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- IX Basic Digital Principles: Modulation
Modulation
Modulation means to vary or change. In wireless we first take a signal, say a telephone conversation, and then impress it on a constant radio wave called a carrier. Once done the voice signal varies or modulates this radio wave. The two go together over the air. A voice frequency in the audible or audio range, what we can hear, thus modulates or varies a constant frequency in the radio range, which we can't hear. That's an important point. Modulation makes voice band and radio band frequencies work together. Different modulation techniques, such as A.M., F.M., P.C.M. and so on, represent different ways to shape or form electromagnetic radio waves.
There are many reasons to modulate a signal in a particular way. Amplitude modulation, like that still used by Citizens' Band radios, produces a simple, robust wave that doesn't use much spectrum or radio bandwidth. It's plagued by noise though and requires high transmitting power. Frequency modulation, such as analog cell phones use, provides better sound but it needs more bandwidth to achieve that quality and is technically more complex to produce. And then there are modulation types just for transmitting digital information. GSM and IS-136 use these schemes.
Amplitude Modulation
Amplitude modulation means a carrier wave is modulated in proportion to the strength of a signal. The carrier rises and falls instantaneously with each high and low of the conversation. Check out the diagram below. See how the voice current produces an immediate and equivalent change in the carrier.
Low frequency commercial broadcast stations in the "A.M band" use amplitude modulation. Most C.B. or citizens band radios use it too. It's a simple, robust method to form a radio wave but it suffers from static and high battery power requirements, reasons enough that few personal communications devices use it.
Frequency Modulation
Frequency modulation confuses many people but it shouldn't. FM is not limited to the FM band. It is not frequency dependent, that is, it can be used at high or low frequencies. That's because it is a modulation technique, a way to shape a radio wave, not a service by itself. The word frequency in FM relates, instead, to the rate at which this method varies a carrier wave, not to any particular radio frequency it is used on. This will become more clear as we go on.
An FM signal quality is apparent by listening to the FM band: low distortion, little static, good voice quality and immunity from electrical and atmospheric interference. It's why television audio and analog cellular use it. FM also exhibits a capture effect, whereby the receiver seizes on the strongest signal and rejects any others. That's unlike A.M, with signals fading in and out. What's more, F.M. needs far less power to transmit a signal the same distance than A.M.
See the difference in the waveform on the right in the diagram below? You don't have the modulated carrier varying in amplitude, as with A.M., but in the number of cycles or rate. Although perhaps not obvious at first, the right hand side does differ from the left hand side.
This diagram above is courtesy of Douglas-Young's brilliant article on modulation. We have an unvarying carrier wave as we do with A.M. See that? But in F.M. the carrier wave is engineered to deliver a uniform output signal. When we impress upon the carrier a audio signal, such as a 440 hertz dial tone, things begin to happen.
Frequency modulation varies the carrier at a rate of 440 cycles per second, matching the original signal. This differs dramatically from A.M., where a wildly swinging sine wave would be produced instead. In F.M. a quick change in audio frequency results in a quick rate change to the carrier. Despite this seemingly complicated operating method, F.M. circuitry after sixty years is now well established, cheap, simple to make, and easily miniaturized.
The July, 1999 Popular Electronics outlined a simple F.M. transmitter kit. It used only one transistor, eleven other parts, and took up no more than a square inch or two. F.M. is now everywhere, developed largely by one man.
Time Out for History!
- On January 31, 1954 a 64 year old man wrote a letter to his wife, dressed for work, and walked out of his 13th floor apartment window, plunging to his death. Colonel Edwin Armstrong, the father of modern radio, and the creator of the first F.M. system, had committed suicide. A brilliant but sensitive man, Armstrong allowed the U.S. military to use his patents royalty-free for the duration of World War II. Before that he played a crucial role in communications during the First World War. He believed, rightly so, that F.M. was a revolutionary operating system and that it should replace A.M. equipment for broadcasting. Tired and despondent after fighting one lawsuit after another against RCA and others, his personal fortune spent on promoting and defending F.M., Armstrong finally gave up and killed himself. Every modern radio has circuits Armstrong designed. And you thought modulation was boring. . .
Frequency shift keying, an F.M. variation
Conventional cellular makes much use of frequency shift keying modulation to send signaling and control messages. It's old technology, in fact, the earliest modems were built with this technique, but FSK works well for what it does. To explain its title, FSK means sending data by slightly shifting frequencies. Simple. Keying, by the way, simply means forming or creating a signal. When you "key up" a microphone you create a signal. You turn on
Frequency shift keying uses the existing carrier wave, say, 879.990 MHz. The data rides 8kHz above and below that frequency. It's just like the earliest modems. 0s and 1s. 0s go on one frequency and 1s go on another. They alternate back and forth in rapid succession. FSK gives you only two states to send information. There's a low limit, then, how much and how quickly you can send information. There is a more efficient way.
Phase Modulation
Three ways exists to modulate a signal: by amplitude, frequency or phase. And although there are dozens of modulation techniques, under the most confusing names possible, all of them will fit into one of these categories. We've looked at amplitude modulation, which changes the carrier wave by signal strength, and frequency modulation, which converts the originating signal into cycles. Now we look at phase modulation, which changes the angle of the carrier wave. Phase modulation is strictly for digital working and is closely related to F.M. Phase in fact enjoys the same capture effect as F.M. First, a note.
A digital signal means an ongoing stream of bits, 0s and 1s, on and off pulses of electrical energy. Like those signals running around the inside your computer. Well, how do we transmit that staccato beat of electrical pulses? Very good. We put it on a carrier wave.
A not to scale diagram of a digital signal
You might think you could send digital without a carrier wave, like the earliest wireless telegraphs but your results wouldn't be good. As Dwayne Rosenburgh, N3BJM, puts it, "Transmitting pure radio frequency energy, with no carrier, is like a spark gap transmitter. Very wideband, very inefficient, and with a limited data rate capacity." Ever had an AM radio on nearby by when you switched on a light dimmer? Blast! That's sort of how an old spark gap transmitter worked. Poorly. But new technology is bringing back an old idea.
Dwayne mentions that ultra wideband technology or UWB does now what old spark transmitters couldn't: transmit without a carrier wave. "UWB transmissions can use 'direct modulation' of the rf energy and transmit pulses without carrier modulation. Think of this as a spark gap transmitter that is controlled, with very low power, and spread across about 7 GHz of rf spectrum. Modern processing technology has been able to allow this type of communication using low rf power and efficient digital signal processing algorithms. Therefore, we can reduce the noise and inefficiencies associated with a spark gap transmitter and create UWB transmitters. Dwayne continues, "This ultra wideband technology exception aside, our radio technology is built on carrier waves. No matter how we transmit RF energy, there is always some type of 'carrier' involved."
Ever hear an A.M. radio station go silent for a minute or two? If they are off the air completely you'll hear static. But if they have simply lost audio for a while you'll hear a slight hum. That's the carrier wave.
For more on carrier waves: what they are, what they do, and why we need them, you may want to read my wireless history series from the start.
Let's get back to phase modulation. How does P.M. represent those on and offs?, those 0s and 1s? By playing with angles.
A continuous wave produced to transmit analog or digital information. The many phases or angles of a sine way give rise to different ways of sending information. More here under Digital principles
Quadrature phase shift keying
Let's discuss the awesomely titled quadrature phase shift keying or QPSK. This scheme, used by most high speed modems, allows quicker data transfer than FSK. And it gives at least four states to send information. There's a good chance you've heard this type as your modem makes a dial up connection. IS-136 uses this technology to enable its digital control channel, allowing PCS like services for conventional cellular. GSM uses a variation called Gaussian Minimum Shift Keying,
Quadrature phase shift keying changes a sine wave's normal pattern. It shifts or alters a wave's natural fall to rest or 0 degrees. By forcing changes in a sine wave you can convey information. You don't stop or abbreviate the sine wave, you change its shape or angle of attack. Check out the diagram below.
As an example, 90 degrees, 0 degrees, 180 degrees, and 270 degrees might be represented by binary digits 00,01,10, and 11 respectively.
You arrange a circuit so that at each point you wish to transmit a bit you force a shift in the sine wave. The receiver expects these shifts and decodes them in the proper sequence. Again, we are putting digital information on a carrier wave. We are shaping a carrier wave to do this, to carry more pulses more efficiently. That's why, confusing though it may be to visualize, we have the make and break, up and down pattern of digital, carried on the smooth, up and down shape of an analog looking wave.
Wireless services use amplitude, frequency, and phase modulation to send both analog or digital radio signals. But what converts an analog signal to digital in the first place? An encoding scheme. Pulse amplitude modulation first measures or samples the strength of an analog signal. Pulse code modulation encodes these plots into binary words, namely 0s and 1s. These binary digits are represented by on and off pulses of electrical energy.
A digital signal thus produced usually modulates the current carrying the signal within a landline. Modulation and pulses, therefore, get digital messages going. Once completed, the resulting digital signal can be sent over the air with another modulation technique for doing just that. We'll now go over some digital basics and then see in detail how pulse modulation works.
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Notes
Modulation and Cellular Radio
A.M. or Amplitude Modulation Types
Quadrature amplitude modulation (QAM): Used in Motorola's iDEN system. Some argue that QAM is a hybrid system, not belonging to A.M., but a cross between A.M. and phase modulation.
F.M. or Frequency Modulation Types
Normal F.M.: Used in all analog cellular radio systems, such as AMPS, TAC, ETACS, NMT 900 and so on.
Narrowband F.M: Was used in the now defunct NAMPS cellular system.
Frequency shift keying (FSK): Used in AMPS for control signaling.
Gaussian minimum shift keying (GMSK): Used by GSM systems.
P.M. or Phase Modulation Types
Quadrature phase shift keying (QPSK): IS-95 and the coming Universal Mobile Telephone System.
Differential quadrature phase shift keying (DQPSK): Used by IS-54, the first North American digital cellular system. I'm not sure if it is now incorporated into IS-136.
Pi/4 differential quadrature phase shift keying: IS-136, Japanese Handy Phone and the European TETRA systems.
Extended discussion on F.M. The word frequency in an F.M. discussion confuses many people. That's because this word is used in three separate but related contexts. Here are the subjects; I describe them separately and then discuss them together after that:
1. Frequency and the original audio signal. Usually our voice, this is the message we want to put on a carrier wave. This is what varies the carrier. The audio signal varies in two ways: strength or amplitude and in frequency. Let's concentrate on the audio. Being in the voice band, an audible frequency might range from, say, 300 to 3,000Hz. I used a dial tone at 440Khz as an example before. For our discussion, let's call this signal the voice frequency.
2. Frequency and the the carrier wave. Easily understood. A radio frequency. For an example, let us use 94.5MHz, something in the F.M. band. A carrier wave stays roughly on the same frequency, give or take, no matter if it is modulated by amplitude, frequency, or phase. This is the radio frequency.
3. Frequency and Frequency Modulation. Whereby our signal is put on or merged with the carrier wave. This is the modulation technique.
Okay, let's take a slight time-out. Look again at our friendly F.M. waveform diagram below. See the channel width an F.M. signal occupies? You have a median point, say 880 Mhz, represented here by 0. With an analog FM cellular signal the radio channel is 30Khz wide. That allows 15Khz of room or deviation above and below its assigned frequency. Following this so far?
"The amplitude (volume) of the input or audio signal (Beethoven's 5th, or whatever) produces the AMOUNT of deviation. More volume, more deviation from the original carrier frequency. The FREQUENCY of the input signal is contained in the RATE of deviation. The faster the signal deviates in frequency, the faster audio is output from the FM detector. Hmm. That makes sense - faster = frequency. Amount = amplitude."
Did you get that explanation from Mark van der Hoek? We have two steps here, amount of deviation and rate of deviation. Let's first discuss amount or amplitude or strength. Whatever.
1) Amount of deviation: The diagram doesn't portray signal strength and FM very well. At first glance the output signal looks uniform. Which it is. But we know that no conversation is of uniform strength. What happens, and I know this is difficult to understand, the carrier frequency moves slightly up up or down to reflect the audio signal strength. That is, the carrier frequency itself deviates slightly from the median line or 0 when modulated. So the radio channel width and the output signal remain uniform, it's just that the carrier frequency deviates within the channel assigned it. Got it?
2) Rate of deviation. Back to simple stuff. This is as depicted above, with the carrier modulated by the audio signal at a rate in lock step to the frequency of the original signal. A higher audio frequency? A quicker rate. A lower frequency signal? A lower rate.
As Mark says in summing all this up, "With F.M. the radio frequency does change, slightly, within a window defined by the information that is pressed onto the carrier wave. Both the amount of deviation and the rate of deviation carry information. The original audio signal varies both in amplitude and in frequency. The rate of deviation in the FM signal carries the frequency info, and the amount of deviation carries the amplitude of the original audio signal." Mark van der Hoek.
Modulation (sung to the tune of the French childrens' song of Frere Jacques)
Modulation, tricky modulation
Modulation, I will soon know you
Modulation is a term
Something I will quickly learn
Tricky term, quickly learn
Tricky term, quickly learn
Ah! (repeat)
(NB: Here, play it on your touch tone phone: 1231, 1231, 369, 369, 9*9631, 9*9631, 111, 111)
Resources:
Douglas-Young, John, Illustrated Encyclopedic Dictionary of Electronics, Parker, West Nyack, N.Y. (1981) p.385 (back to text)
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