Modulation....

[Rohit Chatterjee]

Whats the main advantage of modulating a signal????Apart from FDM and reduction in size of transmitting n receiving antennas is there any other advantage???I had heard sumwhere that a high frequency wave is less attenuated than a low frquency....but havnt found any related information....Is this true????

 

[ke5frf]

Modulating a signal has absolutely nothing to do with antenna size. Zilch. Nada.
The only consideration of antenna size where modulation is concerned would be FM modulation, where the bandwidth of the signal requires (or is made more sensitive with) a broadbanded antenna design...but that has less to do with overall size as much as, perhaps, element radial thickness.

Modulating a signal refers to superimposing intelligence upon a carrier, or varying frequency as a means of modulation, etc. The advantage is obvious, it allows radio (or television, cable, satellite, or internet) to be useful means of communication!

I'm not sure what you mean by frequency and attenuation.
Care to expand?

 

[Rohit Chatterjee]

By frequency i was referring to the frequency of the baseband signal....Now the baseband signal is super-imposed on a high frequency,the frequency of the resultnt wave is also pretty high.....I dont knw about all modulation schemes but this is what happens in AM n FM at least....Now y do we actually have to superimpose the baseband signal on the carrier???Cant we just transmit the baseband signal????Now as far as i know it helps in reducing the size of the antenna,since the size of the antenna should be of the order of the wavelength of the wave for efficient radiation....Moreover it also helps us in using different frequencies for transmitting differnt signals....Is there any other advantage????

I'm not sure what you mean by frequency and attenuation.
Care to expand?

As the wave propagates through the air it gradually fades out...Does modulation in any way prevent or at least reduce this rate of fading out????

 

[ke5frf]

OK.

Telecommunications can encompass a great variety of frequencies depending on the type of data being transmitted.

But I'll discuss audio transmitted upon a carrier.

When a broadcaster speaks into a microphone or when an audio recording track is to be transmitted via a medium such as radio, it must be modulated, or superimposed, upon an appropriate radio frequency carrier. The choice of frequency and modulation scheme may be dependent upon the circumstance of the recieving "station". Some modulation types and frequencies are good for long distance, some better for faster data transfer, some better for local communication, some better for audio/video/data together, and some better for digital links etc.

But in the case of transmitting audio signals through radio, audio "baseband" is too low frequency to be transmitted via electronic means. It is subject distortion and interference. For radio transmissions, well it is outside of the frequency range that is effective for radio. We are talking about frequencies in the hundreds to low thousands of Hz. I think the AF threshold is considered 20,000 cycles/second.

We do transmit AF, when we speak! Or when we hear we recieve it.

Modulating gives us the ability to tranmit audio over long distances via the advantages of radio frequencies!

And your last question about waves fading out. Well, indeed in a manner of speaking modulation reduces the rate of fading out of a signal, but only in the sense that audio waves do not travel very far on their own, and radio gives the advantage of long distance wave propogation.

Do you understand how AF is modulated/demodulated, the AF, IF, and RF, and filtering stages of an amplifier and reciever, etc?

 

[Rohit Chatterjee]

Well i do know a bit about them...i hope ur referring to superheterodyning principle....thats what i know about....n demodulating techniques such as envelope detector n slope detector....n modulators such as square law modulator....if u had sumthin else in mind please feel free to share ur knowledge....i can really do with some information......

 

[blueroomelectronics]

A good book on Radio Frequency would be a smart start.
**broken link removed**

 

[Rohit Chatterjee]

Thnx for the suggestion...i dnt thnk an e-book is available,is it???

And your last question about waves fading out. Well, indeed in a manner of speaking modulation reduces the rate of fading out of a signal, but only in the sense that audio waves do not travel very far on their own, and radio gives the advantage of long distance wave propogation.

Back to my original question....why dont audio waves travel very far on their own???How does modulation reduce the rate of fading out of a signal???

 

[ke5frf]

Audio waves to not travel far on their own because the are audio, not radio. Audio waves are produced by vibrations caused by mechanical means...the low frequency oscillation of which compresses and decompresses the molecules surrounding it...a domino effect which is governed by geometry...as each oscillation of excited molecules radiates from the center of the wave, the earlier waves are pushed out in increasing circumference. The strength of the outermost wave is and its growing circumference have an inverse relationship. In a vacuum, sound doesn't travel. In water, it can travel much further than in air. That's because the density of the molecules surrounding the source govern the progression of soundwaves.

Radio is not mechanical in naure, it is electromagnetic. It creates waves, but the are electromagnetic waves and have to do with the vibration of electrons rather than molecules.

Radio is able to travel further because electromagnetic radiation is not subject to the friction and resistive forces that air molecules are.

And I answered your question about modulation and fading. Modulation has nothing to do with signal strength. Modulation is a means of adding inteligence to a higher frequency carrier, nothing more.

 

[ke5frf]

I wanted to add something about audio.

Audio frequency waves are in fact what the AF section of devices are designed to reproduce electronically. That is what many oscillators and amplifers do. They convert the mechanical energy of a microphone element or recording to an electronic signal, or they convert an electronic signal into mechanical energy for a speaker or playback device.

That electrical signal can produce electromagnetic energy, thus it can radiate...likewise the transducers in the circuit can be subject to audio frequency interference.

That is what causes an alternator to produce whine in an improperly grounded or filtered car stereo system, or how AC hum from fans and motors can cause video ditortion or noise on a CRT screen or speaker system.

In fact, I would venture a guess that early experiments with transmitting audio via electromagnetic energy were conducted with simple AF circuits before modulation techniques were devised.

You seem to be asking why we don't just transmit at audio frequencies. Well, think about it. If we limited ourselves to such a narrow spectrum, we would have a pretty crowded and inefficient means of communication and data transfer! All our cellphones would be jamming each other, our radios would be a horrendous mix of noise and interference, and high speed data would be non-existant.

The radio frequency spectrum is broad though still limited and allows almost limitless "real estate" for our use and manipulation.

So modulation is the key that allows this to occur.

BTW, after thinking about your modulation and signal fading question, I would amend my comments to say that while modulation is not related to signal fading at all, in one respect it does effect signal power with techniques like AM. Output power of an amplifier is stronger when a modulated signal is peaking in amplitude. In that way the modulation does, in marginal conditions, cause a signal to "punch" through the noise floor at times. But if conditions are poor and causing a signal to fade, the entire signal will fade by the same degree including the amplitude peaks.

Modulation has nothing more to do with with signal fading than this.