|This week we are going to cover oscillators, modulation, Emission Designators, mixers, receivers, transmitters.|
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An oscillator is simply an amplifier that has it's output fed back into it's input, this is called feedback. It is Usual for this feedback to occur through a Tuned Circuit, this tuned circuit controls the frequency produced by the oscillator.
The tuned circuit used in an oscillator circuit can either be made up of an Inductor and a capacitor (I.e. An LC tuned circuit as covered in module 3 ) or can Use a Quartz Crystal, this type of oscillator is called a Crystal Oscillator. The Quartz Crystal behaves like a very High Q LC tuned circuit. The main advantage to Crystal Oscillators is that the frequency produced is very stable.
LC tuned circuits often use either variable Capacitance or variable inductance, this allows the frequency of oscillation to be varied, this type of oscillator is called A Variable Frequency Oscillator, VFO for short.
Oscillators are covered on the following Web page
Modulation is the process of varying some Characteristic of a carrier wave in order to transmit information.
The simplest form of modulation is to switch on and off the oscillator in accordance with an agreed signalling method such as morse code, this type of modulation is called Continuous Wave or CW. With CW transmissions the transmitter is in either of two states, ON or OFF.
Other common forms of Modulation are Amplitude Modulation (this includes All forms of Single Sideband) and Frequency Modulation. These are covered in your course book 8-1 to 8-15. The following Web site also covers Modulation.
Emission Designators are an international system of describing types of emission, you will need to know some of these for the Experimenters exam. These Designators are also used on your licence document.
A complete list of all Emission Designators can be found on the following web page.
Mixers are one of the fundamental building blocks of both receivers and transmitters, they are covered on page 8-7 and 8-8 of your course book.
Mixers are circuits that combine two different input frequencies to produce two new frequencies at the output, the new frequencies are the SUM and the DIFFERENCE of the two input frequencies. With most types of mixer the two input frequencies are also present at the output of the mixer.
The output frequencies of a mixer are given by the following relationship
Output Frequency 1 = F1 + F2
A single-balanced mixer suppresses one of the input frequencies at the output and a Double-balanced mixer suppresses both of the input frequencies at the output.
Mixers are covered on the following Web page.
Receivers are covered well by the following web pages.
It is important to understand what each 'block' does and how to draw a basic Superhet receiver block diagram as these will form the basis of several exam questions.
Transmitters are covered in Chapter 8 of your course book (which by now I hope you have read in detail!)
The following web pages cover transmitters in the required detail for the Experimenters exam
Transmitter Theory This page only Covers PEP nevertheless an understanding of PEP will be required.
The following page is a very basic introduction to transmitter generated interference
The following web page covers interference and filtering
|Some additional reading in preperation for next week's module.
Please read Pages
Also read Chapter 4, pay particular attention to the following topics
Two-Tone tests (4-5 to 4-7)