used by hams can be encoded. There are a plethora of digital modes available to the amateur community. As of this writing, a low power mode known as FT8 has a growing number of users. If you would like an introduction to the world of digital ham radio, please see the HF Digital Amateur Radio article on the RadioReference wiki.

Now we move into the non-ham world; basically everyone else. There are numerous modes that will never be copied by the average hobbyist; they use encryption schemes and other methods to make their signals unreadable except to their intended target. However, there are several modes that can be copied. Here are some popular ones:

ALE Stands for Automatic Link Establishment. Also known as MIL-STD-188-141A. By far this is the most used mode worldwide; even hams use it. This is a signaling mode that is used to test the propagation path between 2 or more stations. Sometimes brief messages are also sent

FAX While no longer used for press photos, weather charts are still sent by numerous stations around the world. In addition, the Japanese fishing fleets gets their news reports via FAX (in Japanese)

HFDL Stands for HF DataLink; this name is something of a misnomer as there are many modes that could be described in this way. The one that is most familiar is based on the ARINC 635 protocols, and is used by aircraft to relay specific data to a ground station. There are numerous ground stations worldwide, some of which are in countries that are otherwise hard to hear.

SITOR- B Also known as AMTOR Mode B. The very popular mode NAVTEX is based on these protocols. Often used for marine information by stations worldwide, on both longwave and HF.

Where Can I Go to Learn More?

Non-ham station usage of modes such as those above, and a great many more, are regularly reported in the Utility DXers Forum (UDXF). This is a dedicated mailing list that was set up after the closure of the well-known Worldwide Utility News group. They have their own website, and this page has many links, including audio samples of many modes.

You can read a great deal more about HF digital usage (and some LW usage, too) by reading the DXing Digital Utilities articles on the on the RadioReference wiki. Chapter 6 (References) has numerous links to source material, including web sites with additional audio samples. You will also find an extensive list of software that can decode many modes in Chapter 2 (How Do I Decode A Signal I Find on HF?).

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