LESSON 2
MIXERS, CONVERTERS, AND LOCAL OSCILLATORS
SCOPE.....................................Principles, purposes, requirements,
and
types
of
mixers,
converters,
and
oscillators used in FM receivers.
CREDIT HOURS..............................2
TEXT ASSIGNMENT...........................TM 11-668, para 62-71;
Attached Memorandum, para 2-1 thru 2-5
MATERIALS REQUIRED........................None
SUGGESTIONS...............................Read the assignment in TM 11-668 before
you read the attached memorandum.
LESSON OBJECTIVES
When you have completed this lesson, you will be able to:
1.
Determine output frequencies and the conversion gain of a mixer stage.
2.
Describe circuit devices that can be operated as mixers and converters.
3. Distinguish among the various circuit configurations that are used as local
oscillators.
4.
Analyze the local oscillator circuits.
ATTACHED MEMORANDUM
2-1.
MIXING
a. Methods.
The process of combining RF with an oscillator's frequency to
produce an IF is called mixing or frequency conversion. The two basic methods of
frequency conversion employed with electron tubes are also employed with
transistors. The first method, a transistor that combines an oscillator frequency
and a radio frequency is called a mixer. In the second method, only one transistor
functioning as both oscillator and mixer is used.
This circuit is known as a
converter.
b. Output Frequencies. When a transistor is used as a mixer or a converter, it
is operated on the curved portion of its dynamic characteristic curve. Under these
conditions, when two frequencies are applied to the transistor input, four major
frequencies are produced in the output.
Two of the output frequencies are the
original frequencies that were present at the input.
Another one of the output
frequencies is a frequency that is equal to the sum
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