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					                Review EM waves
An electromagnetic wave is produced by an oscillating charge.
Transverse waves in which electric and magnetic fields oscillate.
                       1               v  f
                   f         v
                       T         T
Amplitude squared is related to….      Brightness
Frequency is related to ….              Color

Speed is …                              C = 3x108 m/s
              How does it work?
How do we put sound or other information on to an EM wave?

We will consider several approaches to this.
          First is also the oldest – amplitude
          modulation or AM.

          Then we will consider frequency
          modulation or FM.

          Then digital and cell phones.
  The Carrier Wave and Signal
Begin with a continuous EM wave at a frequency quite
high compared to audio.
       AM radio: 1 MHz or so.




And we have our sound wave. Here a simple cosine
wave (constant tone).
  Amplitude Modulation
RF Carrier Wave 1 MHz:     Schematic diagrams not to scale.




Signal: 100 KHz (0.1 MHz cosine)




 AM Signal
         Getting the Music Back
   Once we have the signal from the AM radio station, how
   do we receive it and get the music back out?
                                     Rectifier lets through only
                                     positive part of signal.



                                      Signal to speaker is slowly
                                      varying modulation signal
                                      plus fast wiggles that the
                                      speaker does not respond to.

                                                      Radio
                                                      Diagram
Tuned circuit resonates                               From
at the carrier frequency.                             Wikipedia
Frequency Modulation:Better
RF Carrier Wave 1 MHz:    Schematic diagrams not to scale.




Signal: 100 KHz (0.1 MHz cosine)




FM
                 FM Versus AM
AM signal: Amplitude of the EM wave can be affected
by many things.
   Puts noise on the signal.
   Electronics are simple
   Occupies a narrow range of frequencies
   Commercial radio stations operate around 1 MHz carrier

FM: Frequency of the FM signal is less susceptible to
change.
    FM signals are less noisy.
    More complicated electronics
    Occupies a wider range of frequencies.
    Commercial radio stations operate around 100 MHz carrier
              FM Radio Signals
History
    First frequency modulation signals were broadcast in
    the 1930s with carrier waves in the range 42-50 MHz.

    Switched after WWII to 83 MHz – 108 MHz
                   Old radios no longer worked, lots of
                   angry customers!
    Stereo added in the early 1960s.

                   How? Need two signals.
                   Needs to be compatible with old radio
                   receivers.
                  FM Stereo
Clever solution, but not so simple!
Each station has 200 KHz of band width, e.g. from 89.3
to 89.5 MHz.
Sound contains frequencies up to 20 KHz, and really only
up to 15 KHz. Extra room.
             Stereo FM Broadcast
                     Fourier Spectrum




89.3 MHz +

Signals from left and right channel are added together (L+R)
and encoded as FM at lower end of the radio station’s band.

              Mono recievers can detect and play this signal.

L-R is encoded as amplitude modulation in a sub-carrier at 38
KHz above the low end of the band. 89.338 MHz in this example.

A signal at 19 KHz above the minimum is added as well to let
the receiver know this is a stereo signal.
      Digital Meets Modulation
Complicated business! Many clever schemes. Here are two
simple ones related to AM and FM.
Pulse Amplitude Modulation: large amplitude = 1, small
amplitude = 0
          Digital Meets FM




Frequency Shift Keying, here with two frequencies. Can have
more than two frequencies.

OLD 300 bps modems: 1070 Hz for a 0 send, 1270 Hz for a
1 send; 2025 Hz for a 0 receive and 2225 Hz for a 1 receive
Bandwidth in Physics and in Computer Science

A signal with exactly one frequency only goes from time =
negative infinity to time = positive infinity.
Anything else has a range of frequencies.

Even y=5sin(2pt/T) = 5sin(2pft) has a range of
frequencies if it is not on for all time!

        Shorter duration means wider range.

        Thus turning on and off the signal to make ones and
        zeros very quickly (lots of ones and zeros per second)
        needs a large range of frequencies!
Bandwidth in comp sci is bits per second, in physics it’s the
range of frequencies in the signal – very closely connected.
       Cell Phone Technology
The need for mobile communications is not new.

Prior to the development of the cell phone system,
there had been radio phones. Essentially two-way
radios, as are cell phones.
     One communications tower per city.

     High power transmitters in tower and
     individual phones. Mounted in vehicles.

     Lots of blind spots around buildings.

     Limited radio bandwidth meant a few hundred users.

     Quite expensive!
                   Cell approach
Began 1979 with the Bell Telephone System

City is divided into cells.
       Each cell has a base station for broadcasting and receiving
       signals. Each is assigned a portion of the EM band
       assigned to cell phones. Connected to a mobile switching
       station.

                              Transmitters in the base stations
                              are weak and cannot be picked up
                              a few cells away. Phones only a
                              few Watts.
                              Allows reuse of bandwidth –
                              two cells not next to each other
                              can use same frequencies.
Cell System   Call is initiated: Phone
              looks for signal, picks
              strongest. Other cells in
              range ignore.

              Phone assigned a
              channel. Call routed
              to a land line as usual.

              If calling another cell
              phone, that phone
              must be located.

              Call passed off to
              other base stations as
              needed.
  Essential Ingredients
Microphone is connected to an ADC.

Processing of the voice signal is done
in digital form in a Digital Signal
Processor (DSP).

A DAC is connected to the speaker to
produce an analogue signal for the ears.

A microprocessor runs the phone

ROM and Flash Memory store the
OS and other info.

RF circuitry produces and receives
the radio signals.
   A Bandwidth Hungry World!
Seems we never have enough bandwidth.
Constantly want to share more information.
Are there upper limits to the amount of information
we can encode on EM waves?

     Seems yes.
Limits of current technology – but that’s temporary.

Limits on the frequency of EM waves that can be used.

 EM waves above 300 GHz do not penetrate rain very well.
 Unless the frequency is increased dramatically to the
 infrared and visible range, but that has other problems.
                     Summary
Radio communications technology including cell phones is
based on encoding information onto an EM wave.

The basic technology is Amplitude Modulation (AM) and
Frequency Modulation (FM) or closely related Phase
Modulation.
Either analog or digital information can be encoded.

While the basic idea is not very complicated, actual
implementations can be quite sophisticated.

				
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posted:4/10/2013
language:English
pages:20