Modulations

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Modulations
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Modulation

• to tie information to the carrier frequency
• Carrier frequency the RF signal which is
  modulated by the baseband signal
• Two main voice modes
• amplitude modulation (AM)
• frequency modulation (FM, part of angle
  modulation type)

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Amplitude modulation

• Amplitude of main carrier is modulated with
  baseband signal

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A1A

• Simplest mode is on-off keying
• carrier frequency is keyed on and off
• all power is on the fc
• bandwidth is very small
• depends on the speed (150 Hz)
• depends on the rise and fall time ? arupt
  rises/falls cause key clicks harmonics. Clicks
  spread over several kHz, but can be removed with
  smoothing the edges

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AM

• Amplitude is varied
• Transmit power is divided to two parts
• carrier frequency (50)
• sidebands (50)
• bandwidth is 2 x baseband signal BW

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A2A, A3E

• A2A CW where fc is modulated instead of just
  cutting, not used
• A3E voice mode where the modulating signal comes
  from e.g. microphone
• information lies in sidebands
• Very unefficient power is lost on fc and extra
  sideband
• BW is quite high, 6kHz
• fc causes whistling noises
• quality of sound is great combining two
  sidebands minimizes selective interferences and
  interference caused by nearby transmits
• modulation percent overmodulation

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DSB

• Double Side Band
• fc is removed ? no whistling noises
• Still takes as much BW as AM
• Is not widely used

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SSB J3E

• Single Side Band
• fc and extra sideband is removed ? all power is
  on one sideband ? transmit power is four times
  bigger (6 dB)
• BW is as big as BB BW
• Needs to be tuned well to fc, RX and TX should
  also stay there and not drift away...
• Either USB or LSB is used

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Frequency modulation

• Modulating signal changes the carrier frequency
• BW is determined by deviation ?f and BW of the
  baseband signal
• Deviation is small when amplitude of the
  modulating signal is small and big at amplitude
  peaks (pos.neg.)

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F3E

• Bandwidth BW 2 x (?f BWBB)
• e.g. deviation is 3 kHz ... 5 kHz at 145 MHz and
  speech BW is 3 kHz, BW 16 kHz
• BW is large ? used at higher bands
• Less sensitive to interference than AM
  transmittings

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RTTY F1B

• Radioteletype is based on frequency shift keying
• Two different frequencies continuing transmit
  where the mark is on different f than space
• RTTY symbols are created of five bits which are
  equal length ? 32 symbols
• BW is somewhat 300 Hz

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SSTV

• Slow Scan Television transmitting of still
  picture
• Voice modulation the brightness of the picture
  element equals one f at 1 kHz ... 3 kHz
• SSB is used in HF and FM in VHF/UHF
• pros and cons according the modulation type

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FSTV ATV C3F

• Fast Scan TV Amateur TV
• Formed like normal television broadcast
• BW is 5 MHz ... 6 MHz, used only in UHF and SHF
• FMATV uses FM modulation, so it takes 17 MHz ...
  21 MHz. Fits only to gigabands.