Analog communication block diagram; FDM of three signals

Block diagram of an analog communication system

An analog communication system carries a continuously varying message from a source to a user over a channel. The message is first converted to an electrical signal and then modulated onto a carrier.

 [Information source]
         |
 [Input transducer]
         |
 [Transmitter (modulator)]
         |
 [Channel] <---- noise, interference
         |
 [Receiver (demodulator)]
         |
 [Output transducer] --> user

• Information source and input transducer: produce the message, e.g. speech converted to voltage by a microphone.
• Transmitter: amplifies, filters and modulates (AM/FM/PM) the message onto a carrier suited to the channel, then power-amplifies it.
• Channel: a wire, coaxial cable, optical fibre or free space. It attenuates and distorts the signal and adds noise.
• Receiver: selects the wanted signal, amplifies it, demodulates it and filters it to recover the message.
• Output transducer: a loudspeaker or display that turns the electrical signal back into its original form.

FDM system for three message signals

Assume three messages $m_1(t), m_2(t), m_3(t)$, each band-limited to $W = 4$ kHz, and SSB modulation with carriers $f_{c1} = 100$ kHz, $f_{c2} = 105$ kHz and $f_{c3} = 110$ kHz (upper sideband kept, 1 kHz guard band).

 TRANSMITTER
 m1 ->[LPF]->[SSB mod fc1]->[BPF1]--+
 m2 ->[LPF]->[SSB mod fc2]->[BPF2]--+-->(+)--> channel
 m3 ->[LPF]->[SSB mod fc3]->[BPF3]--+

 RECEIVER
          +->[BPF1]->[demod fc1]->[LPF]-> m1
 channel -+->[BPF2]->[demod fc2]->[LPF]-> m2
          +->[BPF3]->[demod fc3]->[LPF]-> m3

Spectrum of the multiplexed signal:

  |  m1  |g|  m2  |g|  m3  |
  100    104 105  109 110   114  kHz
  |<--------- 14 kHz ---------->|

Working:

1. LPFs band-limit each message to $W$.
2. Each message modulates its own carrier. BPFs keep one sideband, so each signal sits in its own band.
3. The adder combines them. The composite signal (bandwidth $3W$ + guard bands = 14 kHz here) is sent over one channel.
4. At the receiver, BPFs centred on each band separate the three signals. Coherent demodulators with $f_{c1}, f_{c2}, f_{c3}$ and LPFs recover $m_1, m_2, m_3$.

Guard bands and good filters prevent crosstalk between the channels.