Introduction to Communications

• It is the transmission, reception, and processing of information with the use of electronic circuits.

  Electronic Communications
• Convert the electrical signal to a signal suitable for transmission over a given communication medium.

  Transmitter
• The medium by which the electronic signal is sent from one place to another.

  Communication Channel
• Accepts the transmitted message from the channel and converts it back to a form understandable by humans.

  Receiver
• These are signals characterized by a waveform that is continuously varying voltage or current.

  Analog Signal
• These are signals used to represent messages via a sequence of discrete values

  Digital signal
• a device produces oscillations

  Oscillators
• Applications for oscillators in electronic communications:

  • High-frequency carrier supplies
  • Pilot supplies
  • Clock circuits
• The net gain around the feedback loop must be unity or greater.
• The Barkhausen criteria states that: The loop gain is equal to unity in absolute magnitude
• The net phase shift around the loop must be a positive integer multiple of 360°.
• LC oscillators are oscillators in which the frequency of oscillation is determined by an LC tank circuit.
• The frequency of an oscillator may be tuned by varying the inductance or capacitance of the circuit.
• Varactors are more convenient substitutes than variable capacitors in many circumstances.
• Crystal Oscillator - The oscillator with the most adequate frequency stability for most radio communications.
• Frequency stability is the ability of an oscillator to remain at a fixed frequency.
• It multiplies the frequency of the input signal and generates harmonics of that frequency.

  Frequency Multipliers
• Harmonic frequency is an integer multiple of the fundamental frequency.
• In Frequency Multipliers, Extremely inefficient and are impractical for

  multiplication factors above 3.
• Mixing is the process of combining two or more signals and is an essential process in electronic communications.
• Two ways of Mixing:

  • Linear Mixing
  • Non-linear Mixing
• Linear mixing is when two or more signals combine in a linear device such as a resistive or inductive network or a small-signal amplifier.
• Linear Mixing also called Linear Summing.
• In communications work, mixing almost always implies a nonlinear process.
• Nonlinear mixing is when two or more signals are combined in a nonlinear device such as a diode or a large-scale amplifier.
• Non-linear Mixing produces Cross Product frequencies
• The cross products are the sum and difference of the two original frequencies and the sums and differences of their harmonics.
• There is an infinite number of harmonic and cross-product frequencies produced when two or more frequencies mix in a nonlinear device.
• The original information or intelligence signals whether analog or digital.

  Baseband Signal
• Transmittal of the original voice, video, or digital signals directly into the medium.

  Baseband transmission
• Incompatibility with the medium for long distance communication.

  Major problem
• A type of transmission where the baseband signal is used to modulate a higher frequency carrier upon transmission.

  Broadband Transmission
• The range of electromagnetic signals encompassing all frequencies.

  Electromagnetic Spectrum
• The number of times a particular phenomenon occurs in a given period of time.

  Frequency
• The distance occupied by one cycle of a wave.

  Wavelength
• Bandwidth - The frequency range over which an information is transmitted or over which a receiver or other circuits operates.
• The Decibel (dB) is a relative unit of measurement corresponding to one tenth of a Bel(B).
• Decibel - Ratio of powers, voltages, or currents.
• Decibel is used for defining amplifier gains, component losses, attenuation, as well as a host of other measurements such as noise

  figure, signal to noise ratio, and many others.
• Electrical noise is defined as any unwanted electrical energy present in the usable passband of a communications circuit.