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

• Amplitude of main carrier is modulated with baseband signal

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

AM

• Amplitude is varied

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

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

DSB

• Double Side Band

• fc is removed no whistling noises

• Still takes as much BW as AM

• Is not widely used

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

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.)

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

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

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

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.