Friday, February 4, 2011

Lesson 28: General Class Exam Course G8A

Lesson 28 deals with the various types of modulation.  AM, FM, PM, and SSB.  This lesson can be a bit obscure if you have not previously studied how signals are modulated to carry information.  I tried to keep things as simple as possible in this lesson and attempted to provide just enough information to help pick the correct answer on the exam.

The four types of modulation that are covered in this section are AM, FM, PM, and SSB.  What sets these various types of modulation apart is how they modify a carrier signal.  A carrier signal is nothing more than a signal with no information on it.  A blank canvas.  Information from a microphone, or other audio source, is combined with the carrier wave, the carrier wave becomes modulated, or transformed, in a way that allows it to carry information.

Amplitude modulation (AM) conveys information by changing the amplitude of an RF signal.  The question on the exam talks about  changing the envelope of an RF signal.  For the purpose of the exam, same idea.  The basic anatomy of a AM signal consists of a carrier wave with a side-band signal at a higher frequency than the carrier (the upper side-band) and a side-band signal at a lower frequency than the carrier (the lower side-band).

Single side-band (SSB) signals are a type of amplitude modulated signal.  To make a SSB signal a modulating signal is combined with a carrier signal in a circuit called a balanced modulator.  The balanced modulator strips the carrier from the new AM signal leaving just the upper side-band (USB) and lower side-band (LSB).

Phase modulation (PM) is a bit tricky.  In phase modulation the carrier is modulated by a modulated signal which shifts the phase angle of the carrier wave.  A simple way to think of this is the modulating signal causes the starting point of the carrier wave to shift back and forth.  The amplitude and frequency of the modulating signal is what causes the phase modulation of the carrier signal.  This is done in a circuit called a reactance modulator.

Frequency modulation (FM) changes the frequency of a carrier signal to carry information.  Like the other forms of modulation, this is done by combining a carrier signal with a modulating signal.  The change in frequency of the carrier signal is determined by the amplitude of the modulating signal.  This is important to note for the exam because the way PM and FM modulate signals is similar.  The carrier wave in a PM signal is modulated by both the amplitude and frequency of the modulating frequency.  With FM, only the amplitude of the modulating signal changes the frequency of the carrier wave.

As always, please feel free to leave an comments, suggestions, or questions in the comments box.  Thanks!

73,
Andy
KE4GKP