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Thread: communication engineering

  1. #1 communication engineering 
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    why does an AM has lower frequency range like 837khz and where as an FM wave has frequency range as 88Mhz?
    why not the frequency of an Amplitude modulated wave be 88Mhz?


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  3. #2  
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    It's a matter of how the FCC (or similar agency in another country) decides to allot the frequencies.


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    I think the am is difficult to be done at higher frequencies because it involves changing the amplitude of the carrier wave which might even increase the SNR, so Am is done at lowerfreq.
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    This kind of frequency was decided by standards.
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    Can any one know what is the Advantages of negative feedback over positive feedback??
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  7. #6  
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    Quote Originally Posted by salman View Post
    why does an AM has lower frequency range like 837khz and where as an FM wave has frequency range as 88Mhz?
    why not the frequency of an Amplitude modulated wave be 88Mhz?
    Its about the newest modulation when those new frequencies becaome available.

    The orginal communication standard was CW (Morse code). That was implements in frequencies now called longwave. The next technology introduced to a new generation was amplitude modulation. The latest practical frequencies for that were tens and hundreds of kilohertz - AM band. The next generation of people saw frequency modulation. The available frequencies were 30 Mhz. But due to shenanagins at corporate levels, those frequencies were takne away. The next latest frequencies were 80 to 100 Mhz. So that is the FM band. Meanwhile between the time of the original FM band and its new frequencies, TV made an appearance. TV was assigned the frequencies about 30 Mhz. Then the new FM radio standard was assigned to next higher frequencies above TV. 88+ Mhz.

    Which each new technology, the next available frequency bands were assigned.

    FM can be implemented at any frequency. As can AM or CW. When American WWII tanks were using AM, the Germans were using same frequencies with FM. It was simply what frequency band was available for standardization when a new innovation was introduced.
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    Quote Originally Posted by Aarom pizz View Post
    Can any one know what is the Advantages of negative feedback over positive feedback??
    Negative feedback is good stuff if you are building regulation or control circuits. Negative feedback isn't necessarily better than positive feedback: if you are building an oscillator, positive feedback is what you would have to develop. A voltage regulator would work only if you used negative feedback.

    In other words, negative feedback might entail mixing an inverted signal with an input signal to an amplifier. Positive feedback would mix in an in-phase signal. An explosive reaction would represent positive feedback.
    "Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense." --Buddha (563BC-483BC)
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  9. #8  
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    Quote Originally Posted by salman View Post
    why does an AM has lower frequency range like 837khz and where as an FM wave has frequency range as 88Mhz?
    why not the frequency of an Amplitude modulated wave be 88Mhz?
    This is fundamental. In AM, the amplitude is modulated, so the modulation bandwidth is about twice the AM bandwidth. In FM, the frequency is modulated, and the frequency variation increased with the audio amplitude. The frequency modulation varies with the design of the modulator. We also have phase modulation, which is a much lower bandwidth than FM but greater than AM.

    Telemetry signals, such as those from rockets, have prescribed channels of varying bandwidths.
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