Wednesday, January 26, 2011

Lesson 26: General Class License Exam Course G7A

Lesson 26 is a big one.  There are 24 questions to cover.  Some of them get fairly deep.  Others are simple Technician Class review.

This lesson covers circuits and schematic symbols.  The majority of the questions are review from the Technician Class exam, but the manner in which they are phrased can throw a wrench into the works.

The first big subject you will to know for this section is the process in a single-sideband phone transmitter.  This is not as hard as you may think.  I did up a quick graphic to help.

A few things to remember: A single-sideband signal is a type of amplitude modulated (AM) signal.  A complete AM signal consists of a carrier signal and two side-band signals, one above and one below the carrier wave frequency.  To make a change the AM signal into a SSB signal, the carrier and one of the side-bands need to be filtered off.

Here is the basic process:
1) Carrier wave oscillator:  A carrier wave oscillator generates a generic signal at a certain frequency.  For our purposes, the carrier frequency does not matter.  It is determined by the desired transmit frequency range, filtering in the transmitter, and the mode of the transmitter.  There is no information on the carrier wave.  It is just a signal at a certain frequency.  Think of the carrier as a blank canvas.

2) Balanced Modulator: If the carrier wave is the blank canvas, the balanced modulator is where it gets painted.  The balanced modulator combines the carrier wave from the carrier wave oscillator with a the audio signal from the microphone.  The end result is a standard AM signal modulated with the voice information on it.  Remember, the AM signal consists of the carrier signal and two side-bands.  The carrier signal is stripped in the balanced modulator so the product is just the two sidebands.  This signal's frequency is most likely not at the desired transmit frequency of the transmitter.

3) Filter:  Now comes the filtering process which converts the side-bands into a single sideband.  The signal goes from the balanced modulator to the filter where the filter strips off one of the side-bands.  If the transmitter is a USB transmitter, the filter will strip the side-band at the lower frequency.  If the transmitter is a LSB transmitter, the filter will strip off the side-band at a higher frequency.  [This is a correction from the original post.  I originally stated that the carrier signal was stripped in this filter.  The carrier signal is actually removed in the balanced modulator leaving the two side-bands as the only product form the balanced modulator.  Sorry... I forgot.]

4) Mixer:  The mixer is where the signal's frequency is changed to the desired transmit frequency.  The mixer combines the single-sideband signal with a new signal produced by an oscillator.  The result of this mixing is the signal's frequency is changed to the desired transmit frequency.

5) Amplifier: The signal is sent from the mixer to the amplifier where the signal's power is beefed up and then sent to the antenna.

Carrier wave oscillator to balanced modulator, to filter, to mixer, to amplifier, to problem.

You will also need to know the process for a superheterodyne receiver.  Here is Figure T6 from the Technician exam.

Remember, the basic skeleton of a superhet receiver is an oscillator, mixer, and product detector.  Filters and amplifiers just add to the basic receiver's performance.  The big function of a superhet receiver is that the receiver produces an intermediate frequency (IF) to ease processing and filtering the incoming signal.

1) The signal is received by the antenna and sent to the Mixer.  Sometimes there is an amplifier between the antenna and the mixer to aid in beefing up the received signal for processing.  The mixer combines the incoming signal with one produced by an oscillator.  The combining of these two signals produces an intermediate frequency (IF) which can further be filtered to improve the output quality.

2) In Figure T6, the IF signal is sent to an IF amplifier.  The choice of where to put the amplifier in Figure T6 was after the mixer vice before.  There are advantages and disadvantages to this which are not important for the purposes of the exam.  This block could have easily been an IF filter, or an IF filter could have been added in addition to the IF amplifier.  The point is that there is something that is processing the IF signal produced by the mixer/oscillator combination before it is sent to the product detector.

3) Block 1 in Figure T6 is a Product Detector.  The product detector combines the IF with another signal from the Beat Frequency Oscillator, similar to the mixer/oscillator process previously.  The resulting new frequency is an Audio Frequency (AF).  This demodulates the IF into a frequency range which, when fed into a speaker, produces understandable sounds (like your voice).  

4) The AF is then fed into an audio amplifier to beef up the AF's power prior to going to the speaker.

A Direct Conversion receiver is a type of heterodyne receiver.  The main difference between a direct conversion receiver and a superhet is there is no IF produced in a direct conversion receiver.  The signal goes from the antenna to the mixer/oscillator combination where it is converted directly into AF.

For an FM receiver, the discriminator performs the same function in demodulating the signal as the product detector does in the SSB superhet receiver.  When you see "discriminator" think FM.

A few definitions:
A Bleeder Resistor helps dissipate the electric field in a power supply capacitor.

Power supply filter networks usually consist of a network of capacitors and inductors.

The minimum peak inverse voltage of a rectifier in a Full-wave or Half-wave power supply should be twice the peak output voltage of the power supply.

Below is figure G7-1.  It should help with the schematic symbol portion of the quiz.

As always, if you have any questions, suggestions, or comments, please feel free to leave them in the comments box!


Monday, January 24, 2011

Lesson 25: General Class Exam Course G6C

In this lesson, we go over integrated circuits (ICs).  This lesson is fairly straight forward, except there are a few definitions that you will need to memorize.  If you remember from the Technician Class exam, Integrated Circuits are semiconductor devices with transistors, diodes, and other electrical components all on a single chip.  They have revolutionized technology by essentially taking and entire circuit and placing it on a very, very small chip.

Here is what you will need to focus on for the exam:

-Linear Voltage Regulators are most often provided as an ANALOG IC.

-CMOS stands for Complementary Metal-oxide Semiconductor and is the most common family of logic circuits.  They use very little power compared to other ICs.

-Operational Amplifiers (OpAmps) are ANALOG ICs.

-MMIC stands for Monolithic Microwave Integrated Circuit.

Please feel free to leave any comments, suggestions, or questions in the comments box.  Stand by for lesson's a big one.


Thursday, January 20, 2011

Lesson 24: General Class Exam Course G6B

Here is the 24th lesson in the General Class series.  This lesson goes over the G6B section from the question pool dealing with a wide spectrum of stuff, from batteries to vacuum tubes.  There is no real rhyme or reason behind the questions however, some of them are a bit deep.  You may want to go over the video a couple of times to make sure you've absorbed the information.

Here is the information you need to focus on:

Peak inverse voltage is the maximum voltage a rectifier (diode) can handle before it will allow current in the non-conducting direction.  Remember, a rectifier is, for the most part, a diode that only allows current to flow in one direction.  A rectifier will block current in the non-conductive direction up until the voltage increases to a point where it will overcome the rectifier's ability to block the current.  The maximum voltage a rectifier will block before it breaks down is the peak inverse voltage.

Junction threshold voltage is another one you will need to know.  Semiconductors are not great conductors, but they're not great insulators either.  It takes a little more push than usual to get current to flow across the semiconductor material.  This minimum voltage required to push current across a semiconductor is the junction threshold voltage.  The junction threshold voltage is different for each type of semiconductor material.

The junction threshold voltage for a germanium diode is 0.3 volts.  For a silicon diode it is 0.7 volts.

As we talked about in lesson 23, because of deficiencies in design, an electric component may have characteristics of another component.  Semiconductors and vacuum tubes are victim to this as well.  When AC is applied to these components, unintentional capacitance can build up within the component.  There are a couple different design developments which help disperse this capacitance:

A Schottky diode is a diode that has a piece of gold or platinum added to it which helps disperse the capacitance which builds up across the PN junction.

A vacuum tube has a screen grid placed between its grid and plate to disperse the capacitance that builds up between those two parts.

Vacuum tubes and transistors have some functional parallels.  The example that is brought up in the exam questions is the common functions between a vacuum tube and a Field Effect Transistor (FET).  Each have three leads.  A triode vacuum tube has the cathode, grid, and plate.  A FET has the source, drain, and gate.  The gate on a FET and the grid on a triode vacuum tube serve roughly the same purpose in controlling current flow through the component.

The other deep question in this section is the regions of a bipolar transistor.  The three regions you need to know are the cut-off region, the saturation region, and active region.  The transistor's region is determined by the polarity and amount of voltage applied to the various leads.  What you need to now for the exam is the cut-off region does not allow any current to flow through the transistor.  The saturation region allows current to flow freely through the transistor.  The active region is somewhere in between.  When the transistor is acting as a switch, you only want the transistor to work in the cut-off (off) and the saturation (on) regions.

Finally there's a bit about batteries.

Nickel Cadmium (NiCad) batteries have high discharge current.
Carbon-zinc batteries are disposable and should never be recharged.
Nickel Metal Hydride (NiMH) batteries are rechargeable.

Like I said, this lesson is all over the place.

If you have any questions, comments, or suggestions, please feel free to leave them in the comments box.


Tuesday, January 18, 2011

Lesson 23: General Class Exam Course G6A

Hello again!  Here is lesson 23 covering the G6A questions from the question pool.  The questions deal specifically with resistors, capacitors, and inductors.  More specifically, quirks with these types of components.  Depending on the situation, sometimes a capacitor may inadvertently act like and inductor, an inductor may act like a capacitor, and a wire-wound resistor may act like an inductor.  How much these components decide to pretend to be other components depends primarily on their design and the frequency of AC.  It is important to understand because these issues can cause a lot of problems, especially in tuned circuits.

Another quirk with resistors is they are temperature sensitive.  Depending on the resistor, as temperature increases or decreases their resistance value may increase or decrease as well.  The other problem is that this change in resistance with temperature is not universal for all resistors.  Some resistors increase resistance as their temperature increases, some decrease resistance as their temperature increases.  Resistors have a temperature coefficient rating (sometimes called tempco) which tells how they react to temperature.  A positive temperature coefficient means resistance increases as temperature increases.  A negative temperature coefficient means as temperature increases, resistance decreases.

There is one question that you will want to memorize, unless you are familiar with filters:

A filter choke is the common name for an inductor used to help smooth the DC output from the rectifier in a conventional power supply.

As always, please leave any suggestions, comments, or questions in the comments box.  Thanks!


Monday, January 17, 2011

Lesson 22: General Class Exam Course G5C

Here is lesson 22 and some math for hams.  This lesson mostly covers how to calculate total resistance, capacitance, and inductance in a circuit.  The formulas you need to know are below.  You may want a calculator for this lesson.

First, a definition you will need to memorize:

Magnetizing current is the current in the primary winding when no load is attached to the secondary winding.

The formulas:

To calculate total resistance for resistors in parallel:
Total R = 1/[1/R1 + 1/R2 + 1/R3 + 1/Rn...]

To calculate total resistance for resistors in series:
Total R = R1 + R2 + R3 + Rn...

To calculate total inductance for inductors in parallel:
Total Inductance = 1/[1/I1+ 1/I2 + 1/I3 +1/In...]

To calculate total inductance in for inductors in series:
Total Inductance = I1 + I2 + I3 +In...

Total inductance and resistance in a circuit are calculated in the same manner for components in series and in parallel.  Total capacitance is calculate in the opposite manner.

To calculate total capacitance for capacitors in parallel:
Total Capacitance = C1 + C2 + C3 + Cn...

To calculate total capacitance for capacitors in series:
Total Capacitance = 1/[1/C1 + 1/C2 + 1/C3 + 1/Cn...]

To find the voltage across the secondary winding in a transformer divide the volts AC connected to the primary winding by the ratio of primary turns to secondary turns.

VAC/(Primary turns/Secondary turns)

The turns ratio for of a transformer is the square root of the impedance ratio.  If you know the impedance you need to match:
(Square Root of Impedance 1/Impedance 2) : 1

Good luck.  As you will probably notice, I'm not a math teacher!  If you have any suggestions, questions, or comments, please feel free to leave them in the comments box.


Sunday, January 9, 2011

Lesson 21: General Class Exam Course G5B

Here is lesson 21 of the General Class Course.  This one is an absolute nightmare.  This section is action packed with formulas to memorize.  To make things more difficult, several of the questions require multiple formulas to answer.  You may want to go over this lesson a few times.  Here is the list of equations you will need to memorize:

Power  P=IE
     Where P is power in watts, I is current in amperes, and E is voltage in volts.

Ohm's Law  E=IR
     Where E is voltage in volts, I is current in amperes, and R is resistance in ohms.

[Because of type limitations, ^2 means to square)                                                                                        
Peak Envelope Power = (Peak Envelope Voltage x 0.707)^2 / R
     Where R is resistance in ohms.

Peak Envelope Voltage = Peak to Peak Voltage / 2

Peak voltage = RMS voltage x 1.414

A 1 dB loss is approximately equal to 20.5%.

Peak Envelope Power = (RMS voltage)^2 / R
    Where R is resistance in ohms.

RMS voltage = Peak voltage x 0.707

dB = 10Log(P1/P0)
     Where P0 is the reference power and P1 is the power compared to the reference power.

Good luck.  If you have any comments, suggestions, or questions, please feel free to leave them in the comments box.


Saturday, January 1, 2011

Lesson 20: General Class Exam Course G5A

Hello again!  Here is lesson 20 and the G5A questions from the exam question pool.  This lesson deals with impedance, reactance, and resistance. 

A lot of the time impedance is associated with antennas and antenna matching.  Impedance is much more broad of a topic.  Impedance matching is important to maximize the transfer of power.  The more closely the impedance is matched between the power source and the load, the more efficient the transfer of power is.  For the purposes of the exam, impedance is the opposition to current flow in an AC circuit.  Resistance and reactance are both types of impedance. 

Reactance is the other new term in this lesson.  Reactance is the opposition to the flow of current in an AC circuit caused by capacitance and/or inductance.  The electric field created by capacitors and the magnetic field created by inductors react to the constant reversal of current direction in AC circuits.  As the frequency of the AC current increases, the reactance (or opposition of current flow) caused by inductors increases.  For inductors, the higher the AC frequency, the higher the reactance.  Capacitors are just the opposite,  The higher the AC frequency, the lower the reactance caused by the capacitor.

Impedance, reactance, and resistance are all measured in Ohms.

If you have any suggestions, comments, or questions, please feel free to leave them in the comments box.  Thanks!