Transmitting Analog Signals

• You have a message signal $m(t)$
• You can transmit this signal as a radio wave using three signals
  • carrier signal $c(t) = Acos(\omega_c t)$
  • amplitude modulation signal (AM Signal), role of a mixer (multiply message signal and scaled carrier signal together) $x_{AM}(t) = A[1+\alpha m(t)]cos(\omega_c t)$
  • frequency modulation signal (FM Signal), requires more complex stuff
• Variations in the amplitude and frequency are taken care of by the AM and FM portions

Demodulating Received Signals (Analog)

• reverse of modulations
• You have a diode + an RC circuit
  • diode gets the top half (positive part) of the signal
  • RC circuit recovers the signal given by “connecting the peaks of the top half signal)

Transmitting Digital Signals

• OOK (turn it on for 1, turn it off for 0)
• FSK (change frequency in accordance to 1 or 0)
• ASK (change amplitude in accordance to 1 or 0)
• can have higher order modulation schemes, allowing to encode more than one bit per symbol

Radio Receivers

• old-school (crystal receiver)
• transistor receiver
• tuned radio frequency receiver
• superheterodyne receiver (using a lot of mixers)

Modern Radio Tech

• amplifiers
• frequency synthesizers/phase lock loops (you can get exactly 100 MHz or 10.3 kHz or …