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Chapter 2 Theory of Operation
Description of Operation
Figure 2
is a block diagram of the major functional parts of the Model 4391A RF
Wattmeter. The Microcomputer integrated circuit shown, controls all the other
portions of the instrument, which fall into two major groups.
Figure 2 Circuit Block Diagram
Keyboard, Range Switches, and Display Group
The keyboard and range switches serve only to pass information to the computer.
The display, of course, returns information from the computer to the operator.
The display, which is comprised of four seven-segment LED digits, is strobed digit
by digit left to right at a rate of approximately one digit per millisecond. This
serves to conserve battery power and drive circuitry while providing scanning for
the columns of the keyboard. Each time a digit is strobed, the corresponding
column of the keyboard is read and if a key is pressed, the computer puts the code
for that key into a memory cell. The nine mode keys select which parameter is to
be measured. The three modifier keys simply modify the way in which the result is
displayed. The range selector switches identify to the computer the nominal full
scale values of the elements used. They have no effect on input sensitivity, which
is determined by the elements.
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