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layman77

How radio works

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Ok, so here's where I've got most of my information on it. This is part where I start to have trouble

 

http://electronics.howstuffworks.com/radio7.htm

 

So, does 98.5, Mhz mean that the wave oscillates at 98.5 million waves per second? Meaning it goes up to a peak then back to zero many times per second? What I don't understand is how you can put information on the wave. If you trasmitted at 98.5, it would just be that frequency, but a sound wave is totally different, it's frequency and amplitude vary quite wildly, not exactly at 98.5, how could you put that on top of a radio wave?

 

This seems to help me a little bit.

 

http://www.qrg.northwestern.edu/projects/vss/docs/communications/1-how-is-data-put-on-radio-waves.html

 

 

Certain fluctuations in the wave correspond to 1's and 0's, but that digital isn't it? Radio's were analog when they first started out. And, would varying the frequency or amplitude make it audible? If you tuned to 98.5 but the frequency frequently changed to put the information on it, then you wouldn't be able to run to that frequency, it would "blank out" as soon as someone spoke, wouldn't it?

Edited by layman77

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Ok, so here's where I've got most of my information on it. This is part where I start to have trouble

 

http://electronics.h....com/radio7.htm

 

So, does 98.5, Mhz mean that the wave oscillates at 98.5 million waves per second? Meaning it goes up to a peak then back to zero many times per second? What I don't understand is how you can put information on the wave. If you trasmitted at 98.5, it would just be that frequency, but a sound wave is totally different, it's frequency and amplitude vary quite wildly, not exactly at 98.5, how could you put that on top of a radio wave?

 

This seems to help me a little bit.

 

http://www.qrg.north...adio-waves.html

 

 

Certain fluctuations in the wave correspond to 1's and 0's, but that digital isn't it? Radio's were analog when they first started out. And, would varying the frequency or amplitude make it audible? If you tuned to 98.5 but the frequency frequently changed to put the information on it, then you wouldn't be able to run to that frequency, it would "blank out" as soon as someone spoke, wouldn't it?

 

Look at swansont's links.

 

To add to that you need to note that analog signals operate on one of two methods of modulation -- amplitude modulation (am) or frequency modulation (fm). The 98.5 MHz which you reference is the carrier frequency, and it is the carrier frequency to which a radio receiver is "tuned". But the actual signal is not a pure sinusoid, due to the modulation. It is via the modulation that information is encoded on the signal.

 

A radio receiver does two primary things. First it isolates one signal from among many by means of tuning to a band centered on the carrier frequency. Second it recovers the information from the modulated signal and converts it to a signal at audible frequencies that is used, after amplification, to drive a speaker of some sort.

 

For more information you can consult electrical engineering texts on signal analysis or communication theory. Howevr, be aware that such texts assume a facility with linear system theory and in particular with the use of the Fourier Transform. The Fourier Transform is particularly important since virtually all of communication theory work is carried out in the "frequency domain" (i.e. the time domain differential equations are converted to algebraic equation in the frequency domain by means of the Fourier Transform).

 

One text that you might consider is Modern Digital and Analog Communication Systems by B.P. Lathi.

Edited by DrRocket

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Look at swansont's links.

 

To add to that you need to note that analog signals operate on one of two methods of modulation -- amplitude modulation (am) or frequency modulation (fm). The 98.5 MHz which you reference is the carrier frequency, and it is the carrier frequency to which a radio receiver is "tuned". But the actual signal is not a pure sinusoid, due to the modulation. It is via the modulation that information is encoded on the signal.

 

A radio receiver does two primary things. First it isolates one signal from among many by means of tuning to a band centered on the carrier frequency. Second it recovers the information from the modulated signal and converts it to a signal at audible frequencies that is used, after amplification, to drive a speaker of some sort.

 

For more information you can consult electrical engineering texts on signal analysis or communication theory. Howevr, be aware that such texts assume a facility with linear system theory and in particular with the use of the Fourier Transform. The Fourier Transform is particularly important since virtually all of communication theory work is carried out in the "frequency domain" (i.e. the time domain differential equations are converted to algebraic equation in the frequency domain by means of the Fourier Transform).

 

One text that you might consider is Modern Digital and Analog Communication Systems by B.P. Lathi.

 

Thanks, but that's quite a mouthful, Well that third paragraph in particular. I don't know most of those terms. Of course I know about wikipedia, but those articles like many on it are very complicated and you need to first know them to be able to understand the article. That's quite a gripe I have with wikipedia. My vocabulary is decent. I'd guess I've heard most English words, but there's no need to be overcomplicated when you don't need to be. That's one thing my Government teacher used to say, that why don't they just write it some everyone will understand it? True, you can't understand some things without first understanding pre-requistites. You need to learn Trigonometry and Algebra to understand Calculus because these maths are part of it, but not always.

 

I hadn't really caught on about it, I hadn't realized that only the amplitude is changed in an FM broadcast, and the changes somehow code to sound (like you said, the second paragraph) that would be a lot simpler to say.

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I hadn't really caught on about it, I hadn't realized that only the amplitude is changed in an FM broadcast, and the changes somehow code to sound (like you said, the second paragraph) that would be a lot simpler to say.

 

Amplitude is changed in AM broadcasts. In FM, the frequency changes. It's right in the name.

 

Basically you compare the signal with a pure tone, and the difference (in amplitude or frequency) are due to the sound encoding.

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does 98.5, Mhz mean that the wave oscillates at 98.5 million waves per second? Meaning it goes up to a peak then back to zero many times per second?

Let's begin with the basics.

 

It's "98.5 MHz" (FM in North America), or something like "1080 kHz" (AM in North America).

 

M = Mega = million (always capital "M")

k = kilo = thousand (always small "k")

Hz = Hertz = cycles/second (always capital "H" and small "z")

 

The unit "Hertz" means "cycles/second", named in honor of the 19-century German physicist Heinrich Hertz, who was the first to demonstrate the existence of electromagnetic waves. It's a nice way of having a simple, one-syllable unit name instead of one that's five syllables long ("cycles per second").

 

A cycle is a change in amplitude from zero to maximum positive, back to zero, then to maximum negative, and then back to zero. Think of your bike pedal beginning halfway up the up-stroke, and its motion during one cycle of the pedal's crank. The frequency of a radio broadcast refers to the "carrier frequency", so called because it "carries" the information meant to be broadcasted.

 

Note — The radio term "broadcast" is borrowed from farming, where it means to sow seeds broadly over a large area — basically in every direction, compared to, for example, planting them in rows or in starter pots. In radio, broadcasting is meant to be received anyone, whereas microwave dishes might sent its signals along a beam, like a headlight or laser, meant to be received by only one other dish in the appropriate position and orientation to receive the signals. This is why broadcast antennas are drawn with radio waves emitting in all directions, and dish antennas are drawn with radio waves emitting in a beam.

 

The term "AM" means "amplitude modulation", which means that the sound/information to be transmitted is encoded in the radio frequency by modulating the amplitude of the carrier frequency. You've held a balloon while speaking at it, so you know that sounds cause objects to vibrate, pushing and pulling at things. Imagine speaking at a disk in a can of carbon powder. When your speech pushes against the disk, it compresses the powder and makes it conduct electricity more. When your speech pulls back on the disk, it relaxes the powder and makes it conduct electricity less. This was how the microphone part of Thomas Edison's telephone worked. Now send your radio frequency electricity through the carbon powder and to an antenna. Your voice will modulate the electric signal's strength (amplitude) which then emits from the antenna as a radio signal, and you have just sent a radio broadcast in amplitude modulation.

 

To "demodulate" amplitude modulation radio frequencies, first you convert it from radio waves into electrical waves — this is what the antenna part of your radio does. Metal does this easily; for example, your bike and the metal clip on your pen are constantly receiving all sorts of radio frequencies. Then you want to pass along only that carrier frequency (and a small range of frequencies around it), and reject all the others — this is what the tuner part of your radio does. Then you want to allow only the "voice" frequencies (about 30 Hz to 10,000 Hz) in the signal through and reject the radio frequency in the signal — this is what the detector part of your radio does. This last electrical signal goes into your speakers which transforms it into sound.

 

While your digesting this, I'll think of a way to easily explain FM modulation and demodulation.

Edited by ewmon

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I think basically the fluctuations are read, frequency and amplitude and some specific patterns in them represent sounds, etc. Like if we were to put a single word repeating on a frequency, what the pattern would be. That would be a good example to show, and certain patterns are decoded.

 

Ok, so frequency means the frequency is varied, and amplitude means the amplitude is varied, and certain patterns mean certain things. The reason I thought that perhaps in FM, the frequency stays the same, but the amplitude varies (even though it is called FM) is because if you set your radio to 98.5FM, the frequency would be changing to make the patterns of someone's voice/sound, etc, so if the frequency is changing wouldn't that mean that for fractions of a second it WOULDN'T be 98.5Mhz, and the radio wouldn't pick it up?

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I think basically the fluctuations are read, frequency and amplitude and some specific patterns in them represent sounds, etc. Like if we were to put a single word repeating on a frequency, what the pattern would be. That would be a good example to show, and certain patterns are decoded.

 

Ok, so frequency means the frequency is varied, and amplitude means the amplitude is varied, and certain patterns mean certain things. The reason I thought that perhaps in FM, the frequency stays the same, but the amplitude varies (even though it is called FM) is because if you set your radio to 98.5FM, the frequency would be changing to make the patterns of someone's voice/sound, etc, so if the frequency is changing wouldn't that mean that for fractions of a second it WOULDN'T be 98.5Mhz, and the radio wouldn't pick it up?

 

When you tune a radio to 98.5 Mhz, it does not mean it can only pick up that exact frequency. This is just the center of the bandwidth range it is sensitive to. This is the reason that there is a limit as to how close in frequency radio stations within transmitting range of each other can be. It keeps you from hearing another station's signal from bleeding into your signal.

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I think basically the fluctuations are read, frequency and amplitude and some specific patterns in them represent sounds, etc. Like if we were to put a single word repeating on a frequency, what the pattern would be. That would be a good example to show, and certain patterns are decoded.

 

Ok, so frequency means the frequency is varied, and amplitude means the amplitude is varied, and certain patterns mean certain things. The reason I thought that perhaps in FM, the frequency stays the same, but the amplitude varies (even though it is called FM) is because if you set your radio to 98.5FM, the frequency would be changing to make the patterns of someone's voice/sound, etc, so if the frequency is changing wouldn't that mean that for fractions of a second it WOULDN'T be 98.5Mhz, and the radio wouldn't pick it up?

 

Frequency is a rather loose term when applied to functions given as a fuction of time.

 

Even a "pure sinusoid" when presented as a physical signal, because it started at a definite time in the past and has run only as far as the present time, is not a pure sine wave. ALL physical signals have a frequency spectrum that is much broader than a single point.

 

To make these notions precise one simply must resort to Fourier transforms and formulate questions and answers in the "frequency domain".

 

Radio receivers do not simply "tune" to a single frequency. Rather they accept a frequency band, centered on the frequency to which we say that they are tuned. The modulated signal is limited to that band and decoded in the receiver to produce the signal that drives a speaker which is then heard as sound. In fact, since electronics does not create perfectly accept/reject bands the situation is slightly more complex and some distortion and noise enters the picture, but the general idea is adequately explained by the simplified picture.

 

If you wish to understand this technology in detail there is simply no substitute for learning the relevant mathematics, which will allow you to read textbooks on the subject such as those previously noted.

 

A slightly more ambitious alternative is to do what I did when, as a high school student I also wanted to understand how a radio works. That alternative is to go get a university degree in electrical engineering. There is no royal road to understanding. You will have to put in some serious effort in order to gain serious understanding.

Edited by DrRocket

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The reason I thought that perhaps in FM, the frequency stays the same, but the amplitude varies (even though it is called FM) is because if you set your radio to 98.5FM, the frequency would be changing to make the patterns of someone's voice/sound, etc, so if the frequency is changing wouldn't that mean that for fractions of a second it WOULDN'T be 98.5Mhz, and the radio wouldn't pick it up?

Unless the 98.5 FM station was transmitting an unmodulated carrier frequency, you're almost guaranteed that the signal is not 98.5 MHz (except when instantaneously passing through 98.5 MHz as it is modulated from less than 98.5 to greater than 98.5).

 

Here's your FM transmitter and receiver.

 

Take your carbon powder mike that you made for your AM radio, and include it in a circuit that uses its resistance to determine the frequency of the oscillator circuit. The circuit is designed to broadcast at 98.5 MHz with the designed value of the mike (no sound input), but as you speak into it, its resistance varies more or less than that value, which varies the oscillator's frequency, which then goes to the antenna as transmits as FM radio waves.

 

In your receiver, you actually tune the tuner to 98.5 MHz, this being a "band-pass filter" that allows through frequencies that are the center frequency ± some set bandwidth. These frequencies then get sent to the detector that extracts the voice/music from the ~98.5 MHz signal.

 

Now comes a detail about filters. Filters do not suddenly filter out all the energy at frequencies outside the allowable band. It may allow through 100% at 98.5, and 90% at 98.49 and 98.51, and 80% at 98.48 and 98.52, etc. So, there's a gradual filtering out as the frequency goes through the high and low "ends" of the pass band.

 

That said, we'll now use a similar filter for the first part of the detector that allows through 0% at 98.495, 10% at 98.496, ... , 40% at 98.499, 50% at 98.500, 60% at 98.501, ... , 90% at 98.504, and 100% at 98.505. We are using this filter to make the signal's frequency modulate the amplitude. Aha. Now we have a signal coming out of the first part of the detector that is both frequency AND amplitude modulated. The second part of the detector treats it just like the AM detector does — it strips away the radio frequency part of the signal, leaving only the voice/music signal that's modulating its amplitude, which you send to the speakers.

 

Disclaimer — Most likely, no radio transmitters or receivers sold today work exactly as I have described here. They are much more sophisticated. What I have given you is what an electrical engineer would make from basic electronics parts if shipwrecked on a deserted island (with Gilligan, the Skipper, and everyone else :rolleyes:).

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FM receivers process signals in a narrow range of frequencies centered at the nominal frequency.

the information (sound waves) in the signal is obtained by subtracting the nominal frequency. This is why separation of .2 Mhz is needed between stations. The maximum sound most people can hear is less than .02 Mhz.

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k = kilo = thousand (always small "k")

 

The kilo prefix is with capital K (Kg for kilogram), as all multiplying prefixes above 1. Prefixes of multipliers under 1 are with lower case.

 

 

While your digesting this, I'll think of a way to easily explain FM modulation and demodulation.

 

A little help:

A discotheque light that flickers its intensity at the loudness of music shows amplitude modulation. To detect it, a photocell would respond with a signal according to the brightness.

 

A discotheque light that changes its color according to the frequency of the music would be frequency modulation. The amount of brightness keeps constant, the color changes. To detect it, a color sensing device would de-modulate it.

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A little help:

A discotheque light that flickers its intensity at the loudness of music shows amplitude modulation. To detect it, a photocell would respond with a signal according to the brightness.

Yes. Intensity is amplitude, and the [instantaneous] loudness is what modulates/controls it. You can actually buy kits with a light modulator/transmitter and a light demodulator/receiver.

 

A discotheque light that changes its color according to the frequency of the music would be frequency modulation. The amount of brightness keeps constant, the color changes. To detect it, a color sensing device would de-modulate it.

It all depends what you want to encode in the carrier wave. If you want to encode frequency, then be prepared to transmit multiple frequencies, and to receive and decode them. It seems prone to poor signal-to-noise (S/N) characteristics.

 

Currently, frequency modulation uses the amplitude of the voice/music signal to modulate the carrier frequency. At any given moment, there's only one instantaneous amplitude of the voice/music signal, so the modulated carrier is only one instantaneous frequency.

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The kilo prefix is with capital K (Kg for kilogram), as all multiplying prefixes above 1. Prefixes of multipliers under 1 are with lower case.

 

Kilo- and hecto- use lower case, though it's not usually a problem to use upper case as it won't cause confusion, unlike a M vs m or P vs p.

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