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!


Thursday, February 3, 2011

Lesson 27: General Class Exam Course G7B

Lesson 27 covers amplifiers, oscillators and digital circuits.  This lesson is a little deep in that it deals with some concepts that the average bear is not extremely familiar with.

Digital circuits process information using binary signals.  This means information is sent to the circuit through a series of ones and zeros.  This is relatively simple for circuits to process because the ones and zeros easily represent and "on" and "off" state, electronically.

Here are the digital circuits you need to know.

Flip-flop.  A flip-flop is a digital circuit with two stable states.  A flip-flop has two inputs: a "set" and a "reset".  They also have two outputs "Q" and "NOT Q".  The "set" and "reset" will each receive an input of either 1 or 0.  Depending on the combination of ones and zeros to the "set" and "reset", the "Q" will either output a 1 or a 0.  The "NOT Q" will produce the opposite of the one or zero the "Q" produces.  The fact that one of the two outputs is a 1 (on state) and the other is a 0 (off state) gives the flip-flop two stable states.

The other circuits require an understanding of the "NOT," "AND," and "OR" logic functions.

The "NOT" function is simply a function which produces the opposite of the input.  So if a 0 goes in, a 1 comes out.

The "AND" function takes two separate inputs and produces a single output.  For the "AND" function, if either one or both of the inputs is 0, then the output will be 0.  If both of the inputs are 1, then the output will be 1.

The "OR" function is similar to the "AND" function except the preference is to 1.  If either or both of the inputs are 1, then the output will be a 1.  If both inputs are 0, then the output will be 0.

These functions can be combined.  There is a question which asks about a NAND gate and one that asks about the output of a NOR gate.  The NAND gate is a digital logic integrated circuit which combines the "AND" and "NOT" function.  The NOR gate simply combines the "OR" and "NOT" functions.  For instance, in a NAND gate, if the two inputs receive a 0 input signal, then the output will be a 1.  The NAND gate takes the input signals and runs them through the "AND" function.  The product of the "AND" function with the two 0 inputs produces a 0 output.  Now add the "NOT" function to the "AND" function's output and the the output becomes the opposite, a 1.  The NOR gate is a combination of the "OR" function and the "NOT" function.  The "NOT" function simply outputs the opposite of the product of the "OR" function.

You will also need to be basically familiar with Class A and Class C amplifiers.  Specifically, a Class A amplifier has low distortion.  A Class C amplifier is very efficient and amplifies CW signals very well.

Good luck.  As always, please leave any suggestions, comments, or questions you may have in the comments box.  Thanks!