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.
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

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