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What is bandwidth?

  1. Aug 20, 2015 #1
    I'm having hard time understand what bandwidth really is? The book i'm using defines it as the width of a signal's spectrum (where spectrum is the range of frequencies in a signal).

    So, for example, I have a composite, periodic signal s(t) = 4/π[sin(2πft) + (1/3)sin(2π(3f)t)]. Then the bandwidth of this signal is 3f-1f = 2f. Lets say f = 1Hz, then bandwidth of our signal is 2Hz. What does this 2Hz represent? Ignoring distinction between absolute bandwidth and effective bandwidth, bandwidth is measuring the difference between the highest and lowest frequency of a signal. So, in our example the 2Hz is saying nothing more than that the difference between the highest frequency and the lowest frequency of our signal is 2Hz? How is this definition of bandwidth useful?

    As a practical matter, how does this relate to transmission media? Can a media that carries signals with a bandwidth of 2Hz carry signals with lower bandwidth also?

    I don't have a strong background in electronics(which is why I'm studying) so any help is appreciated!
  2. jcsd
  3. Aug 20, 2015 #2
    Bandwidth typically refers to the range of frequencies in some form of communication or signal processing that can be accommodated with little energy loss. Electronic circuits for example will only be useful for a certain range of frequencies which would be referred to as its bandwidth. It is often defined as the range of frequencies that can be used (processed or transmitted) whose strength is not reduced lower than 3 dB (about 70%) from its maximum value.
  4. Aug 20, 2015 #3


    Staff: Mentor

    A pure tone (a single frequency) carries no information. To transmit information (sound, video, Interhet, whatever) you must vary the signal. The non-sinusoidal varying signal uses up a portion of the spectrum. If we tried to send two signals over the same frequency, they would interfere with each other, just as two singers singing different songs at the same time.

    However, light behaves differently. Photons can pass through photons without interfering. Therefore, in fiber optics, they are able to push several information signals at once through the same fiber using different base frequencies. That doesn't work with wires, but it does with optics.

    In short, bandwidth is not so exactly defined as is voltage or current. It is a useful concept, but it can be defined loosly.

    Does that help?
  5. Aug 20, 2015 #4
    Thanks for that answer. Are there any texts you would recommend that delve deeper into the subject with respect to Data Communication techniques?
  6. Aug 20, 2015 #5


    Staff: Mentor

  7. Aug 20, 2015 #6


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    Gold Member

    There are two "kinds" of bandwidth, sort of.

    There is the range of frequencies that a transmitter/media/receiver chain can accommodate. For example, an audio amplifier/speaker system may be rated for 20Hz to 20,000Hz (those might be the 3dB corners). Any frequency between those two corners would be passed with minimal degradation.

    Then there is the range of frequencies in the transmitted signal. If you tried to send 20Hz to 40000Hz audio through the above amplifier, the frequencies between 20Hz and 20000Hz would be fine, and the signals between 20000 and 40000 Hz essentially would be compromised.

    Now, these bandwidth characteristics are not brickwall limits. The amplifier will pass some signals above 20000Hz, but they will be altered possibly in phase and amplitude.

    But basically, there is the signal (with its band of frequencies) and the system (each component with its band of frequencies). The difference between the half power points of the lower and upper frequency limits is generally referred as the 3dB bandwidth.
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