## 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 antenna...no 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!

73,
Andy
KE4GKP