Analogue Bandwidth: Measured in Hertz / Kilohertz / Megahertz / Gigahertz

  • Bandwidth is the range of frequencies over which the output voltage is at least 0.7 times the maximum output.
  • Bandwidth is the range of frequencies over which the output power is at least 0.5 times the maximum output.
  • Bandwidth tells you the frequency range over which a circuit or transmission medium will operate.
    Bandwidth is measured in Hz, kHz, MHz, GHz, etc.

    In this image, the 70% output voltage bandwidth goes from 2 or 3 Hz up to about 20000Hz.
    This is a 20 KHz (19998 Hz) bandwidth.The red arrow heads show this.
    This would be typical for an audio amplifier with a good bass response.

    Bandwidth Diagram
     

Example: AM Radio (used on long, medium and short wave up to 30MHz)

  • Bandwidth = 2 fmax where fmax is the highest audio frequency to be transmitted
  • the channels have an audio bandwidth of 4 to 5kHz (kilohertz) - See sidebands
  • the AM radio band spans this frequency range 540-1600 kHz
  • the channels are separated by 10kHz

Example: Telephone network channels have a 2.7kHz bandwidth. This is enough to accommodate frequencies between 300 Hz and 3000 Hz (needed for intelligible speech).

Example: FM Radio (mainly used above 27MHz and on VHF)

  • Bandwidth = 2 x (deviation + fmax)
  • The FM radio band is from 88 to 108 MHz
  • The FM station frequencies are separated by 200 kHz
  • The FM band is wide enough to accommodate 100 stations (this is another example of the concept of BANDWIDTH)
  • The transmitters allow the frequency to deviate (be modulated by) by 75 kHz from the centre frequency
  • This leaves a 25 kHz unused upper and lower guard band. Receivers must be selective enough to reject interference from adjacent stations and this guard band makes this easier to achieve.

Digital Bandwidth: Measured in bits per second.

Example: An Internet connection

  • using a modem 56kb/s is possible - this allows text, poor quality audio and slow image downloads.
  • using ADSL Broadband, the bandwidth is between 512 Kb/s to 8Mb/s - this allows good quality audio, fast image downloads and video in a window the size of a postage stamp.
  • faster versions of ADSL up to 24Mb/s are being rolled out (2006) - this will allow full screen high quality video.
  • Note that b = bit and B = Byte. These are frequently confused.

Modulating or Sampling a Signal

The carrier signal used to carry data or the clock frequency used to sample data needs to be on a frequency at least double the highest data/music/voice frequency.

The Nyquist Frequency also known as the critical frequency is the highest data/music/voice frequency that can be transmitted. This is half the sampling or carrier frequency.

This boils down to low data rates at low frequencies and higher data rates at higher frequencies.

This also explains why optical fibres are so good. The frequency of light is in the region of 1014 Hz. Theoretically data rates of half this figure should be possible if the light emitting and detecting devices and the fibres could ever be made good enough.

Plenty of Samples - This will work - The original signal can be re-constructed fairly well

Signal Sampling

Not Enough Samples - This will not work - The original signal can't be re-constructed

Insufficient Samples