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Modulations

Modulations. 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). Amplitude modulation.

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Modulations

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  1. Modulations

  2. 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)

  3. Amplitude modulation • Amplitude of main carrier is modulated with baseband signal

  4. 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

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

  6. 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

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

  8. 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

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

  10. F3E • Bandwidth: BW = 2 x (Δf + 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

  11. 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

  12. 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

  13. 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.

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